For those asking - red line is Anti-rise (only for the revolt, the others are anti squat)
For those asking - red line is Anti-rise (only for the revolt, the others are anti squat)
Holy shit, so it's pretty damn high on the Revolt. If I loved braking on that bike and thought it was confidence inspiring, then maybe I'm a high AR kinda guy.
Holy shit, so it's pretty damn high on the Revolt. If I loved braking on that bike and thought it was confidence inspiring, then maybe I'm...
Holy shit, so it's pretty damn high on the Revolt. If I loved braking on that bike and thought it was confidence inspiring, then maybe I'm a high AR kinda guy.
My question was meant to be what brake would you use to get down a steep trail the slowest.
Catch berms aren’t steep. They are catch berms. Tight switch backs are steep trails. I’m talking steep shit. Dead Dog.
For sure you’d get spanked once in a while with just the front but if I had to go slow down something I’m using my front. You don’t need the steepest trail in the world to know that you’ll be accelerating with just the rear.
I think I get what you’re saying but still confused on why a 100% anti-rise bike wouldn’t be the most active because it has the least...
I think I get what you’re saying but still confused on why a 100% anti-rise bike wouldn’t be the most active because it has the least amount of chassis change.
Yes. That’s actually something I was just talking to a buddy about and is why bikes like the Raaw seem to work well (for a 4 bar) in a lot of people’s eyes. The higher stack accounts for the pitching sensation. Though, without going down the geo rabbit hole too much I am constantly reminded that the effective stack/anti-rise safety is done with length to the head tube meaning you could active a similar bar height with spacers..?
Brake squat requires the wheel to be torqued forwards under braking to try and keep the rear suspension compressed while it unweights. The corresponding motion is...
Brake squat requires the wheel to be torqued forwards under braking to try and keep the rear suspension compressed while it unweights. The corresponding motion is the wheel wanting to rotate rearwards as the suspension unwinds and extends. This has the effect of adding extra friction damping to your suspension. Either from the disc brake or from the tyre contact patch.
The suspension with the least braking interaction (doesn't try to squat or jack, just reacts to weight shifts) is the most free to move under braking because of the lack of extra friction damping.
100% anti-rise is suspension that actively tries to squat pretty hard under braking. That requires a lot of interaction. I haven't checked to see if any current bikes get close to that number.
Yes squat and antisquat calcs depend on not only the wheelbase but centre of gravity height. Head-tube doesn't matter but bar height will.
I was always told 100% anti-rise is neutral and the bike staying at a constant ride height? That's not the case?
The center of gravity is where I always find issues with the anti-rise debate because it's only a rough estimate based off center of gravity and instant center. That means two different riding styles/center of gravities could have different effective anti-rises.?
I was always told 100% anti-rise is neutral and the bike staying at a constant ride height? That's not the case?The center of gravity is where...
I was always told 100% anti-rise is neutral and the bike staying at a constant ride height? That's not the case?
The center of gravity is where I always find issues with the anti-rise debate because it's only a rough estimate based off center of gravity and instant center. That means two different riding styles/center of gravities could have different effective anti-rises.?
Geometry is preserved but that’s only because the force of your weight shifting forward is equal to the force driving the bike into compression. A perfectly 0% antirise bike would feel like someone grabbing the back of your back wheel, an OTB machine
I was always told 100% anti-rise is neutral and the bike staying at a constant ride height? That's not the case?The center of gravity is where...
I was always told 100% anti-rise is neutral and the bike staying at a constant ride height? That's not the case?
The center of gravity is where I always find issues with the anti-rise debate because it's only a rough estimate based off center of gravity and instant center. That means two different riding styles/center of gravities could have different effective anti-rises.?
Yes, 100% is the theoretical point that the suspension doesn't compress or extend. In my lasso analogy above, it would equivalent to catching it right around the centre of gravity
And yes, published anti squat/rise values assume a certain CoG and it isn't alway the same, so taller/shorter riders or different size bikes will respond a ittle different in the real world, but if you compare 2 bikes with the variables the same then you can roughly guess how they will behave relative to each other
I’d put it out to everyone to think about if they only had one brake to ride a steep trail, what brake would they want? I...
I’d put it out to everyone to think about if they only had one brake to ride a steep trail, what brake would they want? I would choose my front brake to get down the slowest.
I ride in an area with much steeper than average trails and I run my 38 at 94 psi and one spacer for 150 lb rider. Less than that and it dives and I get pitched forward. LSC doesn’t help much when it’s turbo steep for 200m. Many trails are quite linear and steep.
If there are steps and compressions you can definitely push your feet into your back wheel and get some extra traction. But I guess those steps where you get traction would technically not be steep.
@cascade I’m surprised you would use your rear brake enough to get pitched forward going into a rock slab. Perhaps I don’t realize how little braking would make your rear rise.
On really steep trails I am usually using the middle brake. Common for BMX riders, this brake works will but it wears out shoes not pads.
Here's the weight distribution graphic I promised:Any questions?
Here's the weight distribution graphic I promised:
Bike Weight Distribution Downhill
Any questions?
So I went and grabbed my DH bike to see how the CG location stacks up. This is an easy experiment to do if anyone feels inclined to put some real numbers on themselves… measure the weight of you and your bike. In my case this is 190.4 lbs. Then find a nice level spot where you can balance with your bar end ever so slightly on a wall for support. Put the scale under the front wheel and then weight the bike however you see fit. Record that weight. (Total weight-front wheel weight)/total weight is the percentage of weight carried by the rear wheel. You can then find the X coordinate of your CG. With my weight shifted reasonably far back but not stretching it I got 41.4 lbs on the front wheel (78.26%). The bike in question has a 1299 mm wheelbase. (1-0.7826)*wheelbase is the horizontal distance from the rear axle to your CG. In my case here that’s 282 mm. Y height of CG is harder to estimate, but I’d put mine at maybe 645 mm.
Now with the weight distribution problem it’s a little more complicated than where the CG is relative to the front axle. How much braking force can be generated is a massive factor. If we assume as much braking force as friction allows, the angle of inclination actually factors out entirely and the end result is a function of coefficient of friction. In the case of my DH bike, the equation is 0.78-0.50μ. With a coefficient of friction of 0.5, that puts 53% of the weight on the rear wheel. For a tire on dry concrete, μ is 1 so on a super grippy surface you’ve still got 28% on the rear wheel. And if there’s no grip… you’re at the 78% rear wheel bias regardless of angle of inclination.
So I went and grabbed my DH bike to see how the CG location stacks up. This is an easy experiment to do if anyone feels...
So I went and grabbed my DH bike to see how the CG location stacks up. This is an easy experiment to do if anyone feels inclined to put some real numbers on themselves… measure the weight of you and your bike. In my case this is 190.4 lbs. Then find a nice level spot where you can balance with your bar end ever so slightly on a wall for support. Put the scale under the front wheel and then weight the bike however you see fit. Record that weight. (Total weight-front wheel weight)/total weight is the percentage of weight carried by the rear wheel. You can then find the X coordinate of your CG. With my weight shifted reasonably far back but not stretching it I got 41.4 lbs on the front wheel (78.26%). The bike in question has a 1299 mm wheelbase. (1-0.7826)*wheelbase is the horizontal distance from the rear axle to your CG. In my case here that’s 282 mm. Y height of CG is harder to estimate, but I’d put mine at maybe 645 mm.
Now with the weight distribution problem it’s a little more complicated than where the CG is relative to the front axle. How much braking force can be generated is a massive factor. If we assume as much braking force as friction allows, the angle of inclination actually factors out entirely and the end result is a function of coefficient of friction. In the case of my DH bike, the equation is 0.78-0.50μ. With a coefficient of friction of 0.5, that puts 53% of the weight on the rear wheel. For a tire on dry concrete, μ is 1 so on a super grippy surface you’ve still got 28% on the rear wheel. And if there’s no grip… you’re at the 78% rear wheel bias regardless of angle of inclination.
Have a look at Matt Beer's article from PB. At 2:40 in the video you can see his weight distribution changing in real time.
That said, I don't need another Shockcraft branded drawing to tell me if I can use my rear brake on a steep trail. 30 some years of actual riding is a sufficient data point to know the answer is yes. Often and effectively.
That graphic is my old Bergamont Encore (475 reach) and measured weight distributions. DH bikes used to be more rearward, but Enduro bikes have caught up...
That graphic is my old Bergamont Encore (475 reach) and measured weight distributions. DH bikes used to be more rearward, but Enduro bikes have caught up enough that some companies are using the same frames for both!
I think your tyre friction coefficients might be a bit optimistic. Coefficient of 1 and higher required a very sticky and hot tyre on a clean surface. I think 0.8 static is about where you'd be for a sticky MTB tyre on rock/concrete/asphalt. Gravel/dirt is going to vary massively and worth testing. Anything wet is going to be very low.
This is good data from tractors:
Concrete/asphalt/rock static 0.75, dynamic a bit over 0.8. Hard dirt static 0.5, dynamic (really digging in) almost 0.8 Loose static 0.4, really digging in a bit over 0.5
Problem with the back brake is it loses weight as you brake. On flat ground seated you've generally got 2/3 weight on the back wheel. But the harder you brake the more weight shifts forwards and off your back wheel. Which Braking on hard ground (0.8 traction) with the back brake you're going to peak out about 2.6 m/s^2 (0.25 G force) decelleration using the back wheel (depending on geometry). Braking on hard ground with the front wheel you're going to peak out about 8m/s^2 (0.8 G Force) (rear wheel zero weight).
Front wheel on hard ground and firm soil is ~3x more effective.
Add in a down slope and that 3x number gets bigger. To the point where the back wheel can't even hold the bike.
We all get that braking shifts weight forward. What I just walked through is how much weight you can end up with on the front and rear wheels in a certain scenario that involves braking as hard as grip will allow. Downslope does not factor into weight distribution. If you walk through the math you end up with a cos(theta) term in both the numerator and denominator that cancel out. 0.5 is a pretty reasonable number for loose soil such as a rake and ride and that yields about a 50/50 weight distribution on any slope with weight shifted rearward. You can easily end up with it biased even more rearward riding in the wet.
That plot does bring up an interesting thing about braking force on loose surfaces that happens to lend itself towards rear wheel braking. Wheel slip from the front wheel is generally not very tolerable especially if turning is involved but is required for max braking force. On the contrary the rear wheel can slip a ton. If you want hard braking that's less susceptible to the front wheel slipping and washing out maybe it's a good idea to bias weight rearward.
We all get that braking shifts weight forward. What I just walked through is how much weight you can end up with on the front and...
We all get that braking shifts weight forward. What I just walked through is how much weight you can end up with on the front and rear wheels in a certain scenario that involves braking as hard as grip will allow. Downslope does not factor into weight distribution. If you walk through the math you end up with a cos(theta) term in both the numerator and denominator that cancel out. 0.5 is a pretty reasonable number for loose soil such as a rake and ride and that yields about a 50/50 weight distribution on any slope with weight shifted rearward. You can easily end up with it biased even more rearward riding in the wet.
That plot does bring up an interesting thing about braking force on loose surfaces that happens to lend itself towards rear wheel braking. Wheel slip from the front wheel is generally not very tolerable especially if turning is involved but is required for max braking force. On the contrary the rear wheel can slip a ton. If you want hard braking that's less susceptible to the front wheel slipping and washing out maybe it's a good idea to bias weight rearward.
Yup good points! Rubber has the highest grip when there is a small amount of slip. Also your front tyre can't brake AND turn in equal amounts so even if you were braking with the front only, you better hope there aren't any turns on that trail!
And because the friction coefficient (which is different to tractive efficiency...) gets lower as you increase vertical load, if you were to have 100% weight on the front wheel (which you don't) you would still have much less braking available anyway.
Also does anyone know the actual gradient of a "steep" trail? Most people here probably don't need convincing but if you stick your phone on the ground with the surface level app going you would see its no where near as high an angle as some would think
They say Dead Dog is 32 degrees. Definitely not the steepest trail but the key is it’s constant with no catch berms for very long distances. This makes it a good test for this conversation as it’s all about what gives you maximum braking power.
Downslope drives your entire weight distribution. This is due to your weight being above the ground line so slope changes the proportions.The only time your values...
Downslope drives your entire weight distribution. This is due to your weight being above the ground line so slope changes the proportions. The only time your values would cancel out is if COG was at ground level. Which gets close for a split second after an OTB!
Moving your weight back isn't optional on steeps. If you don't do it you're eating dirt.
Here is the resulting weight shifts and max traction on a 20° (36%) downslope:
Bike Braking Dynamics
The front braking is limited by stability (rear wheel lift), the rear is limited by the interplay of forward weight shift and traction.
Slope angle factors out entirely. You need to step through it by summing the moments about the front contact patch. These sketches work fine for reference, but you need to actually do the math. It's pretty easy to see how m, g, and Cos(theta) factor out, but I had it simplify for the sake of it.
Yup good points! Rubber has the highest grip when there is a small amount of slip. Also your front tyre can't brake AND turn in equal...
Yup good points! Rubber has the highest grip when there is a small amount of slip. Also your front tyre can't brake AND turn in equal amounts so even if you were braking with the front only, you better hope there aren't any turns on that trail!
And because the friction coefficient (which is different to tractive efficiency...) gets lower as you increase vertical load, if you were to have 100% weight on the front wheel (which you don't) you would still have much less braking available anyway.
Also does anyone know the actual gradient of a "steep" trail? Most people here probably don't need convincing but if you stick your phone on the ground with the surface level app going you would see its no where near as high an angle as some would think
Yeah it's actually kind of comical and makes you feel like you can't ride anything steep some times. You may very well find you can't slow down on even maintain speed on a 25 degree decline.
Let's see if we can find some common ground on flat braking.Your bike, 1299 wheelbase, 86kg total weight, in rearward position you've got 22/64kg fr/rear split.Using...
Let's see if we can find some common ground on flat braking.
Your bike, 1299 wheelbase, 86kg total weight, in rearward position you've got 22/64kg fr/rear split. Using a 1m COG height (round numbers, can adjust later) I get max rear braking at 3m/s^2 (0.3G). This results in a forward weight shift of 20.3kg and dynamic weights of 42F and 44R kg.
Front brake you can pull 9.5 m/s^2 (just over 1G) IFF you've got the grip. Front is a a shade over 3x as effective as the rear on flat ground.
So I think what’s throwing you for a loop is there is a fundamental flaw in your FBD. What you have drawn is only accurate if the front wheel is pinned to the ground at the contact patch (ie wheel stuck up against a log or something). If your wheel is stuck up against a log or anything else that prevents it from being able to roll downhill then the weight distribution 100% shifts forward as it gets steeper. But this is a fundamentally different scenario than braking force. See the FBD below if you’re actually interested in sorting it out. There is one equation for coefficient of friction where they work out to be the same (when μ=tan(θ)), but that’s it. The rear wheel weight distribution equation for max braking force can be boiled down to (initial weight fraction)-(ycg/wheelbase)*μ. There are no hidden layers or complexity to it beyond that rear wheel and front wheel friction may be different in practice. You can figure out what value of μ would result in zero rear wheel weight by solving (initial weight fraction*wheelbase)/ycg. You’ll find that number to be on the very high end that you’d only see riding things like Squamish slabs.
Those diagrams are illustrations to show people, they aren't intended to be FBD, they've got too many arrows for a start and those arrows aren't to...
Those diagrams are illustrations to show people, they aren't intended to be FBD, they've got too many arrows for a start and those arrows aren't to scale either. If the front wheel was pinned the forces would go through the axle instead of contact patch. But it isn't. On your FBD you've got vectors joining tip to tip under each contact patch. They should be tip-tail which then orients the resulting sum in the directions I have shown.
I'm running all my calculations on a separate spread-sheet. Throw some numbers into your calcuations and see what you get for forward weight shift on that angle static and under braking. Remember if you're braking to decelerate that modifies your gravity vector. D'Alembert's Principle.
It doesn’t matter if they are tip to tip, tip to tail, or tail to tail so long as they are in the correct orientation because vectors are summed based on their components. Same goes for gravity. It’s broken down into components in that diagram such that everything is parallel or perpendicular and can be summed easily. Go look up the FBD of a car braking. This is basic stuff. Static is essentially the same as setting the coefficient of friction to tan(theta) so you’re changing a relationship to that is fundamental to the problem by looking at that. D'Alembert's Principle is why we can ignore the mgSin(theta) portion of the gravity vector which is what I’ve done in the calculations above. What I’ve shown you is correct. You can plug whatever weight distributions, friction coefficients, and CG heights you want in it. There is not a missing bit that causes weight to shift forward more when going down something steeper.
If you don't add vectors tip to tail you get the wrong direction. Yes this is basic stuff. Can you throw some numbers into your diagram...
If you don't add vectors tip to tail you get the wrong direction. Yes this is basic stuff.
Can you throw some numbers into your diagram and tell us what you get for braking force and weight shift?
How they are positioned on a piece of paper only impacts the end result if you create the summed vector by tracing a line from the first tail to the last tip. But I’m not tracing lines so it doesn’t matter.
There are only four numbers to plug in. It doesn’t necessitate a spread sheet. I’ve thrown countless numbers into it that I’ve listed here before. I can come up with weight distributions and friction values that will give pretty much any weight shift imaginable. There’s no point. This was all to show that with a rearward weight bias it’s perfectly reasonable to expect 50% of braking force to come from the rear wheel still. If you have shifted your weight rearward, which usually moves it downward too, and it’s not a super grippy surface that’s perfectly reasonable to expect.
If you don't add vectors tip to tail you get the wrong direction. Yes this is basic stuff. Can you throw some numbers into your diagram...
If you don't add vectors tip to tail you get the wrong direction. Yes this is basic stuff.
Can you throw some numbers into your diagram and tell us what you get for braking force and weight shift?
That said, if you use an initial distribution of 2/3 on the rear wheel, a CG height of 1000, a wheelbase of 1299, and a coefficient of friction of 0.5, you get 28% of weight on the rear wheel under max braking regardless of angle.
See the problem?You've just solved for 28% weight on the back wheel. That's half the 50% you were expecting.I'll have more on this later on. Rest...
See the problem?
You've just solved for 28% weight on the back wheel. That's half the 50% you were expecting.
I'll have more on this later on. Rest of my week is fully booked.
Now if I put in an initial distribution of 0.78, CG height of 650, wheelbase of 1299, and coefficient of friction of 0.5 I get 0.53… there are four variables. I’m pretty sure we don’t ride around fixed to our bikes in one position. You can get even higher if you go with something actually slippery for friction like 0.2.
I'm so glad I left engineering. Lol. This reminds me of discussions with my old boss who loved analysis and wouldn't leave us alone when we had anything vaguely interesting on the go.
Trails have bumps. Bumps give momentary grip. the momentary grip results in a momentary braking force. The momentary braking force results in a momentary decrease in the effective spring rate if you have high AR. So the bike feels softer in the rear which is exactly what you want in the moment when your rear tire hits a bump riding down steep shit.
Trails have corners. Even if the gradient in the corner is no less than the gradient elsewhere, by changing direction you can load up the rear tire more and use it to slow down. Higher AR helps settle the bike and lengthen the amount of time you get to push the rear wheel into the ground before you bounce off again.
Trails have changes in gradient. Steep trails have sections where they're steep or featureless enough where the rear brake does fuck all, and they also have slightly less steep parts where you can get your braking done. Those bits are often still steep, but you can use the rear brake. Saying a rear brake is pointless on a steep trail because it's too steep is like saying a front tire is pointless on a jump track because the tire is in the air.
Who cares about your FBDs, go ride your bike. If you don't think rear brakes matter on steep shit then go ride some steeper, techier shit.
While you guys were working out I went and rode my bike down some steep ass shit. Tried for a brief second to only use rear brake. Instant acceleration. I can’t tell if you guys are taking the piss with this or not.
Each brake is liable to contribute a significant amount to total braking force. Only using one is always going to do noticeably less and the slope angle at which you can no longer hold yourself will get significantly shallower in angle. Sometimes the rear brake will do significantly less than the front, but if grip is bad and you’ve got your weight shifted rearward the front brake might be doing way less than you’d think. There’s a lot of merit to the larger rear brake rotor idea if you’re racing a steep loose track. Kind of ironic to me because Brosnan’s team was messing with different post mounts that they thought would change anti-rise but actually don’t at all.
One other point about anti-rise and steeps. As the trail gets steeper less weight is supported by the suspension so sag drops. That equals less rise available until the shock tops out. So combine that with low anti-rise allowing the bike to pitch forward more and the likelihood of getting your shock to top out goes up. High anti-rise hinders rebound more than anything, but at least you have room available for the shock to rebound.
For those asking - red line is Anti-rise (only for the revolt, the others are anti squat)
Holy shit, so it's pretty damn high on the Revolt. If I loved braking on that bike and thought it was confidence inspiring, then maybe I'm a high AR kinda guy.
Maybe it just wasn’t a Felt
My question was meant to be what brake would you use to get down a steep trail the slowest.
Catch berms aren’t steep. They are catch berms. Tight switch backs are steep trails. I’m talking steep shit. Dead Dog.
For sure you’d get spanked once in a while with just the front but if I had to go slow down something I’m using my front. You don’t need the steepest trail in the world to know that you’ll be accelerating with just the rear.
I was always told 100% anti-rise is neutral and the bike staying at a constant ride height? That's not the case?
The center of gravity is where I always find issues with the anti-rise debate because it's only a rough estimate based off center of gravity and instant center. That means two different riding styles/center of gravities could have different effective anti-rises.?
Geometry is preserved but that’s only because the force of your weight shifting forward is equal to the force driving the bike into compression. A perfectly 0% antirise bike would feel like someone grabbing the back of your back wheel, an OTB machine
Yes, 100% is the theoretical point that the suspension doesn't compress or extend. In my lasso analogy above, it would equivalent to catching it right around the centre of gravity
And yes, published anti squat/rise values assume a certain CoG and it isn't alway the same, so taller/shorter riders or different size bikes will respond a ittle different in the real world, but if you compare 2 bikes with the variables the same then you can roughly guess how they will behave relative to each other
On really steep trails I am usually using the middle brake. Common for BMX riders, this brake works will but it wears out shoes not pads.
Lmao
So when it gets steep enough that a rear brake loses efficacy, you’re gonna crash if you use front brake anyway? Lol
So I went and grabbed my DH bike to see how the CG location stacks up. This is an easy experiment to do if anyone feels inclined to put some real numbers on themselves… measure the weight of you and your bike. In my case this is 190.4 lbs. Then find a nice level spot where you can balance with your bar end ever so slightly on a wall for support. Put the scale under the front wheel and then weight the bike however you see fit. Record that weight. (Total weight-front wheel weight)/total weight is the percentage of weight carried by the rear wheel. You can then find the X coordinate of your CG. With my weight shifted reasonably far back but not stretching it I got 41.4 lbs on the front wheel (78.26%). The bike in question has a 1299 mm wheelbase. (1-0.7826)*wheelbase is the horizontal distance from the rear axle to your CG. In my case here that’s 282 mm. Y height of CG is harder to estimate, but I’d put mine at maybe 645 mm.
Now with the weight distribution problem it’s a little more complicated than where the CG is relative to the front axle. How much braking force can be generated is a massive factor. If we assume as much braking force as friction allows, the angle of inclination actually factors out entirely and the end result is a function of coefficient of friction. In the case of my DH bike, the equation is 0.78-0.50μ. With a coefficient of friction of 0.5, that puts 53% of the weight on the rear wheel. For a tire on dry concrete, μ is 1 so on a super grippy surface you’ve still got 28% on the rear wheel. And if there’s no grip… you’re at the 78% rear wheel bias regardless of angle of inclination.
Have a look at Matt Beer's article from PB. At 2:40 in the video you can see his weight distribution changing in real time.
That said, I don't need another Shockcraft branded drawing to tell me if I can use my rear brake on a steep trail. 30 some years of actual riding is a sufficient data point to know the answer is yes. Often and effectively.
We all get that braking shifts weight forward. What I just walked through is how much weight you can end up with on the front and rear wheels in a certain scenario that involves braking as hard as grip will allow. Downslope does not factor into weight distribution. If you walk through the math you end up with a cos(theta) term in both the numerator and denominator that cancel out. 0.5 is a pretty reasonable number for loose soil such as a rake and ride and that yields about a 50/50 weight distribution on any slope with weight shifted rearward. You can easily end up with it biased even more rearward riding in the wet.
That plot does bring up an interesting thing about braking force on loose surfaces that happens to lend itself towards rear wheel braking. Wheel slip from the front wheel is generally not very tolerable especially if turning is involved but is required for max braking force. On the contrary the rear wheel can slip a ton. If you want hard braking that's less susceptible to the front wheel slipping and washing out maybe it's a good idea to bias weight rearward.
Yup good points! Rubber has the highest grip when there is a small amount of slip. Also your front tyre can't brake AND turn in equal amounts so even if you were braking with the front only, you better hope there aren't any turns on that trail!
And because the friction coefficient (which is different to tractive efficiency...) gets lower as you increase vertical load, if you were to have 100% weight on the front wheel (which you don't) you would still have much less braking available anyway.
Also does anyone know the actual gradient of a "steep" trail? Most people here probably don't need convincing but if you stick your phone on the ground with the surface level app going you would see its no where near as high an angle as some would think
They say Dead Dog is 32 degrees. Definitely not the steepest trail but the key is it’s constant with no catch berms for very long distances. This makes it a good test for this conversation as it’s all about what gives you maximum braking power.
Slope angle factors out entirely. You need to step through it by summing the moments about the front contact patch. These sketches work fine for reference, but you need to actually do the math. It's pretty easy to see how m, g, and Cos(theta) factor out, but I had it simplify for the sake of it.
Yeah it's actually kind of comical and makes you feel like you can't ride anything steep some times. You may very well find you can't slow down on even maintain speed on a 25 degree decline.
Most people feel a lot of exposure over a scree slope and consider them too step to ride, and they're only 30 degrees or so.
How about this, In n Out Burger with only your rear or only your front brake? Out run not included.
So I think what’s throwing you for a loop is there is a fundamental flaw in your FBD. What you have drawn is only accurate if the front wheel is pinned to the ground at the contact patch (ie wheel stuck up against a log or something). If your wheel is stuck up against a log or anything else that prevents it from being able to roll downhill then the weight distribution 100% shifts forward as it gets steeper. But this is a fundamentally different scenario than braking force. See the FBD below if you’re actually interested in sorting it out. There is one equation for coefficient of friction where they work out to be the same (when μ=tan(θ)), but that’s it. The rear wheel weight distribution equation for max braking force can be boiled down to (initial weight fraction)-(ycg/wheelbase)*μ. There are no hidden layers or complexity to it beyond that rear wheel and front wheel friction may be different in practice. You can figure out what value of μ would result in zero rear wheel weight by solving (initial weight fraction*wheelbase)/ycg. You’ll find that number to be on the very high end that you’d only see riding things like Squamish slabs.
It doesn’t matter if they are tip to tip, tip to tail, or tail to tail so long as they are in the correct orientation because vectors are summed based on their components. Same goes for gravity. It’s broken down into components in that diagram such that everything is parallel or perpendicular and can be summed easily. Go look up the FBD of a car braking. This is basic stuff. Static is essentially the same as setting the coefficient of friction to tan(theta) so you’re changing a relationship to that is fundamental to the problem by looking at that. D'Alembert's Principle is why we can ignore the mgSin(theta) portion of the gravity vector which is what I’ve done in the calculations above. What I’ve shown you is correct. You can plug whatever weight distributions, friction coefficients, and CG heights you want in it. There is not a missing bit that causes weight to shift forward more when going down something steeper.
How they are positioned on a piece of paper only impacts the end result if you create the summed vector by tracing a line from the first tail to the last tip. But I’m not tracing lines so it doesn’t matter.
There are only four numbers to plug in. It doesn’t necessitate a spread sheet. I’ve thrown countless numbers into it that I’ve listed here before. I can come up with weight distributions and friction values that will give pretty much any weight shift imaginable. There’s no point. This was all to show that with a rearward weight bias it’s perfectly reasonable to expect 50% of braking force to come from the rear wheel still. If you have shifted your weight rearward, which usually moves it downward too, and it’s not a super grippy surface that’s perfectly reasonable to expect.
That said, if you use an initial distribution of 2/3 on the rear wheel, a CG height of 1000, a wheelbase of 1299, and a coefficient of friction of 0.5, you get 28% of weight on the rear wheel under max braking regardless of angle.
Now if I put in an initial distribution of 0.78, CG height of 650, wheelbase of 1299, and coefficient of friction of 0.5 I get 0.53… there are four variables. I’m pretty sure we don’t ride around fixed to our bikes in one position. You can get even higher if you go with something actually slippery for friction like 0.2.
I'm so glad I left engineering. Lol. This reminds me of discussions with my old boss who loved analysis and wouldn't leave us alone when we had anything vaguely interesting on the go.
Trails have bumps. Bumps give momentary grip. the momentary grip results in a momentary braking force. The momentary braking force results in a momentary decrease in the effective spring rate if you have high AR. So the bike feels softer in the rear which is exactly what you want in the moment when your rear tire hits a bump riding down steep shit.
Trails have corners. Even if the gradient in the corner is no less than the gradient elsewhere, by changing direction you can load up the rear tire more and use it to slow down. Higher AR helps settle the bike and lengthen the amount of time you get to push the rear wheel into the ground before you bounce off again.
Trails have changes in gradient. Steep trails have sections where they're steep or featureless enough where the rear brake does fuck all, and they also have slightly less steep parts where you can get your braking done. Those bits are often still steep, but you can use the rear brake. Saying a rear brake is pointless on a steep trail because it's too steep is like saying a front tire is pointless on a jump track because the tire is in the air.
Who cares about your FBDs, go ride your bike. If you don't think rear brakes matter on steep shit then go ride some steeper, techier shit.
While you guys were working out I went and rode my bike down some steep ass shit. Tried for a brief second to only use rear brake. Instant acceleration. I can’t tell if you guys are taking the piss with this or not.
It's more that, if you're along for the ride anyway then having a rear brake for handling purposes is a lot better than a front brake.
Each brake is liable to contribute a significant amount to total braking force. Only using one is always going to do noticeably less and the slope angle at which you can no longer hold yourself will get significantly shallower in angle. Sometimes the rear brake will do significantly less than the front, but if grip is bad and you’ve got your weight shifted rearward the front brake might be doing way less than you’d think. There’s a lot of merit to the larger rear brake rotor idea if you’re racing a steep loose track. Kind of ironic to me because Brosnan’s team was messing with different post mounts that they thought would change anti-rise but actually don’t at all.
One other point about anti-rise and steeps. As the trail gets steeper less weight is supported by the suspension so sag drops. That equals less rise available until the shock tops out. So combine that with low anti-rise allowing the bike to pitch forward more and the likelihood of getting your shock to top out goes up. High anti-rise hinders rebound more than anything, but at least you have room available for the shock to rebound.
I'm just waiting for the usual Dougal Crash out - which happens every single time someone disagrees with him regardless of who's right.
😏 - Popcorns in the microwave
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