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Universally sized motor powered vehicles on smooth tracks VS rider size specific kinetic energy machines on varied terrain where momentum is everything
I'm 6'2", gimme the juice.
Yea it’s wrong . Also , on this point, on a modern 170 travel enduro bike with high stack , 29inch front wheel then add my height. My head is miles away from the trail. Then add a rutted in uk off camber turn . I find big bikes very hard to balance. 32 inch wheel my head would be touching tree tops. On average , we large folk arent known for our graceful balance. 150 mm fork with a 27.5 wheel is a lot easier to ride them ruts
I just read through this thread and wanted to bring up a few points I haven't read yet.
This is kinda nerdy, but I believe this is the place for it.
There's a widespread idea that the bigger wheels accelerate slower due to a higher moment of inertia, but what is forgotten is larger wheels don't need to rotate as quickly due to their larger circumference (and therefore a lower RPM at a given speed). While there is a real overall increase in mass from a bigger wheel/tire which will impede acceleration; it's not because the wheel is getting bigger. The moment of inertia increase is negated proportionally by the circumference increase/RPM decrease.
IE a 3kg small wheel will functionally accelerate along the ground the same as a 3kg big wheel.
Right now in the XC world a light 29er wheelset is anything below 1200g (no tires, cassette, rotors), and the lightest available 32er wheelset is somewhere around 1450g. I'd wager good money most people have never ridden a wheelset below 1500g of any size, especially if those people consider themselves to have a downhill bias.
I'm building up a 32er and will report back my ride impressions in a few months. I want to keep an open mind before I gather enough experience to pass judgement.
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That being said I'm really excited about it. The decrease in attack/rollover angle is what I'm most excited about. Many of the trails I like the most are bumpy in a way that I believe the 32er is going to make a pronounced difference on.
There is an excellent thread on the customframeforum that is here for those who want to read another thoughtful discussion on 32ers.
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I think the most relevant CON for mountain bikers is going to end up as tire/butt buzz.
Where I live in WY I NEVER buzz my but with my tire. I've been riding full 29ers of various kinds for almost 20 years now. But in WNC where I used to live I'd buzz my rear on the tire during some moves. Tire/butt buzz is going to get much worse with 32 but for a whole slew of riding it may not matter at all.
I was just in Sedona for 4.5 days riding the rowdy stuff there and could see a 32er rear tire causing problems on some of the sustained steep rock rolls (like the section that drops off after the Hangover traverse if you're familiar with that trail).
That being said, 25 years ago when I was a young kid and short 26ers were all we had I remember riding way off the back on trails I'm far more centered on with modern geometry and skills. It could be that as skills and geometry have progressed the need to ride over the back has been greatly reduced.
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My 32er frame has 450mm CS, which is "long" but not insane. I've had 29ers with 450 rear ends and they rode fine, great even. I think it's really easy in the mtb world to get bogged down by numbers when in reality there are myriad solutions to ride trails and have a good time.
I'm really stoked for more tires to come out to open up more possibilities.
Peace nerds,
PH
If I'm riding like a chad and standing up properly, even in steeps, I don't think I should be getting much in the way of butt buzzing. Although when confidence and form collapses, I can see it becoming more of an issue, particularly once 140mm bikes start coming out.
I share probs' concerns about wheel longevity however. 29er rear wheels are a disposable item for me, in a way that 27.5s never were. The increase in flex means that spoke tension, nipples pulling through spoke holes, and weld cracks on alloy rims are all dramatically far more common. It's improved with carbon hoops but not gone, and I can see that issue really rearing its ugly head with even bigger wheels.
One thing I have not seen mentioned in the 32" discourse is brake rotor diameter. 220 has become pretty common on more gravity focused 29er wheels (and 27.5) and wasn't a thing in the 26" times. Obviously the corresponding bigger rotor for a bigger wheel makes sense.
So are we going to see 240 or 250mm rotors on 32" bikes in the future and does the current caliper mounting standard and rotor mounting standards make sense with that bigger diameter? I'm not sure what the limit of the 6 bolt mount is, but maybe centre lock or some other standard makes sense at that size?
Good question, it may become more common.
Though I've never understood the argument that bigger wheels need bigger rotors. Correlating rotor size with rider mass/riding style/terrain makes more sense to me.
On my fatbike I run 160mm rotors because in the cold the small rotors heat up more quickly and the descents are naturally a lot slower. But on my mountain bikes I'll run bigger rotors because otherwise I'll exceed the perceived window of ideal brake temp with warmer ambient temps and much longer and faster descents. Same dude (me) at approximately the same mass, but different needs (unrelated to wheel diameter IMO).
I believe we'll see the average XC rider continue to use 160 and 180 rotors, and enduro and dh riders using 200 and 220mm rotors.
What do y'all think of the bigger wheels->bigger rotors idea?
I think the logic to this theory checks out. Given two different sized wheels at the same speed with the same size rider, the longer lever arm of the bigger wheel is going to apply more torque to the brake caliper at the rotor. Add in the larger contact patch on a 29" tire that helps create more grip and thus applies more torque (compared to skidding), and the fact that bigger wheels and tires weigh more, meaning more rotational mass to slow down.
I agree that bigger and faster riders should scale up their brakes, pads, rotors, etc on any bike, with any wheel size, but if you take the same rider and put them on a bigger wheel size, I think they will need a bigger and/or more powerful brake to get the same level of stopping power. This lines up with my personal experience bumping up wheel sizes. The old Saint brakes with 203mm rotors on 26" downhill wheels and tires were always, always, always enough brake for me, but for me that same brake setup doesn't hack it with 29" wheels and DH tires.
Having said all that, I'm not sure 32" bikes are going to necessitate 250mm rotors, because I'm not sure 32" bikes are ever going to be made for shredding straight down steep hills. Right now, the only 32" tires you can get are XC tread patterns with paper sidewalls. Now, I'm sure someone will make a proper trail casing 32" tire at some point, but I don't think we're ever going to see 32" wheels marketed as a front and rear DH product. And front wheels don't need big rotors as badly as rear wheels do (e.g. the Troy Brosnan theory). Even if we see a 32/29" mega mullet DH bike (and yes, I am confident we will see this atrocity, but until then I pray for this burden to be lifted from my mind), I think 220mm rotors front and rear will be powerful enough for everywhere except Champery.
Some great points there guys, I agree it's unnecessary for xc and trail. In addition to the various reasons for bigger rotors mentioned above, heavy ebikes also matter.
Do ebikes need bigger wheels though? I'm not sure, I also hope we don't get to the stage of 32" being ubiquitous over all disciplines, so maybe we won't need to worry about bigger diameter rotors.
Good points Charlie!
Where I think we diverge here is what kind of machine we think brakes are.
If they are levers, then certainly a longer lever offers greater mechanical advantage.
I think brakes are primarily energy conversion machines. They convert kinetic energy into heat.
In your example of 203 saints not cutting it anymore, the “energy conversionist” would say the 29er with DH wheels allows you to accumulate more kinetic energy, because you're riding faster, than the 203 saints can deal with effectively.
Where I think brakes do act like levers is at low wheel rpm’s, like trials-esque moves. Possibly, IDK that much about trials. But I know V brakes feel good in the parking lot and garbage at speed.
Another useful thought experiment is to think of all the vehicles with high KE, small wheels, and big brakes.
Or imagine what is more correlated; KE and brake size OR wheel size and brake size.
Thoughts?
¿Por qué no los dos? Simple machines can be combined together, so I don't see why levers can't be part of an energy conversion machine.
I'm not convinced by your analysis that the primary reason I needed bigger brakes was increased kinetic energy, because I can remember the trail and day I tried out my new 29" enduro bike with DH tires. I was going exactly the same speed on exactly the same trails with exactly the same people, and the only change was me switching from 27.5" to 29" wheels (with comparable tire and rim weights), but I burned through my 203mm brakes and blew past my stopping points on every single run. I had been running two pot BR-M785 XT brakes for years, which up to that point had been the gold standard for enduro brakes, and I remember I went out and bought Zee's the next day.
I agree that the kinetic energy in my new 29" wheels and tires was marginally greater than my old 27.5" wheels, but I don't think that represents a significant change in kinetic energy when I'm riding. Here's a thought experiment: put a 29er with DH tires and cushcore in the workstand and spin the rear wheel as fast as you can, then pull the rear brake. With even a decent two piston brake and a bedded in rotor, the rear wheel will lock up as quickly as you can pull the brake lever. The kinetic energy of the wheel and tire are not hard to overcome. Similarly, take that same bike and two piston brake for a spin around the block, lean forward, and skid the rear wheel. Again, plenty of braking force to overcome the kinetic energy of the spinning wheel. In fact, the faster you ride into a skid, the easier it is to overcome the kinetic energy of the spinning rear wheel, even though it's spinning at higher RPM's and with more KE. Again, the brake has no problem slowing down the mass of the wheel.
In my experience, my KE as a rider didn't change, and I don't think the KE of my wheels and tires changed significantly, but my brake performance did when I switched to bigger wheels, which leads me to the larger lever/bigger contact patch hypothesis.
Based on the larger lever argument, what does that mean for frame construction and hub/rotor interfaces?
Are we going to end up needing hugely beefed up frames where the calipers attach?
XC, road and gravel have been moving to flat mount, will this smaller mounting area hold up to the large forces but keep weight down?
Will current rotor mounting standards be enough for those that are pushing it? It worked with 29 and 275, but will 32 over come the current standard holding up or make manufactures add considerably more weight for the same standards to hold up?
Not many have a lot of time on a 32" gravity focused bike to test these long term and while I don't think many people are using center lock for hard riding, will they need to beef up the diameter of the spline and/or the thickness of the hub shell under it to take the added torque? It's already pretty thin between the spline and the hubs axles for the forces being put on that area.
And to again beat a dead horse... will 20mm axles come back to add stiffness?
Larger diameter rotors probably also need to be thicker or have a large aluminium spider to stop them getting very flexible. My own experience with 1.8mm 220mm rotors (Shimano RT66) wasn't positive, very flexible and easy to bend.
Fun conversation
Charlie, I’m reading this back to myself and I want to sound curious and not condescending; ala Paul at Mars Hill. But I don’t know if my wording reflects that, please read my words generously in light of the curiosity I’m experiencing.
……….
I too think there are levers involved! All the usual suspension design metrics will have to be looked at when the wheel and brake are moved rearward 30-50mm. Though my position is that braking needs are nearly exclusively related to the need to convert accrued KE into heat in the time appropriate for the application; and that has very little to do with wheel size beyond bigger wheels in some cases allowing faster speeds.
I believe the bulk of the KE increase is due to an increase in velocity. Which is extremely hard to sense, I think most of us have had the sensation of a calm bike feeling slow but going fast and at different times also the inverse case of feeling fast and going slow.
I think we agree that the marginal increase in mass from the larger wheels is not a significant factor. I think that’s what you’re explaining in your second main paragraph. It’s a factor. but because the mass increase is relatively small velocity is squared, a small change in velocity leads to a bigger change in KE.
100kg of body and metal slowing from 40km/hr to 0 takes a lot of energy. 6.17kj Doing so in a short span of time takes a lot of power. Controlling speed repeatedly down a mountain for 1-10 minute sessions at requires huge amounts of KE to be converted to heat.
My top question to the long lever theory as it relates to braking is where is the energy going?
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Thinking more about about the next few posts
Regarding beefing up brakes for 32” wheels; I don’t think it’ll be necessary unless the need to slow down more quickly from faster speeds arises. Like most, I’m expecting 32” wheels to primarily live on the XC/Trail side of the mtb-verse where the big selling point for the big(ger) wheels is a lower rollover angle and increased rolling efficiency. I’m anticipating covering ground more efficiently, not necessarily going faster.
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As an aside, flat mount is terrible; but I get it from a marketing and packaging point of view. Hard to market your frame being 3w more aerodynamically efficient when you have a big IS mount and caliper hitting the wind. Road bikes are a perfect illustrative analog for braking. Road bikes have heaps of traction, descend huge mountains, and also need to stop; but they can mostly get away with smaller rotors because they can afford to dump that energy away as heat over a longer period of time. I say mostly because there are very real concerns over big people absolutely roasting their brakes and putting themselves in danger with undersized rotors and improper brake dragging while descending big mountains on road bikes. …. A Dura-Ace caliper will stop a train, it’ll just take awhile.
Cheers from WY,
PH
Yea I dunno chat gbt anyone ?? Reckon you lot could be boomers
"My top question to the long lever theory as it relates to braking is where is the energy going?"
I'm also really enjoying this discussion, so thanks for bantering with me. I don't have a formal engineering background, so I can't draw you a halfway decent force or free-body diagram to make my case. All I can do is reason backwards from experience and observations in the field. So here goes:
Imagine a bike with two 29" soft compound DH tires at 20 psi and a 160mm four piston rear brake. Now imagine a bike with 20" wheels and BMX street/park tires inflated to 90 psi, with a 220mm four piston rear brake. If you get the two bikes up to the same speed with the same rider on the same road surface and have them grab the rear brake, it's easy to imagine the BMX skidding and the 29" sticky DH tire gripping, which means the 29" bike is going to slow down a lot quicker. But they're both going to slow down eventually, because they're both transferring kinetic energy into heat. One of them is just doing it a lot faster because it's a more efficient energy conversion machine. The mountain bike with DH tires is transferring the energy into heat through the brake pads and rotor, whereas the BMX with the skidding rear wheel and the brake rotor locked in the pads is transferring energy into heat at the contact point between the rear tire to the ground. Because brakes are able to absorb and convert kinetic energy quicker than tires on the ground, the mountain bike wins the stopping competition.
That's my long way of answering your question about where the energy is going- it's still getting converted to heat in both cases. An obvious follow up question would be about my use of confounding variables (tire tread, compound, pressure, rotor size, and wheel size) in my example. For instance, would the 20" wheel still skid with a sticky DH tire inflated to 20 psi? Would the MTB skid with a 29" slick inflated to 90 psi? And what does any of this have to do with rotor size?
I used extreme examples to answer your question about how energy is transferred in different bike and brake configurations, but it's not a good example for parsing out the relative effects of different wheel and rotor sizes. I'm still chewing on that!.
A very inefficient energy conversion machine.
A very efficient energy conversion machine.
I'm have not read this in depth, so bear with me both of you. But, in terms of leverage going to throw another wrench in the works
V-brakes on a rim brake bike. Less clamping force than a hydraulic brake (though, we could toss magura hs33 rim brakes in to make it even better). and your leverage is massive. way closer to the contact patch. Therefore you are able to control speed with worse clamping force (and creating less heat...pretty sure no one ever burned themselves on a rim after a big descent back in the day).
More leverage means less heat needed to create to dissipate the kinetic energy.
I love this example. Totally! Rim brakes use a really big lever (350mm tire radius > 110mm rotor radius) to slow the bike down. I had a set of Ultegra 6700 brakes and levers on an old rim brake bike with alloy wheels, and honestly on a dry day the braking power was incredible. Those were some of the best feeling brakes I've ever had on any bike (in the dry).
Rim brakes are still creating lots of heat (by converting KE to heat), it's just that aluminum is a great heat sink, and rims are exposed to air that's rapidly rushing by them, so they're very effective at shedding heat, making them a great heat conversion device. This is also why carbon rim brake wheels suck at braking and are so scary on big mountain descents- carbon fiber stores a lot more heat than aluminum. Carbon wheels get hotter and hotter and hotter the longer you brake, whereas it's pretty hard to fade out a rim brake wheel with a good brake track and fresh brake pads. Not impossible, but pretty hard.
And at @Zuestman, they were FSA Energy wheels on my old road bike. They were the cheapest wheels in the FSA line up, but they worked fine for me. Lots and lots of miles on those wheels. Thank you!
The Blister Review folks are doing a series of interviews with folks on their Bikes & Big Ideas podcast about people in the industry and 32 inch wheels. The most recent one interviews Daniel Yang of Neuhaus Metalworks, a frame builder and bike designer who’s actually built and ridden a few 32” bikes. He had a really interesting perspective on it, but something he said stuck with me re: the “why do people say 32s would be good for tall people” question: the increased wheel radius means you can create bikes with a deeper BB drop without turning your chainring into a plowshare. BB drop is one of the few aspects of bike design which we can’t really scale proportionally, so it’s a unique opportunity to explore that part of the design space.
I've said this elsewhere, but for the anti-32 crowd...it's well understood that geometry is the single most important aspect of bike fit and function.
Wheel diameter is the geometry of wheels.
Haha, never thought I'd be the one accused of being a robot
, we have a robot already!
I think the v brake perspective is super relevant here!
From an energy conversion perspective it could be argued that the same amount of heat is being produced but it's spread out over a much larger area so the temperature is lower.
But I like this vein of thinking a lot Zuestman. In my world I think about optimal "feeds and speeds" a bunch, maybe the wheel size discussion is super related. Bigger wheels= lower rpm at a given speed -> different braking response? I could see the efficacy of pad on rotor being unexpectedly (to me at least) affected by the rpm of the rotor. IE maybe the brake is able to convert less/more KE to heat disproportionately with respect to RPM.
Testing all of this would be fun.
I like the test you propose!
Education by experience is worth a lot. IMO the factors at play here are too nuanced for a practical use of force diagrams. I think it's likely the differences you've pointed out are the critical things. As you pointed out the traction available by big soft tires allow the KE to be converted to heat when an inferior setup for a given environment would just break traction and no KE->heat conversion would occur. On good tarmac, a GP5000 can handle incredible braking forces without breaking loose, in the presence of sand and wet roots it'd be a disaster.
FWIW I used to teach HS physics, and that distilled world doesn't say a whole lot about nuance. In that clean world the coefficient of friction of our two interacting materials are of primary concern. In that world tire pressure doesn't really even make a difference bc at a lower pressure the contact patch of your tire gets proportionately bigger, BUT the pressure has gone down, and the normal force remains constant. bc Force=Pressure x Area. Clearly, there is more going on. Maybe the bigger contact patch allowed by lower pressure makes it more likely that at least a bit of the tire is touching something solid and grippy. Maybe the shape of the contact patch is really important, etc. All that to say experience is worth a lot in a world with heaps of variables like a trail even if experience is messy and it can be difficult to pinpoint the "why" behind the "what" we've felt.
It could be time for some experiments!
+1 for experimentation.
It's been a long time since I took a physics class, but at that time I wanted to be an aerospace engineer. Take that for whatever it is.
I'm thinking the larger wheel diameter doesn't contribute to excessive braking forces by way of a longer lever arm. (But additional weight, angular momentum? Yes.)
Hear me out: the forces acting on the brake pads and the brake disc are equal and opposite. The brake pads are trying to accelerate* the front wheel in the opposite direction of its rotation, while the disc and the total momentum of the bicycle + rider are trying to overcome the braking forces of the pads. For whatever net amount of slowing that the rider wants to experience, there will be a total amount of friction and thus heat generated that results (hopefully) in the bicycle slowing to the desired speed. The heat produced is directly proportional to the braking force and the total acceleration* of the system. This heat may vary from one wheel size to another.
Now, technically, the lever arm involved here measures from the edge of the brake disc to the center of the hub. Changing the rotational dimensions of the wheel attached to the braking system doesn't change the braking dynamics of the forces involved at the brake pads and disc. (Except in the sense that there is more rotational momentum and angular velocity, as mentioned above.) In fact, there is some amount of reduction in the heat, as a larger wheel will be rotating more slowly in RPM and thus also across the swept area of the disc than a smaller wheel for any given MPH demonstrated by the bicycle as a net system.
That said, I'd be interested to see the results of some testing to determine where the forces are coming from. I'm innately confident that a larger wheel would create more heat than smaller one, but I think it is because the additional momentum generated by a heavier, larger wheel that is more resistant to acceleration* is the reason, not a longer lever arm. In short, the lever arm measures from the disc to the hub, not the tire to the hub.
I'm open to critique. Would love to hear an angle that I haven't considered.
*For the purposes of physics equations, we consider a change in velocity "acceleration." (Even if we are talking about braking, which most people would consider a "deceleration.")
A lot of this talk sounds like the old adage that the difference between theory and reality is that in theory there’s no difference between theory and reality. But in reality there is.
Bring ‘em on. I don’t see myself buying one, the LAST thing my local trails need is a way to make them easier. But who cares if the companies wanna try selling em.
32" Maxxis Dissectors on some bikes at Taipei Cycle Show....
I believe there was a Neko Mullaly youtube video where he talks about testing 250mm (or larger) brake rotors made by galfer or reverse components (couldn't seem to find the video). He was a fan.
Did find these 254mm rotors on sale though:
https://www.bike24.com/p2421668.html
"MonsterEnduro 32er" sent from dirtysixer via email - has dissectors
Had an interesting chat yesterday, during a group ride with a product manager from Conti. Apparently management of the bicycle division at Conti is not yet convinced that 32"-wheels will become a mainstream wheelsize, let alone are even here to stay. The guy told me that he personally believes this to be a repetition of the 650B-Plus situation. Conti anticipates very little consumer demand for various reasons, not least of which being a sort-of innovation-fatigue within cyclists. Also, apparently a lot of brands expect and anticipate the UCI to pretty much immeaidately ban 32" wheels for XC competition, should they turn out to be a clear competitive advantage and are thus hesitant to fully dedicate themselves to the development of 32"-compatible product (frames, hubs, rims, forks, etc.)
Super mullet enduro bike from Alutech with Dissector front tire.
Wheel- and component manufacturer Newmen has some interesting, fact-based opinions on 32" wheels after doing a lot of lab testing:
https://www.newmen-components.de/en/32-Inch
They insist that 32"-wheels are over 30% less stiff than 29"-wheels and that 32"-wheels thus require a different hub standard with wider flange spacing. Thicker gauge spokes or increased spoke count apparently unable to solve the stiffness problem.
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