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The size of the cog corresponds to how fast the chain length has to be growing in order for the freehub to “catch up” with the wheel speed. In a really low gear the chain has to be going super fast to spin the freehub fast enough. Not too different from how you’ll spin out if you drop it into gear 1 while going full speed. Whether or not to count the chainring is a whole other question. If you’re strong enough then you can pretty much say it’s rotationally fixed in which case its size doesn’t matter.
As for speed I completely agree. I can think of very few spots in local trails where I ever get going that fast.
You wouldn’t experience pedal kickback*
*until you use your brakes right, when your wheel is rotating at as little as 0m/s?
As soon as you lock up all bets are off course. To be honest I’ve never really ridden anything where I found pedal kickback to be annoying beyond when the wheel is locked up. Even going back to back with and without chain. There’s a part of me that actually kind of likes being able to use chain tension to keep it a little higher. Let the cranks rotate backwards a little on landing and the impact is lessened using more travel. Keep feet entirely flat and push through it for more efficiency.
I feel that heavy braking is the largest contributor/least discussed for pedal kick. The situation where you might be locking up the rear brake but still using a good amount of travel is the exact situation where you’d really want your suspension performing at its best
Despite your description I still couldn't see it, so I drew it with values. Three hypothetical chainring/cog combos, two with the same 3:1 final drive ratio but different sized rear cogs, and two drivetrains with the same size rear cogs but different final drive ratios (3:1 vs. 1:1). I'm not an engineer so I'm happy to be wrong, but I came to the opposite conclusion, and it looks like, in terms of pedal kickback, a bigger rear cog is better for the same gear ratio. Please chime in and point out if my values, arrows, or math are wrong.
The arrows represent where chain growth is happening and the direction of forces occurring in the drivetrain. It looks a little weird at first, because horizontal chain velocity is the value I kept constant across all three models. I kept horizontal chain velocity the same because (roughly speaking) that's the direction where acceleration happens in a harsh suspension compression as the rear axle moves away from the BB. It also looks weird because I drew it on MS Paint. I didn't calculate the real value for "x" because I didn't care, but you could if you wanted to. You'd have to measure the diameters of different cog and chainring sizes. Here are my takeaways:
1. The blue drivetrain (big rear cog, high gear ratio) will only experience pedal kickback at lower speeds relative to the other two. If your rear suspension compresses and your rear axle moves away from the BB at a velocity of "x" meters per second, the blue drivetrain will experience pedal kickback at all trail speeds where the rear wheel is spinning at 90 rpm or lower (roughly 8 mph with a 29" rear wheel). By contrast, the red drivetrain with the same gear ratio would experience pedal kickback at speeds up to 24 mph (rear wheel spinning at 270 rpm or less). The blue drivetrain won't have pedal kickback in the 8-24 mph range, which is preferred.
2. However, at a speed of 24 mph or less, the red drivetrain (small rear cog, high gear ratio) will produce less pedal kickback at the crank than the green drivetrain given the same suspension compression. At similar trail speeds (rear hub rotational velocity) and impacts, the green drivetrain will accelerate the cranks more (a velocity of 270 rpm vs. 90 rpm for the red drivetrain), which results in more radians of rotation at the crank for the same radians of rotation at the freehub.
3. I need to find an alternative to MS Paint.
Am I way off base or does that check out?
The key is that in a likelihood of experiencing kickback. The smaller the tooth count, the faster the shock can compress before you experience any kickback. The way to look at it is “if the chain gets pulled at some speed v, how fast does the freehub rotate and is that faster than the hub?” The smaller the tooth count, the faster the freehub spins for that given speed. How much exactly your cranks get rotated by an instance of kickback is a massive guess because in reality neither ends are fixed.
Smaller cog equals worse for pedal kickback in the sense that it is more likely. So making everything bigger is helpful. I think that aligns with what you wrote. To be clear, to me low gear on the cassette equals high tooth count.
[EDIT: after your latest post it sounds like we're in agreement] "The smaller the tooth count, the faster the freehub spins for that given speed." But if the freehub spins faster than the hub, it will engage the hub, creating kickback, right?
Smaller tooth count ==> faster freehub spin ==> catches up with hub
Larger tooth count ==> slower freehub spin ==> doesn't catch up with hub
You are correct, reading more about it mine was an oversimplification. Still, if pk is just a function of gearing, frame design, wheel speed, and wheel vertical acceleration (is it?) we could still calculate what speed you need to be going to not feel pedal kickback while the suspension is moving at a certain speed. In theory, with data acquisition you could even calculate the % of time you spend while being susceptible to pk during a full run. That'd be interesting to see.
Edit: sorry I somehow missed a lot of posts discussing this.
Saddly some events didn't allow me to do all the testing I wanted to do this weekend, keep in mind that I also had to ride with a different shock since mine blew up last week.
- Lower chain guide: basically the same as Dolface, quieter and you can visually see your derailleur moving much less when cycling your suspension (I don't have an upper pulley). Now for the noise reduction it often misslead you to think the bike is also smoother, and I didn't have a chance to test a lower pulley position that wouldn't reduce lower chain growth but would still keep the chain more stable.
- AS to PK to dynamic behaviour: I wanted to test my frame with a 38t ring instead of 30t (gear ration held constant, so 17t vs 13t cogs) to see if reduced AS values impact the bike dynamic behaviour (pumping, jumping) and if I could tell the difference with reducing my PK from 8° to 3.6°. I can confirm that AS has no business in how well your bike will pump and jump, and that an AS value of ~50% at SAG is horrendous to get the bike up to speed when needed. As for the PK reduction it is hard to tell as I didn't get a chance to move back to the 30t ring to compare. Since I had to borrow a shock for the weekend I can't compare with my 2 years of experience on that frame.
Another thing I would consider to explain why chain/derailleur less runs make suspension feel better is unsprung weight. I'd be curious to see how a run would feel with no chain/derailleur but the equivalent weight strapped to the rear axle, tight or slightly loose to mimic the chain moving around.
What is SAG an abbreviation of?
ahah good point. I guess I learned about sag before learning how to speak english and never questioned the use of the term, which is actually just an english word.
50% SAG = 50% Screen Actors Guild. Duh. You should know that, you're in BH
@Blake_Motley Agreed, steep chunky stuff is really the only place I notice it.
I'll just use a jack shaft and an extra chain with 1 to 1 cogs. Easy easy.
taking all recent discussion into account, I'm thinking this with an ochain will be my next ride.
And an e13 Sidekick hub for safety purposes.
Their rear suspension design must be horrible to warrant such a design to combat pedal kickback. Cant blame the owner. 1000 iq design.
Back to tech rumors!
An interesting drivetrain on the KTMs!
Looks like it’s wireless and the derailleur seems very close to the rear triangle…
Eleven speed linkglide too
Edit: Here's one that isn't blurred out, not that you see very much more.
Edit x2: oh...
This one is TRP E.A.S.I. A12 as pictured thiugh
There's also this in the regular bike catalog.
The text which has had a filter applied, when copied, spits out this:
R.DERAILL. Shimano XTR Di2 M9250-12 GS shadow+
SHIFTLEV. Shimano XTR Di2 M9250
CRANKSET Shimano XTR M9200-12 Carbon 32T
BOTTOM BR. Shimano MT900 pressfit PA29 MTB
SPROCKET Shimano XTR M9200-12 / 9-45
CHAIN Shimano M9100-12
So that's Shimano's first(?) carbon cranks confirmed?
And the brake name is in full view, the new protos we've seen aren't saint? M9220 seems it would be the 4 piston variant.
Sorry Shimano, but there's more:
Shimano Deore XT Di2 M8250-12 GS shadow+
Shimano Deore XT Di2 M8250
Shimano Deore XT M8200-12 30T
Shimano MT800 pressfit MTB
Shimano Deore XT M8200-12 / 9-45
Shimano M8100-12
And M8200 brakes. So at least XTR and XT will be moving to the new lever shape?
Still 12s, Carbon cranks (at least in xtr trim), and they don't look like direct mount derailleurs.
9-45t seems like an odd choice, arent 10t sprockets inefficient enough already?
The source of the pics?
9t is lighter though.
The carbon crank is a Race Face.
Is this a joke? I don't get the point of this.
The more, the merrier! Why limit yourself with a 10t when you can go 9t?!?! This way people can go even lower on the chainring and still have a good-enough gear ratio for high speeds, together with a GS-derailluer for better clearance (as those SGS fuckers are too damn long!!!).
Clearly not.
Download the catalog yourself and check.