Mountain Bike Reviews Forum banner
1 - 6 of 6 Posts

·
Registered
Joined
·
253 Posts
Discussion Starter · #1 ·
Technical questions/discussion for enginerds :D

The Rohloff manual states that the gear hub excerts a torque on the frame while running in other gears than 1:1. The output torque approaches 100% of the input torque as the gears get lower. The torque tries to turn the frame backwards in lower gears, which should have some impact on the rear travel design.

1. What is the reason for this torque? Although I understand some basic mechanics (preservation of torque and energy) I can't really understand why this torque is present. Could someone explain this in a simple manner?

2. What is the net effect on a "normal" full suspension frame when running in a low gear? Let's assume a single pivot frame for simplicity. The chain force will try to compress the rear suspension, but to my understanding the hub torque would try to extend the suspension (in gears below 11). Will these effects almost cancel completely in the lower gears, where the output torque approches 100% of the input torque? Or is this more complicated, since the chain torque on the rear wheel around the rear suspension pivot depends on how far off center the chain pull is versus the pivot point (not well explained but hopefully understandable)?

3. Would there be a risk that the more advanced, modern designs like DW link would pedal worse with IGHs, since they are designed for the forces that are present derailleurs?
 

·
mmm, carbon
Joined
·
148 Posts
The torque that the manual is referring to is between the hub and the frame (or frame component when we're talking rear suspension). As such it doesn't involve any interactions with the suspension. Put another way, the torque is trying to twist the frame apart, not actuate the suspension. This is the reason they mention it in the first place; to emphasise the forces which the frame must withstand in order to be compatible with a hub gear.
 

·
ups and downs
Joined
·
15,600 Posts
You'd have problems with suspension interaction if you tried to run the Rohloff hub with a torque arm on a Horst link suspension, but normally the torque will just be trying to tear the disc brake mount off or the dropout apart (depending what method you're using to hold the hub from counter-rotating.

The torque comes from the planetary gear assembly, it essentially changes the leverage the cranks have available to make the rear wheel rotate, so as it does that torque multiplication, the hub axle is the fulcrum for this virtual lever, as the multiplied torque from the cranks rotates the wheel forward it tries to rotate the frame backwards with the same force (equal and opposite force and all that). Luckily the rear wheel is free to roll otherwise the frame might be in trouble.
 

·
Registered
Joined
·
253 Posts
Discussion Starter · #4 ·
The rear hub applies a torque to the frame. Since the frame does not have rotational acceleration, there must be an equal but opposite torque on the frame. The only possibility I see is that the front wheel supplies this torque. Therefore, when the hub tries to rotate the frame backwards, this is countered by less normal force on the front wheel, while a forwards torque gives higher normal force on the front wheel. The opposing torque has to be transered to the rear part of the frame, and I don't see how that can be done without affecting the rear suspension.
 

·
Cassoulet forever !
Joined
·
2,261 Posts
whow, some strange answers here !

1 At a steady state, the torque you put on the cog + the torque betwenn the frame and the hub + the torque on the rim = 0
So when you are not in 1:1, the torque on the cog and on the rim are not the same (that is the goal of a transmission !) and so there is a torque created between hub and frame

2 It depends on the frame design. This is just the opposite, but similar in nature, to the torque created by a brake disk. So if you have got a suspension designed to be independant from braking forces, then no effect. If not, it will have an effet.
On a monopivot there will be an interaction. How much is not easily to guess

3 Yes. The mechanical problem is not the same, so the optimisation made shall be adapted. But i don't think it would lend to huge changes.
 

·
Registered
Joined
·
30 Posts
I do not fully understand everything written above, so my question might be silly.
Does anybody knows, do we have such torque on pinion frames ? Does the gearbox apply any torque on the bottom bracket area when not on 1:1 ratio ?
 
1 - 6 of 6 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top