dave42

I'm not referring to balancing a bicycle.

Can using a low gear create enough torque at the hub to cause hub failure(stripped hub threads, shattered freewheel or freehub pawls, broken spokes or spoke flanges, etc.)

Are there any reliability issues with 36 tooth or larger cassette cogs?

Reason I ask is that I'm considering building a drivetrain with a 14 inch low gear. Maybe even a tad lower.

I would like to be able to travel at a walking pace(2 to 2.5 mph) at 60 rpms.

Conventional wisdom seems to be that 20 gear inches is a good low gear for touring, and that 80 rpms is a good climbing cadence. Which means kansas or walking up every hill(around here).

So, 14 as a low and 100 as a high would be ideal loaded touring in the appalachian mountains, at least for me.

Any thoughts on the mechanical issues?

bradtx

dave42, Offhand I'd guess that a steel freehub to resist galling and the lowest 3 or 4 cogs pinned together to also resist galling the freehub splines. I have to add a properly torqued cassette.

Depending on the RD's capacity for chain wrap, there maybe some combinations that'll have the jockey and tension pulleys colliding. Not to mention that the RD will need to be compatible with a 36T cog.

Brad

Brad

wphamilton

I don't think that you're creating more torque, so no worries.

jyl

Mountain bikes often have 24 x 36 x 26", which is pretty darned low, and get thrashed in those gears, up steeper slopes and with more aggressive pedaling, than you'll do on your loaded tourer.

Bill Kapaun

Would there be any difference with a strong rider using a slightly higher gear to go up the same hill?

I think any potential problems might arise if you try to mash it from a dead stop while pointed up a very steep hill or pulling a heavily loaded trailer and mashing from a dead stop.

FBinNY 

There are two ways to look at rear wheel torque loads. There's what you can put into the system, so following that logic lower gears might mean higher torque loads. OTOH- one can look at the output side, ie. the amount of torque needed to get you up a hill, or for the acceleration involved coming off a line.

Now, consider that input and output torques on the wheel have to be equal, so while a lower gear could mean higher torques, the reality is that it doesn't, and the torques are set by your weight and the slope. Therefore, the highers torques happen not with the lowest gears but the strongest riders, and if rear wheels can handle Pros climbing the Alps, they can handle you regardless of what gear you use.

dave42

I think I I follow you. You are saying that, given the same power output, using a lower gear does not increase input torque at the hub?

Edit: Damn, I had to think about it. Now, I follow you a little better.

davidad

I have a 20-34 on my tourer with no problems.
A lady friend has a trike with an 11 inch gear and no problems. I believe she has over 25k miles on it now.

prathmann

These two statements appear to be contradictory. The loaded tourist on some of the steeper climbs in the Appalachians is likely to have a heavier body, bike, and certainly load than the pro rider in the Tour and is also likely to be going up a steeper slope (albeit a shorter one). So the torque on the rear wheel needed to maintain his (slow) pace up the climb is going to be more than the torque needed to maintain the (much faster) pace of the pro rider up the less steep but much longer climb in the Tour.

Now the pro rider will certainly be putting out more power since this involves the product of the torque and the speed at which the wheel is spinning and his wheel will be spinning much faster than that of the tourist struggling to get his heavy body/bike/load up the hill. But I agree that I wouldn't worry about the effects on a normal good quality hub and cassette/freewheel - although I'd advise some caution and checking the specs if using an internal gear hub.

wphamilton

He'll need to be putting out more power than the pro then. Power in = power out.

headloss 

Shimano had to redesign their deore level mtb hub because the freehub was failing when a 36t cassette was combined with a 29"/700c wheel. I think failure is more likely in a mtb-ing scenario than a touring one, but who knows.

FBinNY 

Yes, I allowed myself to mix the issues of pure slope, where the torque would be a function of slope angle and payload, and the issue of accelerating include on a climb.

But my point, was to point out that wheel torque was limited by both input and output considerations and capped by whichever was lower. OTOH I'd still be willing to bet that the highest wheel torques happen with powerful sprinters coming off a line.

dave42

Thanks. That's good to know. I've spent the evening reading about torque as it applies to chain-driven power transmissions. I regret my lack of formal education...but, when and if I get my head wrapped around some of the issues, i'll add more.

This stuff fascinates me.

Now, if I had a company credit card and a bar full of Shimano r&d guys, I bet the education would commence rapidly-er.

prathmann

No, as I had already stated in the post to which you replied:
"Now the pro rider will certainly be putting out more power since this involves the product of the torque and the speed at which the wheel is spinning and his [the pro's] wheel will be spinning much faster than that of the tourist struggling to get his heavy body/bike/load up the hill."

I.e. the tourist in a low gear on a steep slope may well be generating more torque than the pro in the Tour even though the pro is generating far more power.

noglider 

I don't know the answers to the questions here, but I have noticed that freewheels with bigger cogs get screwed on tighter than those with smaller cogs. I suspect that the lower ratios do give an opportunity to add torque to the wheel.

FBinNY 

They do, but it's an indirect process. Lower gears allow the rider to climb steeper hills, so the output limit is raised.

The torque can't be higher than what's needed on the output side, nor higher than what the rider can produce on the input side, (where lower fears do make a difference) whichever is lower.

fietsbob

I tested a Mountain tamer Quad 16,26,36,46.. in a MTB. it worked but even the 16:28 low in a 26" wheel had the chain feel stretchy, It wasn't of course rubber

But it felt like that

and I quickly figured out it would really only be practical in a 3 wheel trike like a tadpole recumbent

restarting in a Hill was Almost impossible because the Momentum went away before I could clip my other foot in,
and climbing so slow the risk of just falling over remains.

so for a Bike with 2 wheels, once my low is about around 20" I Use My 2 feet and walk it.




Freewheels screw into a shoulder on the hub . after that its the Pawls that transmit your torque to the whole Hub.

cassettes too, the pawls do that job ..

Little Darwin

Calling all physicists...

Some in this thread have explained that the torque doesn't change with lower gearing, but the question then to me is what does change? I am no physicist, but something obviously changes in the realm of power transfer, or we wouldn't have different gears!

I know that it has been said that if one has a lever long enough, and a fulcrum on which to rest it, one could move the world. Admittedly, if the lever was long enough, it wouldn't move the world very far, but it would move.

Consider the fact that if a series of pulleys is used, I can lift an item larger than myself (such as an engine on a chain hoist). Without the pulley system I could climb the rope/chain without moving the object on the other end... Well, I can't climb a rope, but the concept still works.

So, intuitively, something has to change, since levers and other "gearing" enable me to move things I couldn't normally move. If the thing that changes isn't torque, then what is it?

How come I can apply more torque with a longer torque wrench, but if I do it through gearing, the same rule doesn't apply? It seems to me that torque is exactly the thing that changes, but I don't have the academic background to know for sure.

fietsbob

The Leverage changes .. think of the circle that happens to be divided in half inch increments of teeth numbers ..

the differences in those sizes can be described as the length Ratio differences on either side of the Fulcrum point in a Lever ..

FBinNY 

The discussion is limited to the torque within the wheel. Moreover, it's not about the actual torque which depends on rider pedal pressure and gearing and/or the slope and rider weight (both are always equal within the wheel) but the maximum that needs to be considered.

Since rider strength is an unknown variable I suggested looking at the output side of the equation, or the slope and weight which are more easily quantified. Lower gearing gives the rider more leverage and so increases the amount of torque he can generate for a climb, (discounting acceleration). So that means he can climb a steeper hill.

prathmann

I don't think anyone in this thread has actually said that - and if they did they were wrong. FB correctly said that the output torque at constant speed will be determined by the slope of the climb and the total weight (bike, rider, load). Using lower gears will allow a given rider to carry a heavier load and/or climb a steeper slope before having to get off and walk. Therefore these gears will also increase the possible output torque of that rider.

Little Darwin

Thanks for the clarification... I guess my 40 year old high school physics kicked in when I thought I read that torque didn't change... Maybe I'll remember to read more closely in the future.

noglider 

Aha! Good explanation!

Bill Kapaun

I think we are confusing torque with force?

If we were to place the front wheel into a wall and with enough gear reduction, we could easily break "parts".
Since we are talking a scenario where the bike can move, you can't build up that much force/torque.

FBinNY 

Yes, that's basically it, though if I wanted to split hairs, I'd object to the wording "build up...". In any case, I don't think it's a matter of confusion torque with force.

In case anyone is still confused, here's an analogy that might clarify things.

A bunch of friends come to you and say they're planning to use a jib and pulley to raise a piano the 3rd floor window and into a friend's apartment. You could ask how many of them or how strong they are, or you could simply say "how heavy is the piano" since the tension in the rope can't be higher than that. Of course, that doesn't mean they'll succeed, that depends on them, but it does ensure that the rope won't break.