StodaD

Is it preferable to ride a tt bike with a slightly shorter crank length than that of your road bike?

My cycling coach advised me to do this and i did it. He said to me that it wqw beneficial for Timetrialling. After et making the change I dont know if it makes any difference to my Timetrialling or not.

I was thinking if it would put less pressure on my hips and thighs riding in the TT position using shorter cranks but then again would more leverage on your crankarm make you push more power out through the crankset?

What do people think?

rubiksoval

TT bikes usually result in lower front ends which result in more acute hip angles. If you're having trouble sustaining the position, or producing typical power, then shorter cranks can open up the hip angle and allow more typical power or the ability to hold the position.

Longer cranks arms do not let you put out more power. Your aerobic abilities let you put out more power.

Doge

If you pedal smooth circles shorter is better. Stomping, then slightly longer than the road bike may be in order.

Junior rides the same or wee longer (175) as on the road bike (172.5 and 175). He rides 170 on the track.
"True" spinning is rare and generally requires a much higher rpm as the muscle just put out different torque (force) at different part on the O'Clock.
He can generate more power from a stomp at the lower part of the stroke 3 O'clock to 5 O'clock. Tested multiple times and recorded by old fat guy non-racers like me.
The pedal stroke and length depend on the event. For TTs, he stomps, so same or 2.5mm longer cranks.

Voodoo76

Agree in the hip angle limiter being a reason for potentially going shorter. This trend was driven by the Tri world. The idea being you can run more effectively after a TT on shorter cranks. An issue we don't have to deal with.

I've given 165's a try and didn't really feel any advantage. They feel about the same till I get tired. Then I feel like I can battle fatigue, stay on top of a gear better on 172's. No real numbers to back that up.

burnthesheep

Rubi is correct.

I can't fully agree with Doge though. It's been proven, with lots of SRM or other quality dual side PM data, that the whole "perfect circle pedaling" of pros is a myth.

As your power increases, the ratio between the power leg and inactive leg grows less significant.

Think about it. If you're at 100w and inactive leg is negative 10w, that's 10%. If you only make 200w and your inactive leg has a negative 10w component, that's 5%. If you have a threshold of 400w, that's only 2.5%. Even a cheapy meter like a left-only stages somehow gives an efficiency readout. Not sure how, seems crap to me. But, it does register in the 90's % when "giving it the beans" versus worse when at low power.

So, pedaling in circles isn't it. I'm going to go with leg length, foot length/size, fore/aft cleat position, or a morphology driving it one way or another. I'm sure there are fore/aft cleat position differences between people that would make up for 5mm on a crank. And that's a measurement irrelevant to hip angle or vertical "stack". Take a person that likes 175's and move their cleat way forward and see if they still like them so much.

Going from 175 to 165 plus going to a low stack pedal set like a Speedplay zero can get you nearly a 1/2" of hip angle improvement doing nothing else.

However, in terms of actual lever length and "feel" it ain't much difference.

Some of my mechanical engineering nerd math......
400w is about 1/2 horsepower. 1/2 horsepower at 90rpm cadence is about 30 ft-lb of torque.

So the actual force for each crank length would be:
175mm = 6.89 in = .574 ft = 52.26
172.5mm = 6.79 in = .566 ft = 53.00
165mm = 6.50 in = .541 ft = 55.45

With any of those you're probably within 1rpm to 2rpm cadence change from having dead equal pedal forces between the extremes of crank length.

Meaning the feel thing may be bunk, ignoring morphology. This is why I guess it's a person's body shape and fit that makes something "feel better".

Doge

Depends on the amount of arc you apply the power. If from 3->5 O'Clock over 60 degrees (assuming no power for the other 300 which is false) it is about 180 ft-lbs (using your math) of torque.
This feels closer to correct with the running/stomp pedal method. If using the stomp method, the higher opening of the hip angles is not about power, rather bringing the pedal around.

Coach saw a power change using Q-rings on the road. In theory non-round Q-rings might help here, but again only if rotated to your preferred power position and if stomping on a small arc, the circle vs the shaped rings are not much different.

burnthesheep

I can't comment on the stomp thing, it's out of my bank of reading on Slowtwitch I could pull from.

However, lots of meters read "wrong" or "different" on Q rings because the algorithm in the meters make certain assumptions, one of which being that pedal velocity is constant.

That's certainly not true on an oval ring, the more oval the more pronounced.

Here:
https://powermetercity.com/2016/07/2...al-chainrings/

So, I'd take nothing as a 1:1 comparison when talking about power numbers for a person swapping to/from oval rings. I'd go by "feel" preference, placebo, or go with results. As you can't rely on the raw data, since it's not accurate due to the alteration of what the meter is seeing for pedal velocity in the calculation.

Doge

Good point on the PM. I'd think a crank vs hub meter would give different results here. I wonder if pedal PMs would. We have ever only used round rings.

For ITT always and only use the clock on purpose. In an ITT the power is just one of thee components and a raw power number does not say a whole bunch if it cost position.


The course could drive crank length too, although most think of an ITT being kinda flat.

The Haggerstown Nationals ITT course coming up in a couple weeks is not flat, and I think shorter would be better going out and longer better coming back, but I don't know how to do that other than a bike swap (which they did at worlds). I forgot who swapped and who didn't, but it was hard to determine if it mattered.

I just read the article. They essentially say that.

Getting slightly off topic, but as there is reason to vary power over a hilly course. That was part of the reason for the statement that the crank length may vary too. On an uphill TT you will see more longer cranks than on a flat TT.