Brompton Crank Length

[mitchibob]

I have short legs. Probably need to get measured properly, but a tailor once said I had a 27" inside leg.

I love my Brompton, but for slightly longer periods in the saddle, I have been finding I start getting some hip pain. Also, even after various saddle positions, find that at higher cadences, I'm bobbing about a lot.

This is on a 2016 M2L.

I'm considering some significantly shorter cranks for my leg length, but at the same time, I don't want to spend a fortune and hate them, or get the wrong size front cog to go with them. But want to do some longer rides with longer stops between lights than London parks, and would really be interested in whether shorter cranks might help my short legs, and whether there are sensibly priced alternatives to try.

 

[Yellow Saddle]

I have successfully eased similar problems for customers with short cranks. The shortest available at the time was 165mm IIRC. Perhaps it was 160. This was for a racer and the customer was fussy about brand and groupset status and all that. I'd imagine there are plenty choices available in the UK and it is worth a try. What is your current crank length?

 

[mitchibob]

I think it's 172.5mm for the current brompton

 

[Yellow Saddle]

I think it's 172.5cm for the current brompton

I'm no anatomist, so I cannot visualise how short a 27" inside leg is. I assume it is shorter than mine because of my trouser size. But, 172.5 is longish - right for average-height males. Try and find a 165mm bike to test and see if you feel more comfortable. That's of course easier said than done, since those are pretty much in the minority.

But as an aside,bobbing about at high cadence is not a crank length issue, that's a muscle memory issue. Your large muscles - gluteus, hamstrings and quads have to learn to fire fast and switch off even faster. This comes with time and in remedial cases, with conscious effort in controlled sessions on a trainer. The bounching comes from the return leg still having muscle tension after the crank has moved past the lowest point for that leg.

Shorter cranks typically sort out the problem where the rider feels his/her legs come up too high and simultaneously extend too much. The latter is visible as a rocking pelvic movement when viewed from the back. It also typically exacerbates saddle sores.

 

[simon.r]

Standard cranks on my 2017 Brompton are 170mm.

I've fitted a 160mm crank on the left, as I have a particular knee / leg issue. Spa do a good selection: http://spacycles.co.uk/m2b0s109p0/Parts-and-Accessories/Components-Gears-Chainsets

One thing to look out for - as the new crank is thicker than the standard Brompton crank, the pedal doesn't fold.

 

[Brommyboy]

If you have the standard suspension then that could be the reason for the bobbing: a firm version is available. Also try to pedal more evenly rather than emphasizing the power stroke, which would 'pump' the suspension. If your leg length is 27", is that trouser length which is shorter than leg length? Probably a 165mm crank may be better for you. Spa cycles have several available to consider.

 

[12boy]

If you have a newer brompton the chain ring can be switched to the new crank arms. As Kell remarked in an earlier post high RPMs on a brompton do result in some bobbing, and while you can train yourself to spin more easily I find pedalling fast with little resistance, as in going down hill, does tend to make me feel off balance. The switch to a firmer suspension block will reduce bobbing and if you have a softer one, a few Jubilee clips ( we call them hose clamps) will stiffen the block and allow you to see if that helps for little expense. In my experience short cranks arms are best for spinning while longer ones create more torque and therefore are best for those who like to pedal more slowly but with more effort. 5 mm (165 vs 170) is less than 1/4 inch so the difference is not great. I find that extending the seat post so when the pedal is fully down my heel barely reaches the pedal makes the most difference for comfort and knee pain. If my saddle is too high I will roll from side to side to reach the bottom of the stroke and too little will make my knees hurt since I am applying power when my knee is bent uncomfortably. Once used to a saddle height I find even an 1/8 of an inch change feels wrong. The other thing that affects comfort is the fore and aft adjustment of the saddle. I put mine all the way back so the handlebars don't feel too close. So, including the jubilee clip if you have the softer suspension block, there are 3 things you can try before springing for a new crank arm set.

 

[mitchibob]

I reckon my 27" inside leg at 5'7" helped me win a large pub bet that my 6'10" basketball playing friend would be no more than 6" taller than me if we were sitting down. It was about 4.5"... ish. Won me £80 worth of drinks. When I buy the shortest Levis, I need to have them taken up 2" to make them look normal, for me, and others might wear them shorter. Definitely, I'm a bit of a short-arse.

 

[mitchibob]

Standard cranks on my 2017 Brompton are 170mm.

I've fitted a 160mm crank on the left, as I have a particular knee / leg issue. Spa do a good selection: http://spacycles.co.uk/m2b0s109p0/Parts-and-Accessories/Components-Gears-Chainsets

One thing to look out for - as the new crank is thicker than the standard Brompton crank, the pedal doesn't fold.

That definitely looks like a well priced set to try. Thanks for that.

Folding pedal isn't so important since I already compromised my brompton by putting on SPD pedals.

 

[mitchibob]

If you have the standard suspension then that could be the reason for the bobbing: a firm version is available. Also try to pedal more evenly rather than emphasizing the power stroke, which would 'pump' the suspension. If your leg length is 27", is that trouser length which is shorter than leg length? Probably a 165mm crank may be better for you. Spa cycles have several available to consider.

I've toying with going to the firm block, as it's a cheap replacement, but slightly worried that as much as I'd enjoy the stability and power benefits, my back would hate me for it.

 

[12boy]

I have mine tightened up a lot and bumps from the back tire don't bother me, at least as much as pogoing does. What does though has been vibrations to my hands. I have tried 3 different ergo grips, and although I had worn the cork Ergons to the point where bits of cork are falling off, I have returned to them. I have some GP2s but the bull horns are just to small for me. I have some cut down generic bar ends, just short enough to clear the fold, and I twined them with paracord and then shellacked them. Durable and the cord soaks up a lot of vibration.

 

[Brommyboy]

12boy you might find that the hand tingling from vibration is due to leaning on the grips, rather than sitting back to take the weight off your hands!

 

[12boy]

You may be right...From November through March 20 + mph winds are the norm here and being lazy I like to minimize their effect by leaning over. This position is pretty much the same on all my bikes, with my favored hand position being the handshake one with the palm parallel to the frame. Bar ends let me do that and the Ergon grips help a lot. In fact, changing hand positions in itself helps a lot. I have S bars and even those seem higher than I would prefer. Do you folks who ride higher bars and who need the saddle all the way back to get the room you need find that the front end feels twitchier since there is less weight on the bars?

 

[Kell]

I failed my A-level physics so I may be about to make a fool of myself, but surely if you put your saddle further back, there's more weight on your arms...

 

[12boy]

I see your point but I was thinking the further back your weight was on the bike the easier it would be to do a wheelie. I was thinking also the reach down for lower bars would throw weight forward. My 700 c bikes all have long chain stays but folders seem to have relatively short ones so sitting back further would have more effect. I guess my lack of physics classes of any sort comes into play here.