Olympic BMX

[nofuse22]

I am not trying to be facetious, but watching the BMX riders spinning what looks like 120rpm in the olympics, the obvious question is - why don't they simply fit a bigger gear? surely would be quicker... or is the gear ratio regulated?

[Hard-Drive]

Spooky. I just came on here to ask precisely that! I mean it's not like they need a gear to accelerate out the gate with...its on a bloody great slope. It does seem that a bigger gear would pay dividends...but I guess there must be a reason!

[kazste]

Only know this because of the commentary but they keep saying that's it's because acceleration is more important than top speed.

[Hard-Drive]

Ah well, that's me told then.

Either way, it looks like a remarkably quick way to end up in A&E!

[sjg]

Acceleration is everything - no good having a couple of extra mph top end if your rivals are already 10ft+ ahead of you. It may be a great big slope but they're all pedalling hard all the way down it. Getting in front is also a massive advantage as it's far harder to pass later on - that would be even worse if you can't accelerate out of the corners so well.

Most racers can manage 180rpm cadences, by the way.

[kazste]

I want to know what keeps them so steady at the start gate. I can do a track stand but their not even having to try and balance, is it purely the friction caused between the front wheel and the gate?

[Watchman]

kazste said:

I want to know what keeps them so steady at the start gate. I can do a track stand but their not even having to try and balance, is it purely the friction caused between the front wheel and the gate?

Yep, that's it.

[anonymous-user]

Had a look at some pictures and Philips was running something like 48:16 in the final!

[Markp13]

When you see it in slo mo the faster ones are pedalling before they have landed.

(the British girl didn't employ this method)

[Watchman]

Markp13 said:

When you see it in slo mo the faster ones are pedalling before they have landed.

(the British girl didn't employ this method)

She did in one of the slo-mos I saw. I think it was in the final too.

[Bacon Is Proof]

Watchman said:

Markp13 said:

When you see it in slo mo the faster ones are pedalling before they have landed.

(the British girl didn't employ this method)

She did in one of the slo-mos I saw. I think it was in the final too.

Yep, I pointed it out to a colleague when she did the time trial.

[raceboy]

Yeap, as has been said before it's all about the 'snap' out the gate a 44/16 is the norm.

http://www.youtube.com/watch?v=nlXavqvgf6A

[Watchman]

raceboy said:

44/16 is the norm.

Surely not these days?

[splodge s4]

Bacon Is Proof said:

Watchman said:

Markp13 said:

When you see it in slo mo the faster ones are pedalling before they have landed.

(the British girl didn't employ this method)

She did in one of the slo-mos I saw. I think it was in the final too.

Yep, I pointed it out to a colleague when she did the time trial.

Ah I remember doing that around 1983ish, we used to call it an 'ET' pedalling in the air just like Elliott did in the film ...

[raceboy]

Watchman said:

Surely not these days?

Park/Street no, but for racing yes.
The whole 'ET' thing is a gimic thing the idea being that you land pedalling earlier, only really see it at the top classes, the class I race in nobody would be doing it.

[Watchman]

raceboy said:

Park/Street no, but for racing yes.

I did notice their chainwheels seemed larger than those on the bikes at the local skatepark but I didn't think they'd have a 16 - not a freewheel is it? Must be a modern cassette. So what's the purpose behind having a larger set of sprockets - must be a weight penalty there?

[a11y_m]

Watchman said:

Must be a modern cassette. So what's the purpose behind having a larger set of sprockets - must be a weight penalty there?

Total guess, but possibly more chain wraparound the rear cog thus minimising chain loss and maximising power transfer over a greater number of teeth? Or could be me talking bks.

[raceboy]

Yeah, there are a few technical reasons but a lot is just to do with 'evolution' of riding, park bikes needed a smaller front ring to reduce smacking it on park lips, ledges and rails so the small rings got developed, in racing that is not needed, a lot of riders also use MTB style cracks with only the one ring obviously but these can weigh a lot less than a BMX crank and you can change the ring size for different tracks although at 'adult' level the 44/16 is pretty much 'standard'
There are still some people riding freewheels but cassette hubs with a 16 tooth driver is much more common as they have much more engagement.

[Watchman]

Thanks for this.

Another Q (if you don't mind) - one of the commentators started to talk about questions they'd received into the BBC about BMX, one in particular about the gear ratio and whether it was fixed by regulation. He never got to finish his explanation. Do you know if it's fixed by regulation or have the riders simply decided that 44/16 is still the right ratio? I ask because I have learned that riders now use clips, so I'd be surprised that a slightly higher ratio wouldn't benefit. They all seemed to be spinning out.

[sjg]

Lots of interesting data here: http://www.trainingandracingwithapowermeter.com/20...

Don't confuse "spinning out" with just pedalling really, really fast. As I said above, acceleration matters and any slight advantage in a longer gearing for more speed is going to be outweighed by not getting such good acceleration out of the gate and corners.