Compact Cranks

LJS is putting together a list of components for the new Cervelo P3 carbon bike he wants to buy. Can one of you please answer this question (remember I'm asking a question on something I know nothing about!)

He is interested in the new VumaQuad compact crank, a 4-arm spider that comes in choice of 50/34 or 53/39 chain ring in a 110mm bolt circle and 172.5, 175, and 180mm lengths. Which of those lengths would you recommend and why? What is the difference?

If it's relevant, LJS is 6ft tall and weighs 75 kilos. Leg length is 87cm.

No smart-ass answers please - there's a lot of money tied up in this bike, and where money's concerned, I'm deffo interested!

'Normal' crank length is usually 172.5 which is fitted on standard bike builds as far as im aware..  Thats what I have on my road bike.

When I was fitted for my P3C I was advised to go for 175's as being a taller rider they were the best fit for my dimensions.  I cant advise any more than that really i.e. crank length should suit your dimensions.. however it also has a marginal impact on rotational force required i.e. longer crank = less force but larger pedaling circumferernce.

In terms of the crank set (standard double 39/53) or (compact 34/50), it would be a matter of preference and mainly determined by rider strength and the likely terrain you'd be riding.

I would question the need and suitability of getting a P3C if a compact chainset was needed.  A P3C is a no compromise time trial bike designed to optimise speed over flat to undulating terrain but is certainly not ideal over hilly courses.   Contrasting this a compact chainset shouldnt be necessary except for anything but the most hilly courses.  i.e. I had my road bike converted to a compact for IM Lanza last year, is isnt a course id ride my P3C on.

thanks barley - that's the sort of info I was looking for!

Do you like your P3C?

IW - not sure if this is helpful or not but my TT bike has a double and 172.5 cranks (I'm tallish and long bodied and femured) and I much prefer the longer cranks (old bike had 170mm).  Can begin to imagine riding it on a hilly course as its things is fast and undulating!  For hilly riding (and I do a lot) instead I have a road bike with a compact 50/34 (I think) and a 12/27, although I think now the 27 maybe a bit of overkill!

I love it though ive only ridden it once (IMFL), the roads are to grimy to take it out here yet but no doubt i'll be venturing out around May.

Theyre certainly not cheap but in my opinion worth every penny, I justify that on the basis of it being my hobby and the amount of hours of pleasure im sure to accumulite astride the beauty.Brooklyn being fitted and that included copious amounts of measuring, swapping over and reswapping every component imaginable.

At 6ft 1in I was fitted out with the 58cm frame which surprised me a little as my research had indicated the frames can come up slightly big and riders of my size often went for 56cm.  I relied on the judgement of R&A's bikefit guru's though whose references to previous customers was like a who's who of pro's.

This was the build I went for..

Dura Ace throughout, 53/39 12-23 (cassette specifically for IMFL)
Zipp 606 (Tubs) & Continental Competition tubulars
Vision Tech carbon base bar and Carbon Pro clip ons
Deda Newton stem
Fizik Arione Tri2 saddle
Look Keo carbon pedals
XXX lite carbon bottle cage

I am reaaly quite gangly and tall and I run 170 cranks on my roadie. Why? ... I find it easier to maintain a higher cadence with shorter cranks. I think its to do with the total distance your leg has to travel in a circumfrence. I dont know if that has any scientific proof however. Ive never run a compact but am toying with the idea for Nice. Usually the spider is smaller on a compact so you cant fit standard rings, so if youve found a crankset that you can interchange standard and compact setups on I would think seriously about it, as it gives you great options.

Cougie - maybe, he does have a surfeit. Certainly won't take anything off his titanium Litespeed, he loves that bike! I'm very much against another new bike - I keep telling him it's not the bike that makes it, it's the rider! Between us, we already have 5 MTBs, 5 tri bikes, and 4 roadies. To my mind - more and better training is more worthwhile than spending a 4-figure sum on yet another bike. Still, like barley said, it's a hobby. We don't spend much on stuff outside of sport. I want a new wetsuit this season.

Bassy - as he still wants to run his other bikes,  interchanging parts isn't going to figure much in the equation.