Compact Cranks

Question for our pirate bike experts

14 messages
Bionic Ironwolf    pirate
02/03/2008 at 17:00

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!

PSC    pirate
02/03/2008 at 17:05
There is plenty on the web about crank lengths... clicky thing not sure about the other stuff... but there is a similar conversation going on on the numpty bike thread at the moment.
Bouncing Barlist    pirate
02/03/2008 at 17:16

'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.

Edited: 02/03/2008 at 17:17
Bionic Ironwolf    pirate
02/03/2008 at 17:20

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

Do you like your P3C?

02/03/2008 at 17:27

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!

Bouncing Barlist    pirate
02/03/2008 at 17:29

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.

Bouncing Barlist    pirate
02/03/2008 at 17:34

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

Edited: 02/03/2008 at 17:36
cougie    pirate
02/03/2008 at 17:55
how does ljs climb ? Does he feel he needs the lower ratios that the compact will give him or not ? What does he run at the moment -a standard double?
I dont think the crank length makes much difference - i would prob go for the 172 standard and maybe 175 compact - but i cant tell the difference - i had 175s on my bike when it was the rage twenty years ago.
What is so good about these cranks ? Would it not be cheaper to go with the standard groupset ?
Bionic Ironwolf    pirate
02/03/2008 at 17:56
Thanks guys. Barley - LJS typically rides a 58cm, so probably would fit that one. Your components are more what I would expect. Thanks.
Bionic Ironwolf    pirate
02/03/2008 at 18:01
cougie, he was just wondering if there is any advantage with the new one. He does struggle a bit on climbs and currently uses a triple, same as me. Is thinking about 50/34. I agree with you, seems like a big expense for something that may not be what he needs.
cougie    pirate
02/03/2008 at 18:16
well he is not a heavy weight - i think a compact may be of use to him then - but he needs to compare the gears he has now against what he will have - or he could just swap the kit over onto the new frame ?
02/03/2008 at 19:10

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.

Bionic Ironwolf    pirate
02/03/2008 at 21:03

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.

cougie    pirate
02/03/2008 at 22:34
To be honest - a training camp would be cheaper and probably of more benefit ? As you say IW - its the rider and not the bike, but bikes are v shexy !

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