I have a 2010 Tarmac Comp with the Rival gruppo. When I got the bike (my first real road bike), I found the stock 53/39 w/ 172.5’s a bit too much to drive. I hindsight I should have stuck with it but I had the crankset swapped to a compact (50/34) Sram Powerglide rings on 170mm Bontrager Race X Lite crank arms with GXP. I recall the LBS that did the swap had to change the BB. I’m having two issues now:
- The cranks seem to be positioned inboard (close to the frame) and I think there is a chainline issue in that the chain will drop to the outside (from the bottom) when I’m in the 50 and the 11. I did not have this issue with the stock crankset.
- Climbing is not so cool as I tend to mash instead of spin. My tri bike has the 53/39 on 172.5’s and I love (well, less hate) climbing on it.
Here are my questions…
- Would I do better going back to the 172.5’s on my road bike, or go back to the 53/39, or both? I was actually think of going with a 52/36 with either 172.5 or 170, probably Rival OCT or Force level.
- What bottom bracket will work here? Does the drive train problem I’m having come from the GXP bracket or is it a problem with the type of crankset or an installation error?
The LBS that did the work is over 4 hours away so they are no longer in the mix.
Any help/advice is much appreciated.
Crank arm width is the “q”. Most top of the line cranks have a narrower “q”, meaning the cranks are closer to the frame. Different cranks require differing bottom brackets, but ultimately the crank itself is designed to be a certain width. Frames have either a 68mm or 73mm BB shell, but the smaller 68mm often requires spacers to be added on many bottom brackets to bring the spacing up to 73mm. Long story short, the bike won’t change the design of the crank width because the bottom bracket will simply be “adapted” to fit correctly.
The “q” has nothing to do with dropping your chain. The shop probably didn’t adjust your front deraileur after the crank swap. Change the front deraileur settings and you will be fine.
As far as crank length, a longer crank would probably make you a masher… a smaller crank helps you “spin” more. I think this is more in your head. Unless your really tall… you will likely run a 170 or 172.5 crank length and it is just personal preference. As far as a compact crank vs. standard 53/39 gearing, that is all relative to what you need to get up a hill! If you still have problems climbing with a compact, switch your cassette to one with a 25 tooth large ring. If that is not small enough, you can try a cassette with a 27 tooth, but it may not work due to excess chain slack… at that point, you might need to get a triple chain ring front crank. Then, you need a different front and rear derailuer too! I beleive tri shifters will still be ok, since they are friction shifting on the front, but a road bike would also require a new triple front shifter.
Sounds like the front mech limit screw just needs tweaking slightly. Chek the parktools website for a good walk through on how to do it. Dead simple and best if you do a trial and error tightening the screw a quarter of a turn at a time. Bit poor of the LBS though.
Re the 170/172.5. Then there’s 2 schools of thought. one is that even the best cyclists would struiggle to notice a 2.5mm difference. The other is that it is important. However, being blunt then with a compact on then unless you’ve got a 11-21 cassette on the back then the reality is the limiter is your strength, and not the crank length. The solution is cheap though. Hit the hills, learn to love the hills, learn to get the pace right and the technique sorted by doing a weekly hill session where you jsut do reps up and down a hill for an hour. Mix it up so that ideally you have some medium climbs to practice climbing seated, and some that are out of the saddle. But you gotta get some technique and muslces on those legs / that core.
Both Bontrager and SRAM use the Truvativ GXP bottom brackets, so there was no initial need to swap the BB’s when you bought the new cranks.
As stated, the chain drop issue is not sue to the cranks, rather is a front deraileur issue caused by the outer limit screw being too open. Simply tighten and go out and ride. As a masher, you’ll probably be happier with longer cranks. Gearing isn’t as much of an issue as you’ll never have too hard of a gear when going uphill in your compacts. Instead of 39x25, you just crawl at 34x21.
Just wanted to say thanks for the advice. I worked on it this evening using the Park Tools instructions and it’s shifting like a dream now.
Glad it’s working for you now. Just a note, in the future you may find that the shifting goes astray. Don’t be temtped to change those limit screws! The mech and frame are now calibrated, and aside from a large lump of dirt getting stuck on the end of the screw, then the reason that the shifting deteriorates from here is cable stretch. Quite normal with new cables (even pre-stretched) in the first 200 miles, then should be fine to the point where bad indexing is the first warning before you get a cable snap. So don’t be tempted to retension in a years time, or whenever, that’s time for a new cable. Retensioning is a short term fix that will then leave you in the mire when it actually snaps.
That website is a goldmine for all cyclists, from casual fettlers to full bike builders. Sure, it’s in their interests to get people to do the work at home and buy the tools, but there’s nothing complex on road bikes / triathlon bikes (MTB suspension I admit is very very complex and in the realm of the pros), so get stuck in 
Frames have either a 68mm or 73mm BB shell, but the smaller 68mm often requires spacers to be added on many bottom brackets to bring the spacing up to 73mm.
Only mountain bike frames will have a 73mm wide bottom bracket shell.
Road bikes do not need to have any spacers added. Just screw the cups in and install the cranks.