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Alternative to SRAM front mechs required.

rafletcherrafletcher Posts: 1,235
edited August 2011 in Workshop
So, I bought a secon hand Planet-X carbon pro that was equipped with SRAM Rival. The shifting of the fron mech from small to large chainring was really really stiff. Far more effort required than with my Campag Veloce on another bike.

So I've stripped the bike down (it was filthy, and needed new BB and headset anyway) and when I'm building it up will use Gore Ride-on cables to minimise friction.

On the Planet-X the front mech cable is routed through a small tube in the BB shell part of the frame (due I guess to fat carbon frame elements) but the curvature from the cable guide and into the tube looks no worse than on my steel frames.

So I looked at the mech. I've cleaned it up and it seems to be moving ok - and compared to another (SRAM Force) mech I have doesn't seem particularly stiff.

BUT I have noticed one major difference between the SRAM mechs and the Campag Veloce one - and that is the length of the "lever arm" that operates the mech. Put simplet the distance between pivit point and cable attachement point on the Campag mech is 20mm, and on the SRAM mechs (both) is 10mm or maybe a bit less - meaning something like twice the force need to be applied to shift up. I don't think this is compensated for at the lever end either.

So, given that the SRAM mech is indexed is there an altrnative to them that I can try that will work with the double-tap shifters - do Shimano front mechs have the same "movement to cable pull" ratio as SRAM? And do they have longer lever-arms?

Edit - Shifters are SARM Rival, which may make a difference? Although I'd have though all SRAM front mechs would operate with all SRAM shifters (but then again....)

Posts

  • esspeebeeesspeebee Posts: 174
    Unfortunately, doubling the length of the lever arm as you want to do will also double the cable pull required to make the same shift. It looks like you're asking for the impossible.
  • rafletcherrafletcher Posts: 1,235
    Point taken, but only if the operating geometry of the mech stays the same - a bit like rear mechs where the same cable pull results in different movement of the mech between say a 9 and 10 speed groupset although differences are tiny here I accept.
  • mrolimroli Posts: 3,622
    Sram/Shimano front mechs are compatible - I've used Force/Ultegra and have Rival/105 on my cross bike (just because I had the bits lying around, not for any other reason). I don't know if they solve your issues with "stiff" shifting (I haven't ridden Campag for comparison), but the compatibility is fine. Rear mechs not compatible though.
  • rafletcherrafletcher Posts: 1,235
    mroli wrote:
    Sram/Shimano front mechs are compatible - I've used Force/Ultegra and have Rival/105 on my cross bike (just because I had the bits lying around, not for any other reason). I don't know if they solve your issues with "stiff" shifting (I haven't ridden Campag for comparison), but the compatibility is fine. Rear mechs not compatible though.

    Thanks for that. The force required to shift on my Campag set-up is similar for both front a rear, and indeed the rear SRAM shift is about the same - and I expected the front SRAM to be a similar force as the rear, but it was much stiffer. Could just have been the cable route, but it looked ok, as did the cables I pulled out. And I've not read in any review of the Planet-X that front shifting was an issue.
  • phy2sll2phy2sll2 Posts: 680
    rafletcher wrote:
    The force required to shift on my Campag set-up is similar for both front a rear, and indeed the rear SRAM shift is about the same - and I expected the front SRAM to be a similar force as the rear, but it was much stiffer. Could just have been the cable route, but it looked ok, as did the cables I pulled out. And I've not read in any review of the Planet-X that front shifting was an issue.

    I've had one of these for about six months now with SRAM Rival fitted. I had a lot of trouble getting the shifts right at the front. Have you tried the following (apologies if I'm teaching Grandma how to suck eggs):

    1) Shift into small / big (front and back respectively)
    2) Loosen off high limit screw all the way
    3) Set barrel adjuster on the down-tube half-way
    4) Release the cable pinch bolt and pull the cable through very lightly! then tighten bolt back up again
    5) Increase the tension using the adjuster until you get a snappy shift up
    6) Check it again in the small ring at the back
    7) Set the high limit screw as loose as you can possibly get away with

    One of the problems I was having (and this sounds a little like what you're experiencing) was that the cable tension and high-limit screw were 'working against' each other, the effect of which was that the shifts felt very heavy and a lot of effort was required to move the lever through the full range (also, very loud 'click!' sounds when changing down). Letting as much of the tension out of the cable as possible was how I fixed that.

    The other problem I had was that the chain didn't like to stay on the big ring when shifting up (at the front, or even more hilariously on fast descents, shifting up at the back would cause the chain to drop off the front!) If I tightened up the high-limit, it just refused to shift. In the end, I had to 'narrow' the derailleur cage a little and the whole things plays nicely now.

    Front vs. rear? I guess the front is still a little stiffer than the rear, but this has been my experience on all bikes and I think intuitively it makes sense as the front derailleur has to move further and the difference in number of teeth between the small and big rings at the front is far greater than between any consecutive cogs in the cassette.
  • rafletcherrafletcher Posts: 1,235
    Aha phy2sll2, first hand knowledge, just waht I needed. It may well have been the limit stop vs cable thing (though tbh the shift felt still all through the throw) - I never checked a) cos I'd bought it off a "proper" cyclist so it should be set up ok and b) it was manking filthy (I had to use a screwdriver blade to break off chunks of hardened dirt/grease from the jockey wheels!) so I was stripping it to clean anyway. And maybe he'd had similar issues to you re chain skip.

    Anyway when I build it back up I'll take note of your comments, thanks again. Getting front mech cages parallel to chainrings is an art, what with all the different cage profiles you could choose to line up.
  • esspeebeeesspeebee Posts: 174
    rafletcher wrote:
    Point taken, but only if the operating geometry of the mech stays the same - a bit like rear mechs where the same cable pull results in different movement of the mech between say a 9 and 10 speed groupset although differences are tiny here I accept.
    Yes, if the rest of the geometry is different so that the mechanical advantage stays the same, then the cable pull will be the same. So then will the force required on the cable.

    If your front mech is too stiff, then you need something that moves more easily, not that has longer levers. Or, as has been mentioned, better adjustment.
  • phy2sll2phy2sll2 Posts: 680
    rafletcher wrote:
    Aha phy2sll2, first hand knowledge, just waht I needed.

    Good luck and PM me if you need help - setting up that DR drove me up the wall.
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