Hollowtech left arm cap help

Recess
Recess Posts: 58
edited March 2012 in Workshop
Hi. I use a Tiagra crankset on my commuter bike, and have sheered off four of the plastic end caps over a two year period. Probably due to some horizontal force, but as it left me cycling the last six miles with only my right leg today when the left arm kept coming off, I figure it's time to move on.

So. As this has never happened with my Ultegra cranks on my 'Sunday' bike in two years of riding, I want to go for a metal end cap like on Ultegra cranks. It doesn't seem that there's an option to get one for Tiagra, as it's smaller. so does anyone know what, or if model numbers on 105 use the larger, metal caps? (like used on my Ultegra)

Thanks in advance.
John
Success is merely a failure to imagine more...

Comments

  • nicklouse
    nicklouse Posts: 50,673
    If you are snapping the preload bolt then the cranks have never been fitted correctly or are now so damaged that they will continue to work their way off the axel.

    Get them inspected and if ok fitted correctly and don't ride with a lose crank arm.
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  • desweller
    desweller Posts: 5,175
    You've misunderstood how the crank is attached to the crankshaft.

    The end cap is there purely to eliminate axial play in the crankshaft during assembly onto the bike. Once that's done, the pinch bolts are used to clamp the crank onto the crankshaft. The end cap plays no part from here on in.

    If it's failed it's because it was not correctly fitted, not because the parts are low quality. As Nick says, get them looked at and replaced if they're beyond repair, but there's no need to go chasing after expensive Ultegra-level parts.
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  • keef66
    keef66 Posts: 13,123
    +1 to that. It's the same principle as the top cap on the headset; just tightened enough to preload the bearings, then the two pinch bolts do the job of clamping the crank on to the axle.

    You could even take the end cap off after installation.

    No mechanical difference between Tiagra and Ultegra

    Sounds like it's been badly installed from the start. If the crank's been working loose I suspect the internal splines are mashed up and you'll need a new crank if it's ever going to fit properly. If you shop around you might find a chainset cheaper than a crank though.
  • Recess
    Recess Posts: 58
    Ok, thanks folks. The cranks are about three years old now, and due to the Glasgow weather, have been on and off several times to clean/replace bottom brackets. And like I said, they've worked their way off a good few times. So I think the splines might well be knackered.

    Ribble are selling the 5650 105 for £65ish so I've just bought that to replace the old Tiagra one.

    Thanks for all your help.
    John
    Success is merely a failure to imagine more...
  • keef66
    keef66 Posts: 13,123
    Follow the installation instructions to the letter! Thin layer of grease on the axle when pushing it through the BB, whack it with the flat of your hand to make sure it's fully through. Lightly grease the splines and fit the crank.Preload the bearings with the plastic end cap, finger tight only. Make sure the little spacer / retainer thing is correctly inserted in the gap in the crank, grease the pinch bolts and progressively tighten them to the required torque.
  • Cleat Eastwood
    Cleat Eastwood Posts: 7,508
    DesWeller wrote:
    You've misunderstood how the crank is attached to the crankshaft.

    The end cap is there purely to eliminate axial play in the crankshaft during assembly onto the bike. Once that's done, the pinch bolts are used to clamp the crank onto the crankshaft. The end cap plays no part from here on in.

    If it's failed it's because it was not correctly fitted, not because the parts are low quality. As Nick says, get them looked at and replaced if they're beyond repair, but there's no need to go chasing after expensive Ultegra-level parts.

    Theres a vid on youtube of a mechanic setting up some hollotech cranks and he says that over tightening that little cap can preload the bearings slightly and he says as a common fault most people over tighten it thinking its that keeps the cranks on, so yes you're right.
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