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Dragging Disc Rotor Issues
Hi Guys,
My bike has started to develop an issue with the rear disc rotor rubbing against the pads.
I've tried readjusting the caliper alignment & this sorts the problem when the bike is unweighted, but as soon as I sit on the seat the problem returns. ie, the rotor runs freely with no sound when I lift the rear wheel & spin it, but when I jump on it for a test pedal, the rotor starts scraping again. Also, if I shift my weight forward over the handlebars the sound dissappears, but returns again as soon as I shift my weight back over the rear wheel. I can't work out what is going on here. It seems like something is flexing that shouldn't be... any ideas?
At first I thought that the real wheel was the culprit but I've had it trued (only out a few mm in one spot) & the mech said that it only had a couple of spokes that needed tightening.
The bike is a stock 2008 Giant Trance 2, bought new in Jan 09 - the only mods I've made are the tyres.
http://www.giant-bicycles.com/en-AU/bikes/mounta...
Any help to track down the source of this problem would be muchly appreciated, Ta.
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if the problem only occurs when the rear is weighted, it must mean there is some deflection occuring between the disc rotor and the caliper when weight is applied. Have you check if your wheel is sitting properly in the dropouts? Wheel clamped tightly in dropours? Is there any play in your hub?
The wheel alignmnet can not have been the cause as the disc is attached to the hub, not the rim or spokes.
assuming you have a duallie, perhaps there is a small deflection in your swingarm?
or perhaps the caliper is not bolted on perfectly straight - although i think this is unlikely.
Get somene to sit on the bike and setup your caliper positon while the rear is weighted
thats all i can think of.
This could very well be a loose swingarm. It's not the same as being out of alignment and is a wear and tear issue and not a manufacturing fault.
Find all the pivots on your bike and it's clear that they are held in with a hex key bolt. Inside is the bearing. All bikes, no matter how fantastic the brand, will have pivot bolts that slacken off during use. It's the owners responsibility to ensure they are tight, and a shops' responsibility to tighten them during a service. However they can slacken off pretty quickly. Don't Lock-Tite them because they need to be undone should bearings require replacement in the future.
So simply pop in a hex key on each side and tighten and do this frequently. There are two obscured by the cranks so they'll need to come off to get to them.
If you want a shop to do it all it's quite labourious so don't argue about the labour charge just get it done.
These bolts also correctly load and align the bearings and promote long bearing life.
Another cause could be loose hub cones, causing the wheel to be loose in the drop out even though the qr is secure. When tightening the cones, don't go crazy, easy does it...
Even though your brakes are entry level Shimano the rotors will still be straighter than anything from Avid so warp shouldn't be the issue here.
Best of luck..
I had a similar friction problem that seemed to be dragging unweighted and better weighted. Turned out to be a worn out cone in the hub. If you have a cup and cone rear hub that hasn't been serviced for a while it might be worth getting it look at. If you take the rear wheel off and spin it while holding the axle you may be able to feel if it is loose or grindy.
I thought I had a dragging disc issue, and like you, no amount of rotor truing or caliper alignment would fix. Still that disc-dragging "swish-ting!" noise coming from the rear, no matter what I did. No slop in the hub cones either - it was a cartidge bearing hub. So, finally one day out of frustration I removed the caliper off the frame and taped it to the seatpost and pedalled down the street.
"Swish-ting!"
Aha!
Turned out, it was loose-ish spokes sliding in the Mavic straight-pull hub under load and all that was needed was some attention to spoke tension. Viola! No more noise.
Thanks for all the suggestions, guys.
I'll report back when I get time to have a proper look over the bike.
Time's a bit short at the mo, as I'm moving house in a few weeks - 8 years of clutter to sort through........
However, Hawkeye - i'm fairly certain that its the disc rotor & not a spoke issue, as the sound is more of a constant "metallic scraping/grinding" sound rather than a "swish-ting" sound. I'll try your method of taping the caliper to the seat-post to confirm that its the rotor first, then work my way through checking that the axle is sitting in the dropouts correctly, the hub cones, & then tighten up all of the pivots bolts.
Hopefully, i'll be back to the sweet sound of silence!!
Finally sussed out the cause of the noise -
It wasn't the disc rotor at all, but that plastic spoke protector thingy on the drive side.
I'd accumulated a good deal of deep gunk between the sprockets that my regular cleaning hadn't removed, and it was causing it to make the noise. I managed to scrape out most of the gunk after degreasing with a small guage allen key & VOILA!!!
However, I don't quite get why it was only occurring when then rear wheel was weighted. (???)
Anyway, is the plastic thingy serving any purpose? I'd like to remove it so the problem doesn't return as soon as the sprockets gunk up again. Is there an easy way to remove this without having to take off the cassette? I don't have a chain whip tool or a lockring attachment. If I try to kludge something I'm sure to regret it.
from a poorley adjusted derailleur pushing the chain into them.
Leave it and just clean it occassionally
The plastic thingy is to stop your chain going into the spokes - which is very bad.
However, if you maintain your bike regularly this should never happen.
Ensure your limit screws on your derailleur are properly adjusted.
so get ride of the plastic thingy, cos it colects dirt and looks crap
Delicious,
Not on the main topic of the overall thread, but I feel I have to comment on your sweeping generalisations on pivots and loctite.
I do not agree 'ALL pivots slacken off during use'. This has NEVER happened to me across a number of dual suspension bikes, and should not happen to MOST bikes.
I agree some bikes have design issues that may cause occasional loosening, but on the whole they shouldn't, I also agree they should be checked during service for correct torque, but not continually 'nipped up'. Nipping up 'cracks' the threadlock, and may be why you have issues with loosening. If they do move (tighten), they need to be undone, re-loctited and retensioned.
I also do not agree about 'not using' loctite. It is designed to prevent loosening of threads AND for removal. There are 3 different grades for screws, nuts and studs. (222, 242 and 262). I highly recommend the use of 222 and 242 as appropriate on mountain bikes, particularly pivot points, brake rotor screws, derailleur idler pulley bolts etc. With Loctite (or similar threadlock compound), bolts can be tensioned to the manufacturers recommended torque values, and you can be confident they will not come loose. Many people over-tighten bolts and screws to feel secure, but this can stress the components and ultimately lead to failure, or deforming of the threads, which will also lead to loosening.
[end of rant]
Yep, Loctite (insert favoured brand in here) for the win!!!