Strange Disc Brake Problem

[MrJamie]

This is a bit of a weird one, to me at least. I took my bike out for a short ride yesterday and noticed that when i braked the wheel seemed to move left every time and proportionately to the braking force, ie. gentle pad contact had no effect but harder braking would see the tyre move left considerably.

It's hard to tell exactly whats moving, but it looks like the top of the wheel is deflecting left (rotor side) maybe 5-8mm and then springing back as braking stops. Ive had a google and done some reading, I found it's not uncommon and in various guises with lots of different solutions.

>I tried to move the rim with the bike upside down, but there doesnt seem to be any play at all in the hub, also spins very freely so bearings seem okay.
>The rim looks true and the spoke tensions still feel even, its only been on a month too.
>Seems to happen the same with suspension or lockout.
>The suspension fork is rated upto 180mm which is the rotor size im using, I cant see any visable flex but i guess a very small angle could still move noticably at the top.
>Its probably worth noting that bike plus rider is about 140kg so probably larger than normal momentum and forces involved. Id read from Sheldon Brown about how disc brakes apply massive forces and apply forces in silly directions opposite the dropouts.
> I reseated the wheel in thedropout and closed the QR and still moved under braking. Im wondering if it could be the QR skewer not being tight enough. Im a bit ham fisted so have been trying not to overtighten things. The way the wheel looks to be deflecting, ie. pivoting leftwards and what ive read from Sheldon B. about the forces trying to push the caliper upwards and the wheel away from the dropout would agree with this. I havent really had a good test, but this seems to have improved things, but im not sure how tight i can go with a skewer.

Any advice/warnings/stories/jokes welcome

 

[HovR]

Im wondering if it could be the QR skewer not being tight enough. Im a bit ham fisted so have been trying not to overtighten things.

I'm not sure if this would be the issue, but unless you have a particulaly expensive QR skewer I'd say just tighten it as hard as you can using your hands only (i.e. no extension bars!). I tighten the rear QR as tight as I can on my Dawes (horizontal dropouts) and have never been able to break a skewer.

 

[potsy]

My Tricross had this problem Mr Jamie, lbs swapped the OE skewers for some 'beefier' ones, cured the problem but I always make sure they are done up very tight.

 

[mrandmrspoves]

In addition to the advice above, I would suggest checking the disc calipers to ensure that the inner(nearest the wheel) piston hasn't seized.

 

[MrJamie]

I hope it's the same thing Potsy, I wouldn't mind replacing the skewer, I was just worried that my front wheel might lock or come out of the dropout if something failed

Mrandmrspoves, i don't know if thats just for hydraulic disc brakes, but on the mechanical bb7s, the inboard pad doesnt move, the caliper just pushes the outboard pad and squeezes the rotor against the inboard pad. The caliper is still tightly mounted on the fork, the rotor runs freely between the pads and the brakes work well, just the wheel moving is a bit unnerving!

Thanks for the replies, hopefully i'll take it out tomorrow and test it properly

 

[mrandmrspoves]

I hope it's the same thing Potsy, I wouldn't mind replacing the skewer, I was just worried that my front wheel might lock or come out of the dropout if something failed

Mrandmrspoves, i don't know if thats just for hydraulic disc brakes, but on the mechanical bb7s, the inboard pad doesnt move, the caliper just pushes the outboard pad and squeezes the rotor against the inboard pad. The caliper is still tightly mounted on the fork, the rotor runs freely between the pads and the brakes work well, just the wheel moving is a bit unnerving!

I was thinking of hydraulics - but as you have confirmed that the inboard pad is fixed I suspect that's your problem. If you loosen the caliper so that the inboard pad gives minimal clearance - it will not be able to distort when the brake is activated.It may be that the pads need replacing too. Some mechanical discs do have a dial to adjust the inboard pad position as it wears - which solves this problem.
Do ensure the QR skewer is tight too though.

 

[MrJamie]

I was thinking of hydraulics - but as you have confirmed that the inboard pad is fixed I suspect that's your problem. If you loosen the caliper so that the inboard pad gives minimal clearance - it will not be able to distort when the brake is activated.It may be that the pads need replacing too. Some mechanical discs do have a dial to adjust the inboard pad position as it wears - which solves this problem.
Do ensure the QR skewer is tight too though.

I did think that at first, but the outboard pad pushes the top of the rotor to the right, the opposite way the wheel is moving, but I checked anyway. The bb7s have inner and outer dials, so I've got the inboard pad 1 click away from rubbing the rotor, so the rotor deflects as little as possible like you suggest When the brake is pressed and the bike is static, you can see a tiny deflection in the rotor but nothing in the wheel, which made me think its got to be something giving under the braking forces.

To check pad wear do i drop the wheel out and release the pads from the caliper and just replace them when the pads look like theyre running out? It really should be fine for a while though, I've not been out for any distance recently.

Thanks again

 

[mrandmrspoves]

If you have already adjusted the inboard pad so there is minimal deflection and if the wheel is moving over to the right under braking....and your qr skewer is tight and no play in axle bearings - then I can only conclude that the wheel is warping under the force of braking. On the front wheel the spokes should be equally tensioned on both sides of the wheel. It may be that the spokes on the left side are a little slack. In answer to the question regarding pad wear you can often see how thick the pads are without removing them just dropping the wheel out - but if the wheel is moving to the right and only the outboard piston moves it seems like distortion has to be the answer.

 

[andrew_s]

When using disc brakes the front QR should be as tight as you can get it with your hands only. Have the lever at the disc side and align it with the fork so you can see if it starts to move round.
Shimano QRs give a stronger grip than others, and are what you should use if you change.

Movement will tend to undo the QR even when clamped, and if left long enough without re-tightening the wheel can be forced out of the forks by the brake despite lawyer lips. The consequences are obviously unpleasant if you are going any speed, and even at low speed it may well mean new forks (as it did the time I saw wheel ejection happen).

On the rear wheel the brake forces push the wheel into the dropouts so there is no problem.

 

[HovR]

Have the lever at the disc side and align it with the fork so you can see if it starts to move round.

If doing this ensure that aligning the lever with the fork isn't inhibiting the QR's ability to completely close, as is sometimes the case. I prefer to position my QR levers either just fore or just aft of the fork for this reason.

 

[MrJamie]

If you have already adjusted the inboard pad so there is minimal deflection and if the wheel is moving over to the right under braking....and your qr skewer is tight and no play in axle bearings - then I can only conclude that the wheel is warping under the force of braking. On the front wheel the spokes should be equally tensioned on both sides of the wheel. It may be that the spokes on the left side are a little slack. In answer to the question regarding pad wear you can often see how thick the pads are without removing them just dropping the wheel out - but if the wheel is moving to the right and only the outboard piston moves it seems like distortion has to be the answer.

Just to clarify, the top of the rotor flexes slightly to the right as you would expect from the one sided caliper, the top of the wheel pulls the opposite way slightly to the left (or it looks like it's the top). I dont have a spoke tension measuring tool, but squeezing them in pairs from the same side,they all feel tight on both sides, ill go back to that if the skewer tightness isnt the problem though.

When using disc brakes the front QR should be as tight as you can get it with your hands only. Have the lever at the disc side and align it with the fork so you can see if it starts to move round.
Shimano QRs give a stronger grip than others, and are what you should use if you change.

Movement will tend to undo the QR even when clamped, and if left long enough without re-tightening the wheel can be forced out of the forks by the brake despite lawyer lips. The consequences are obviously unpleasant if you are going any speed, and even at low speed it may well mean new forks (as it did the time I saw wheel ejection happen).

On the rear wheel the brake forces push the wheel into the dropouts so there is no problem.

Im really hoping it is as simple as the QR, it would explain why it tilts left (rotor side) and when the QR is undone the wheel can move a similar amount either way, with just the dropouts keeping it inplace. I have the QR lever pointing to the back of the bike as per instructions that I think were implying that has lowest chance of being knocked open/snagged, it was still perfectly horizontal so I dont think its rotating, but its entirely possible it wasnt done up very tightly in the first place as I've been trying to avoid over-tightening things. I think the current QR is Shimano, but ill look into getting a better one if tightening is the issue, maybe a Shimano XT, for peace of mind if nothing else.

Thanks again all

 

[MrJamie]

It does look to be the QR skewer, i tightened it up much much tighter and it looks near enough fixed. I think theres a very very small shift under very heavy braking, but nothing like it was yesterday, its really not noticable if its even there at all. Having read more about it I cant believe that theyve engineered front brakes so that the force tries to pull the wheel out of the dropouts. Theres so much information about it online, some people saying you shouldnt use QRs at all on front disc wheels.

I presume the skewer is a Shimano M475 like the wheel hub, so im not sure how much better an XT would be and the XTR skewers are £35+. Ill leave it for now and keep my eye out for a bargain on skewers

Thanks

 

[mrandmrspoves]

Glad it's sorted and cheaply.
Hydraulic brakes are extremely powerful when compared with side pulls (and even their predecessors) yet the design of wheels has hardly changed as brake design has improved.

 

[HovR]

It does look to be the QR skewer, i tightened it up much much tighter and it looks near enough fixed. I think theres a very very small shift under very heavy braking, but nothing like it was yesterday, its really not noticable if its even there at all. Having read more about it I cant believe that theyve engineered front brakes so that the force tries to pull the wheel out of the dropouts.

Glad you got it sorted.

Some frame manufacturers (Cotic, with their RoadRat for example) are starting to put the disc brake mount on the front of the fork for greater axle security - Maybe we'll start to see major fork manufacturers follow suit.

 

[02GF74]

The consequences are obviously unpleasant if you are going any speed, and even at low speed it may well mean new forks (as it did the time I saw wheel ejection happen).

how can that happen? fork drop outs are recessed so that even when the skewer is undone, the wheel cannot drop out.