Disc Brake Safety Issues
A collection of old articles I wrote about issues around disc brakes, weak quick releases and inadequate hubs:
So, what’s the deal?
Basically, there has been a lot of discussion in the bike press and industry recently, about the safety of using front quick release hubs in conjunction with disc brakes.
For a lot more info on the theory, have a look at James Annan’s website, but the basics are this:
Disc brakes produce a lot of leverage, because they have a smaller diameter than the wheel, and they only act on one side of the fork. Because of the position of the disc caliper and the alignment of the dropouts, there is a force acting to push the wheel out of the forks. The theory is that this force is either powerful enough on it’s own to force the wheel out under hard braking, or that it causes the quick release skewer to vibrate loose – either way, the wheel is ejected from the bike at speed.
Why all the discussion?
Disc brakes have been basically added on to existing designs of fork and quick release – before James Annan brought this to the attention of the industry, this particular problem had not been considered in depth.
But it’s not that clear cut. There has only been one documented case of this effect – Annan’s own, which was a tandem with custom-made forks. There have been other possible cases, but they have not been verified.
There have been many discussions on online forums about the merits or otherwise of the theory, but no-one has performed any rigorous tests. Some companies have issued statements, but for legal reasons they do not say much. The CPSC in the States is investigating, but they have not been the most balanced cycling test body on previous occasions.
How the system works…
The important parts in the system are the hub axle with serrated locknuts, the quick release skewer with serrations on the faces of the skewer, and the fork dropouts.
The serrations on the axle dig into the inside faces of the dropouts. They are harder than the dropout material, so they cut grooves for themselves. This prevents movement of the axle, either up or down.
Forks are springy, and the grooves cut by the locknuts are quite shallow, so without help the serrations will slide out of their grooves. So the quick release skewer pulls the two dropouts hard onto the axle. The skewer is only in tension – there is no shear force at all on the skewer.
Under static conditions, this is all that would be needed. But in the real world, a bicycle is subjected to a lot of vibration – a simple skewer would quickly loosen up. So the skewer has serrations which also bite into the dropouts, to stop it unscrewing. These serrations are only resisting the vibration – they have nothing directly to do with holding the wheel in.
So is James Annan’s theory correct? The following is my opinion…
His initial analysis is correct, the force diagrams and calculations he details are accurate, and as someone with a degree in Physics, I have no problem with that part of his theory.
I am less sure about the consequences of the basic maths. These are my reservations:
In Annan’s analysis, the two forks legs are assumed to be independent – the force from the disc brake is assumed to only act on the left dropout. I do not believe that this is correct in practice. Imagine what happens if the axle starts to come out on one side only. First, the disc rotor will have to be deflected – this requires quite a bit of force. Then the skewer will also need to be stretched by an amount proportional to the displacement of the axle. Thirdly, the axle cannot move far before the rim of the wheel contacts the inside of the fork leg – in most case I tested, it is not possible to remove the axle on the left side only.
So, basically, the force generated must be enough to overcome the strength of the serrations on both sides of the fork, not just the left side.
The lack of documented cases. I have talked to a number of technical people at various manufacturers, and they have never seen this problem with their products. Yes, they could be covering up – but these were off-the-record discussions with non-lawyers so I do not think so.
The only recorded case was Annan’s own. This occurred with a custom-made tandem on it’s first outing. The rider weight of a tandem is double that of a solo bike, and the maximum braking force is more than doubled (it is harder to lift the back wheel). The forks in question were rigid steel forks of a particularly thin construction – forks I would not use on a tandem without a disc brake, let alone with.
But all this is theoretical. I believe that there is a possibility of a problem with some combinations of components. Unlike James Annan, I do not believe that there is a fatal design flaw with all combinations of disc brake, fork and quick release.
These are the first preliminary results – this page will be updated as I get more data. I’m doing this in my spare time, and you can help…
Part of the theory is that disc brakes cause quick release skewers to loosen off more quickly than they do with other types of brake. To test this, I did the following:
My usual commuting bike is equipped with disc brakes. It has titanium quick release skewers, and forks with no “lawyer tabs”. In over a year of use, I have never had to adjust the quick release. It was tightened as hard as I could get it by hand.
I retightened the skewer using the “90 degree rule”. This rule is often quoted for quick releases – you tighten the nut so the lever starts to get tight when the lever is at 90 degrees to the wheel (straight out). So this was looser than I previously had it, but still “hand tight”. The lever was on the right side (opposite to where I normally have it, but consistent with advice from SRAM and others).
I then rode the bike on my usual commuting journey – 6 miles per day, on and off road, including cattle grids and speed bumps taken at speed. Every day I loosened then tightened the lever, and recorded the angle at which the lever began to “bite”. This turned out to be an accurate way of measuring the state of the skewer – I also tried using a torque meter on the skewer, but this was less accurate.
I repeated the experiment on a V-brake-equipped bike – the same skewer was used over the same route. The results were as follows:
Disc bike: after 3 days, the “bite point” was 80 degrees – the experiment was halted.
V bike: After a week of use, the “bite point” was still 90 degrees.
Conclusion: From the above, there seems to be an effect from the disc brake which causes the quick release to loosen. There could be other explanations (for example, I tried to brake in the same way on both bikes, but since discs are more powerful, I may have braked harder on the disc bike).
Since this disc bike was used with the same skewer for over a year with no adjustment, it is apparent that this effect only occurs if the skewer is not tight enough. There is a certain critical limit – below this limit, the skewer can loosen – above this limit, it won’t. The limit is dependent on skewer design and rider weight.
It is also important to note that in the experiment I had the lever on the right side, whereas I normally have it on the left. How important this is will need to wait for further tests…
Update: Out of interest, I have now been trying a new setup – using hte same QR on the same disc-equipped bike, tightened in the same way, but with the QR lever on the left side instead of the right.
This setup has now been in use for over a week with no loosening. So now I am going to repeat the first experiment to make sure that I did not imagine the loosening that occurred…
Want to help? I know that two weeks’ riding does not make for excellent statistics – I could really do with some help to get more data. If you want to help, try repeating the experiment above. Tell me if your bike has discs or V-brakes, what make of fork and skewer you use, and (approximately!) how heavy you are. Tell me how many miles to takes for the skewer to start loosening (if at all). Tell me how you tightened the skewer – the “90-degree-rule”, the “hard as you can by hand rule”, or the “stand on it with all your weight rule”. Was the lever on the left or right side of the bike? Thanks…
There are three levels of solution, depending on how paranoid you are feeling at the moment…
Level 1 paranoia: (cheap or no-cost DIY solutions)
- Use a good quality branded skewer from one of the major manufacturers – make sure that it has good serrations and that the lever action feels smooth and has a definite “clunk” as you close it.
- Fit the skewer with the lever on the left side. This means that the entire skewer has to turn, instead of just the nut, making visual checks easier.
- Close the lever as hard as you can manage by hand – it should leave a definite imprint in your palm (if you use your palm). If you don’t have that much hand strength, lay the bike on it’s side and stand on the lever, but don’t go OTT.
- Make sure that the lever is not touching anything when it’s closed – it should not be touching the fork leg, for example. I know it’s not as neat, but closing the skewer so it points horizontally backwards is the best way.
- Use a releasable cable tie to sttach the lever to the fork leg, to stop it flipping open.
Level 2 paranoia: (cheap low-hassle aftermarket solutions)
- Several manufacturers are producing (or have on the drawing board) various types of locking skewer. These could be a good idea, assuming that they lock with enough force, as they’d prevent any vibration loosening.
- Use a solid axle nutted hub instead. The clamping force of a 10mm nut is much higher than that of a QR skewer, which is why track bikes use them.
Level 3 paranoia: (serious bombproof solutions)
- Use a bolt-through front hub. This requires new forks and a new wheel, but is totally solid.
- Several manufacturers are introducing forks with forward-facing dropouts – this means that the ejection force is no longer pushing the axle out of the slot.
- Use forks with the caliper on the front of the right fork leg. This alters the angle of the force so it is no longer a problem. This would require a custom fork, of course.
Personally, I’m a level 1 person…