[Closed] Disc brake questions

https://www.bikeman.com/bicycle-repair-tech-info/bikeman-tech-info/1638-51mm-international-standard-74mm-post-mount

I had this disk/dropout arrangement on the back of a tandem (2000-era Ventana, dropout design basically pre-dated disks). It did slip occasionally under severe braking but never caused any real safety problems as all you get is a spot of wheel rub. It improved over time as the QR clamp dug indentations in the dropouts. I did the QR up tight as possible, but I wouldn't deny there is a small risk involved. Losing a rear wheel on a tandem probably isn't as big a risk as a single bike due to much longer wheelbase. I wouldn't expect to see a bike with this design nowadays.

As for the front, I wouldn't touch it with a bargepole unless I was very confident there was a substantial lip that would retain the QR under all conditions. If you flip it open, does the wheel come out, or do you need to unscrew it also?

Good quality QR skewers are fine. Shimano steel ones are extremely reliable. XT are slightly nicer, but the cheap ones are just as effective. Properly tightened QR skewers are fine, just check them before each ride (although I have no experience with recumbents).

Mechanical disks are easier to set up if you are used to rim brakes. Hydros have better lever feel because they don't have cable friction. I would always go for hydros, the performance is worth it. Only problem is likely to be hose length, but that's a one-off thing.

Thanks very much. It looks like it has IS mounts.

I've included some photos to give an idea of the angle and position of the dropouts and mounts.

As I said, the dropouts are open. Does it look like using disc brakes might be potentially dangerous in any way given the position of the mounts and the dropout angles and types?

LOL. You feel lucky, punk?

Ha! Ha! If I was lucky I wouldn't have ended up with this bike!

So, you wouldn't advise using disc brakes on it? What are the main reasons for not doing so? I had a bad feeling about it myself, hence I asked, but I don't know the technical reasons.

Anyway, thanks for stopping me from spending another small fortune on it!

I would replace that QR skewer, whatever you do. It looks pretty, but I think you'll find the Shimano ones clamp much more tightly.

It should be possible to put a couple of spots of weld on the bottom of each dropout to act as lawyer lips and prevent the wheel moving even if the QR isn't particularly tight.

What are the main reasons for not doing so? I had a bad feeling about it myself, hence I asked, but I don’t know the technical reasons.

When disk brakes are mounted behind the axle, braking forces try to lever the axle downwards (i.e. the opposite direction that the rotor is moving at the point where it contacts the brake pads. This tries to eject the wheel from the fork. If you have a disk braked bike handy, try removing the front QR skewer or axle, and pushing the bike with the front brake applied. You'll see that the wheel gets ejected downwards. If the brake was mounted ahead of the axle, it would force the wheel into the fork.

On top of that, having a brake on only one side causes complex twisting forces on the fork, so there is a theory that this can cause QRs to work loose. In the olden days, people used to like to pimp their bikes out with lightweight QR skewers. A lot of those are poorly designed and do not clamp the fork strongly enough for safety. On top of that, people (apparently) used to grind down the lawyers lips to speed up wheel changes. That's obviously a pretty dumb idea. Shimano QR skewers seem to be the gold standard, so I would toss that skewer you have and fit a Shimano one.

IMO, it will probably be okay if you fit a good skewer and make sure it's closed tightly before each ride. However, the consequences of that being wrong are almost certainly a serious crash. How much do you value your teeth?

you can get hex key skewers instead of QRs, which would give me a bit more confience in wheel tightness. you could also machine a little of the dropout material away on each side, so that when the skewer is done up it can't slide forwards at all (as in, if the skewer has a 14mm bolt head, drill a 14mm indent, ~1mm deep, where the wheel will sit)

you can get hex key skewers instead of QRs, which would give me a bit more confience in wheel tightness.

This is controversial. The theory of QRs working loose is that the fork twists when you apply the brakes and this twisting slowly loosens off the skewer. A hex key skewer won't help this, assuming the theory is correct (which was the subject of long, bitter arguments back in the day). QR skewers are much easier to check because you don't need to fish around for tools, so the argument is that they are actually safer. I personally don't know what to make of this, it was never really resolved and the industry turned to through axles to eliminate the problem. I've never had a Shimano QR skewer work loose in 20 years of using disk brakes, and all my bikes still use them.

^ covers it.

Does it look like using disc brakes might be potentially dangerous in any way given the position of the mounts and the dropout angles and types?

In theory yes, maybe. In practice, unlikely. As Hols2 says the problem I see is the QRs themselves. The difference in clamp force between those and a Shimano or DT RWS is significant.

If you were to be using the bike for 1000s of miles w/o touching the front QR you might see some creep. I'd check how much bite the QR gets into that Ti dropout, probably not much - roughen up the dropout surface if there's a way to do that. An actual wheel ejection situation would need a loose QR first. Do the QR up properly and check it now and then. To put this risk in perspective as far as valuing my teeth - carbon forks bother me more than wheel ejection, since one is an unseen fault and sudden failure, the other has a preceeding fault which is something I can see and check for and would be likely to cause a slip or some noise before sudden ejection.

How much do you value your teeth?

It's a recumbent. Hip gravel rash and a broken elbow, perhaps.

This is controversial.

Yes, they won't prevent the precession from blade twist. But I like them because you can use a torque wrench to set them consistently and check them. A QR check is done by feel, a Hex key is good for a less mechanically confident rider - it's suprising how many get QR use very wrong (not relevant to posters here) and I think that makes a hex key a better bet on lower-price bikes with discs where TAs aren't used. I'm going a bit OT though.

I can remember the arguments about wheel ejection on usenet in the early noughties and thought at the time the risk was minimal as long as you did your quick releases up properly and kept an eye on them. I'd certainly not worry about it on a recumbent as the main risk on an upright appeared to be unweighting the front wheel after the quick release had already loosened.

I’d certainly not worry about it on a recumbent as the main risk on an upright appeared to be unweighting the front wheel after the quick release had already loosened.

Actually with a recumbent or a tandem there's potential for more braking force applied at the front wheel since there's much more leverage keeping the back of the bike down, hence more chance the QR friction force is overcome by the brake and wheel forces. The chap who raised the alarm on disc ejection originally had it happen to him on a tandem.

the main risk on an upright appeared to be unweighting the front wheel after the quick release had already loosened.

The major concern was that the geometry of the brake mount meant that braking forces would eject the wheel under hard braking.

The major concern was that the geometry of the brake mount meant that braking forces would eject the wheel under hard braking.

That was the concern but at the time there didn't appear to be an real world cases of actual ejection, only quick releases loosening.

Thanks very much for that.

Also, thanks for the warning about the skewers. I was probably going to replace them, but now I definitely will. I bought them from a local bike shop to tide me over, and that was all they had. I normally use Shimano or Campag QRs or axle nuts, depending on the bike.

It sounds like discs might still be an option if I make sure the wheels are clamped in tight, and keep an eye on them. Also, try to modify the dropouts to reduce the risk of the wheels dropping straight out.

I have a front fairing for the bike, and always intended to streamline it a bit, so there is the potential for high speeds on descents.

Another option is drum brakes, but I'm not sure about the forces being put through the forks, plus they would need custom brackets made up to secure the brake arms. Also, I'm not sure about braking performance from 70mm drums.

The other option is to buy Paul's Motolite brake arms (also expensive) and have slightly larger (451) 20" wheels built. I think the Motolites could reach the larger rims. However, tyre selection is extremely limited in that size.

The bike has bosses for V-brakes, but they're positioned for an obscure 20" wheel for tubulars (432), and I just can't get decent, reliable rims or tubs, hence the reason I'm looking for another wheel/brake solution.

If using disc or drum brakes I could use the smaller 20" (406) wheels and tyres, and there's a lot more choice of tyres in that size.