Radial Disc Brakes
I wonder if you could help my understanding of what "radial" refers to in radial (disc) brakes, and why they are better.
The motorcycling media frequently make references to a bike model improving by adopting radial brakes. A remark made in this weeks MCN about the water cooled BMW R1200GS is a good example.
I can see the visual difference between radial disc brakes and "normal" disc brakes. I can see the benefit in easily moving the calliper out "radially" by using equal spacers, to allow larger discs in adapting a production machine for racing. But is there an intrinsic benefit other than this adaptability?
I assume the moving of the calliper out on spacers is where the "radial" bit comes in? There doesn't seem to be anything else any more radial about them than older set ups. The axes for the mounting bolts aren't radial. The cast lower mounting strut always seems to fan out in radial fashion I suppose, although I can't see any innate theoretical necessity for it to be like that.
Is it because the mounting arrangement is wider and withstands higher forces transmitted through it? Opening the way for a more powerful calliper design? And larger discs.
I've now read your article on brakes in the technical archive. Sorry I didn't think to check there first. If I've got this right, I can now see 4 benefits in this design type. (only 3 of them apply to Telelever equipped bikes)
1) From your article: the radially disposed cast "arms" absorb the higher loads of modern brake calipers. Full reason explained in your article.
2) My own observation: (telelever equipped owners can look away) these same cast arms are well suited to the very short cast ends of modern upside down forks. In fact, presented with modern long calipers (evolved this way as explained in your article), situated a fair distance away (due to discs with large outside diameters and large inside diameters, as explained in your article) and a very short cast tube to attach to, the component pretty much designs itself.
3) As I've mentioned in my post, and Granitehead concurs, I've heard that spacers can be used to set the calipers out even further to enable even larger disc diameters.
4) A bonus (but still a benefit): They look damn cool! Not only that, they look "right". Usually a good sign.
Yup, the core advantage is the whole set-up is more rigid, rather than having the calliper connected at only one end of its body so it's waving like a flag, being held firmly at both ends means the mounting is more rigid and there's less flex in the calliper body itself. This lets you use higher pressures for more power without increasing distortion and spurious movement, which interferes with feel and progression.
You're right too in that it suits modern inverted forks very well. But larger discs are avoided in fact where possible as they increase gyroscopic forces which interfere with the steering by increasing the inertial moment of the steered mass. This is due to the mass of the callipers being further from the centre of the wheel as well as the larger diameter discs. So generally you'll find 300mm and 320mm discs on the fronts of sports bikes, and you could deal with those with a conventional calliper mounting too. Designers prefer to use the smallest discs they can while still generating sufficient braking power - as in all engineering, it's always a compromise.
The name comes from the manner of mounting each calliper in effect on two aluminium arms (cast into the fork body) which extend and are aligned radially from the wheel hub.
I mentioned Telelever above. It's interesting that the latest telelevers in the spy shots look exactly like upside down forks. Is that a fashion thing, driven by the need to keep sales from the steadily rising competition (with regards to GS models). Or is it the way evolution works regardless of fashion. In other words, we want the benefits of radial calipers, so we design the bottom of the fork leg to accommodate them. That means beefing that bit up. As a consequence we find that the loadings on the fork leg proper have changed, so we can reduce them down. The result. They look like telescopic upside down forks! May as well colour them accordingly.
Or a combo of the two?
Sorry, have to disagree Kev. It's nonsense to pretend that the radial mount is in someway stiffer. Either way the caliper position will be maintained better the closer it is to the forkleg, but not as well the other end.
The real advantage in the use of radial mount calipers is the ability to easily change the disc size. That doesn't necessarily mean fitting larger discs since in racing it is sometimes desirable to use smaller discs, but not always and the radial mount allows the disc size to be easily and rapidly changed which is simply NOT possible with conventional axial mount calipers.
This concept has now spread across onto road bikes (as it always does) but to pretend that the reason for their use is anything other than the convenience of using different disc sizes is utter tosh.
Radial master cylinders are another area of misconception. Brake fluid doesn't care which way it's being pushed and the different orientation of the actual cylinder makes no difference whatsoever. However, these newer M/Cyl designs do allow a much better mechanical advantage than with the older conventional design, with the lever pivot able to be much further from the point of force on the lever (i.e. the hand).
Neither of these innovations are bad, far from it, but lets just be honest about their origins and recognise that their benefit on road bikes is essentially just bling.
It's great to have some input and thoughts on this. I'm looking forward to hearing Kev's response. In the meantime I've been pondering a bit more on this anyway and here's how I see things to date.
Engineering is a bit like politics, there is rarely just one answer, usually a compound effect. For my money, there's a little bit of everything that has been said so far involved here.
1) With the move to upside down forks and the desire to keep fork overlap as good as possible, balanced against the need for decent fork travel, the cast end of the fork is quite short. You'd be hard pushed to attach a long caliper to it without ending up with a shape very similar to what we now commonly see. I agree, I see no firm reason for the casting to be precisely in the form of 2 perfectly "radial arms. In fact I've seen quite a few designs that have nothing radial about them, except for the fact that the caliper, if moved in or out on spacers, will move in a perfectly "radial" manner or direction.
2) If the need to mount the caliper so as to enable it's movement on spacers arose first, then you would still end up with a shape similar to what we now see. How else would you anchor the far end of the caliper? In conjunction with the ever shortening fork leg end, you have your first compound effect.
3) If we look at the forces involved in severe braking and think how the fork leg would try to bend, I think the bottom cast "radial arm" anchors the bending force quite well. I may be wrong, so I welcome input and correction if that be the case.
4) Calipers have got longer and they are now attached at the extreme ends for the very reason you have given. Discs are now getting smaller in outside diameter as Kevin says, to keep gyro forces down. But this is only made possible because braking pressure is able to keep going up. It has to. Overall braking performance is getting better, even though discs are tending to get smaller. Ever more pressing need to anchor the caliper better and protect the fork leg from bending than ever before.
5) Consider how upside down forks are put together at the bottom end. You have the steel tube pressed into the cast end. It's not one homogenous piece like it used to be. If I'm right with my analysis in 3) then without the cast radial lower arm maybe there would be more tendency for the two parts to separate.
6) Bling! As you've already said. A strong identity. But for my money, that's last on the list. The political bonus. The engineering has been done by that stage. I think the "radial" moniker is a misnomer, but hey, that's marketing. What else would you call the breed?
Anyone know when, and on which bike, did radial mounted calipers first appear?
I'm guessing the late 90's or early noughties.
I know we're not in the same league performance or design wise, but the BMW calipers of the mid seventies look uncannily similar in appearance. I wonder if if any inspiration was taken from these? BMW changed to a set up typical of jap bikes in the early eighties, so maybe the earlier set up didn't offer the technical benefits it was supposed to. It looked a heavy arrangement.
Radial brakes with upside down forks.
The format lends itself well to incorporating the galleries and mounting for the remote damping chamber/reservoir in the latest breed of forks
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