Rim Vs Disk? Why not Both?

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reohn2
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Re: Rim Vs Disk? Why not Both?

Postby reohn2 » 1 Sep 2016, 10:42am

bobc wrote:"handbrake" - my trike back brake lever incorporates a small button which locks the brake on. A trike of course can run away for miles....

"four brake levers" - not necessary, there are several solutions for driving 2 brakes from one lever. I have used brake levers that actually incorporate a balance bar for 2 cables, and there are widely available in line "one cable in, 2 cables out" widgets (these don't balance so adjustment is more important, but in practise not difficult - this solution is used on the front brake(s) of my trike...)

The question is,is it necessary to have two brakes on one wheel ?
On tandems it can be,especially in high mountains with big descents but one of those brakes needs to be a drag which needs to be a drum with a fair bit of heat sink and which can be set at a variable measure,preferably on a friction lever,with the other two brakes used to bring the bike to a halt as per usual.The drag can also be used as a parking brake.
I question the need for two brakes on one wheel especially if they're rim brakes which will generate more heat on one rim.of course if one is a disc and one a rim on the same wheel that problem is solved.
I know trikes use two brakes on one wheel but that's a compromise due to not having rear brakes,though that's not an insurmountable problem and some trike do have rear brakes.
Two cable brakes from one lever needs more power on the lever for the same clamping force per brake and an unequal cable adjustment can lead to trouble.
I know that for some people the problem is insurmountable for a couple of reasons I can think and that is unavoidable,but usually it's undesirable.
IMO if a bike needs three or four brakes to stop it(other than a drag on a tandem)it needs two effective brakes not an extra two brake to make up for the ineffectiveness of the other two.
We've been riding tandems for more than 20 years and outside of long mountainous type descents I've never found the need for more than two brakes even with an all up weight(bike,luggage,riders) of at times 190+kg.
If someone wants a third brake it's up to them,need in normal use is another matter.
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Gattonero
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Re: Rim Vs Disk? Why not Both?

Postby Gattonero » 1 Sep 2016, 11:27am

In a nutshell: adding a brake should not be a substitute for an inefficient brake that is already there.

When there is a problem, one should work to sort it out, not adding more stuff and thinking that's a good way.

One of the very few times there is need for an additional brake, is for utility bikes, or tricycles/bikes for people with disabilities, or a tandem (the latter, usually has a large rear disk brake only to slow down)
It is by riding a bicycle that you learn the contours of a country best,
since you have to sweat up the hills and coast down them.
Thus you remember them as they actually are...

rmurphy195
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Re: Rim Vs Disk? Why not Both?

Postby rmurphy195 » 1 Sep 2016, 6:09pm

Brucey wrote:
rmurphy195 wrote:Not seen any frames or forks with both disc mounts AND rim brake mounts tough! Perhaps I haven't really been looking.


almost every suspension fork had both mounts for about a decade or more.

[edit; also, some touring bikes have come with both mounts on a rigid fork. BTW I have a few 1-1/8" steerer touring (will take 700x40C) forks with both canti and disc mounts if anyone needs one.]

cheers


Clearly not looking (my excuse is - not my kind of bike)! Frames as well?
Brompton, Condor Heritage, creaky joints and thinning white (formerly grey) hair
""You know you're getting old when it's easier to ride a bike than to get on and off it" - quote from observant jogger !

reohn2
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Re: Rim Vs Disk? Why not Both?

Postby reohn2 » 1 Sep 2016, 6:12pm

Gattonero wrote:In a nutshell: adding a brake should not be a substitute for an inefficient brake that is already there.

When there is a problem, one should work to sort it out, not adding more stuff and thinking that's a good way.

One of the very few times there is need for an additional brake, is for utility bikes, or tricycles/bikes for people with disabilities, or a tandem (the latter, usually has a large rear disk brake only to slow down)

Now why didn't I make it that simple for myself :?

Our Cannondale tandem has 203mm rotors front and rear,with BB7 road calipers,they stop it PDQ,and we've toured some pretty hilly terrain,though not long Alpine type descents.
They've never failed us :)
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Brucey
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Re: Rim Vs Disk? Why not Both?

Postby Brucey » 1 Sep 2016, 6:54pm

rmurphy195 wrote: Frames as well?


for a while quite a lot of hardtail MTBs came with both mounts on them on the frame as well. Seems to be less common now.

cheers
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Brucey
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Re: Rim Vs Disk? Why not Both?

Postby Brucey » 1 Sep 2016, 6:57pm

re tandem brakes; I think it is prudent to have three brakes on a tandem because one brake isn't really enough to slow you down safely on a hill. One brake is what you end up should you start with two brakes and lose one.

cheers
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reohn2
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Re: Rim Vs Disk? Why not Both?

Postby reohn2 » 1 Sep 2016, 7:27pm

Brucey wrote:re tandem brakes; I think it is prudent to have three brakes on a tandem because one brake isn't really enough to slow you down safely on a hill. One brake is what you end up should you start with two brakes and lose one.

cheers

I find myself agreeing with you,though there are many that don't,we have three of them :shock:
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tatanab
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Re: Rim Vs Disk? Why not Both?

Postby tatanab » 1 Sep 2016, 7:43pm

reohn2 wrote:I know trikes use two brakes on one wheel but that's a compromise due to not having rear brakes,though that's not an insurmountable problem and some trike do have rear brakes.
The reason lightweight trikes have two front brakes is because British law sensibly recognises that on a lightly loaded machine with a high centre of gravity the rear brakes will achieve little because a wheel will lock up very easily. This is because the wheel is at the end of a dirty great outrigger (a big lever if you like) so it is very easy to unweight a wheel. Heavy trikes and tandem trikes do indeed benefit from rear brakes, and even on the lightweight side there are some who like to have them. The double front brake has been in use since at least the 1940s, but even then there were options for rear brakes which were largely eschewed by the lightweight fraternity due to weight. The double front brake also maintains the legal requirement to have independent brakes. It is all in the rules.

Brucey wrote:re tandem brakes; I think it is prudent to have three brakes on a tandem because one brake isn't really enough to slow you down safely on a hill. One brake is what you end up should you start with two brakes and lose one.
My tandem days were spent largely with 1940s frames and equipment until I had accustom built frame in 1981. At that time Mafac cantis had all but disappeared and the Japanese ones had not come onto the market, so the frame was built for 3 Campag sidepulls - one on the front and two on the back. The rear two worked from a twin cable Mafac lever which worked fine because the cable lengths were pretty much the same. Later I added a front hub brake worked from a Weinmann alloy straight bar lever mounted underneath the normal drop bar front brake lever. For normal braking I could just use the sidepull. For more serious braking I could go to the drops and use two fingers on each of the two front brake levers (hub and sidepull) which was pretty effective although the steepest hills we rode in were places like Cotswolds/Quantocks/Mendips, so generally short and sharp

reohn2
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Re: Rim Vs Disk? Why not Both?

Postby reohn2 » 1 Sep 2016, 8:13pm

tatanab wrote:
reohn2 wrote:I know trikes use two brakes on one wheel but that's a compromise due to not having rear brakes,though that's not an insurmountable problem and some trike do have rear brakes.
The reason lightweight trikes have two front brakes is because British law sensibly recognises that on a lightly loaded machine with a high centre of gravity the rear brakes will achieve little because a wheel will lock up very easily. This is because the wheel is at the end of a dirty great outrigger (a big lever if you like) so it is very easy to unweight a wheel. Heavy trikes and tandem trikes do indeed benefit from rear brakes, and even on the lightweight side there are some who like to have them. The double front brake has been in use since at least the 1940s, but even then there were options for rear brakes which were largely eschewed by the lightweight fraternity due to weight. The double front brake also maintains the legal requirement to have independent brakes. It is all in the rules.

I was aware of that.
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Binkyboy
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Re: Rim Vs Disk? Why not Both?

Postby Binkyboy » 2 Sep 2016, 11:05am

If you look at a slowed down version on youtube video of that awful crash to the leading rider at the end of the last Olympic Women's Road Race, you'll see that her rim brakes were good enough. https://www.youtube.com/watch?v=oxzkTotjZoc
Having taken the wrong line into the bend, she braked so hard that she locked the rear wheel, which slid sideways, then the front wheel locked and tipped her head first over the bars and into the air. The gutter that Chris Boardman was so scathing about, was nothing to do with the crash.

Disk brakes would have done nothing different.

pwa
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Re: Rim Vs Disk? Why not Both?

Postby pwa » 2 Sep 2016, 12:02pm

Binkyboy wrote:If you look at a slowed down version on youtube video of that awful crash to the leading rider at the end of the last Olympic Women's Road Race, you'll see that her rim brakes were good enough. https://www.youtube.com/watch?v=oxzkTotjZoc
Having taken the wrong line into the bend, she braked so hard that she locked the rear wheel, which slid sideways, then the front wheel locked and tipped her head first over the bars and into the air. The gutter that Chris Boardman was so scathing about, was nothing to do with the crash.

Disk brakes would have done nothing different.


I think the gutter will have played a part by removing the option of running wide and relatively gently tumbling into the bushes.
But your main point is undoubtedly correct. The brakes did 100% of what any brake could do. No additional braking power could have been applied.

On my tandem I can lock the wheels with the vee brakes alone. The third brake (disc) is not there to provide more braking power. It is to take a share of the heat generated when braking on long, steep and twisty descents where "letting go" is not an option. It is important, therefore, that this third brake is not working on the rear rim like the rear vee brake. An Arai brake would be even better but does not fit on a Rohloff hub, unfortunately.

reohn2
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Re: Rim Vs Disk? Why not Both?

Postby reohn2 » 2 Sep 2016, 12:30pm

Binkyboy wrote:If you look at a slowed down version on youtube video of that awful crash to the leading rider at the end of the last Olympic Women's Road Race, you'll see that her rim brakes were good enough. https://www.youtube.com/watch?v=oxzkTotjZoc
Having taken the wrong line into the bend, she braked so hard that she locked the rear wheel, which slid sideways, then the front wheel locked and tipped her head first over the bars and into the air. The gutter that Chris Boardman was so scathing about, was nothing to do with the crash.

Disk brakes would have done nothing different.


The height and sharpness of the edge on top of the kerb* most certainly played a very big part in the seriousness of the injuries sustained by the rider.
The brakes did the job they were designed to ie;stop a wheel turning to slow down the machine,disc brakes would have done exactly the same in the same situation.
The problem in this case is that it was a race and the bend was over cooked and then panic braked,all with with a view to winning a gong and nothing to do with what sort of brake being used.

*which was arguably an unsafe road for a race of this sort to be held on.
Last edited by reohn2 on 2 Sep 2016, 12:59pm, edited 1 time in total.
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Keezx
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Re: Rim Vs Disk? Why not Both?

Postby Keezx » 2 Sep 2016, 12:38pm

Ontopic:
What do we think about a hybrid braking system with a disk in front and caliper rear?
Advantages:
-Stopping power in rain.
-No rim wear front
-Use existing frame
-Use existing rear wheels

reohn2
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Joined: 26 Jun 2009, 8:21pm

Re: Rim Vs Disk? Why not Both?

Postby reohn2 » 2 Sep 2016, 1:01pm

Keezx wrote:Ontopic:
What do we think about a hybrid braking system with a disk in front and caliper rear?
Advantages:
-Stopping power in rain.
-No rim wear front
-Use existing frame
-Use existing rear wheels

That's not a bad braking system as the front brake is the most used and the rear is least effective.
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Brucey
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Re: Rim Vs Disk? Why not Both?

Postby Brucey » 2 Sep 2016, 5:26pm

but there may be a subtle difference; simply because rim brake blocks are a bit squashy and tend to melt when used hard, I think you are perhaps a bit less likely to get to the point that you are 'doing a stoppie' with a rim brake than with many disc brakes, even if both are sufficiently powerful in normal use.

It is well known that the fork needs to be stiffer/stronger if you have a disc brake fitted, but if you use the full brake power regularly (regardless of the brake type per se) then quite probably the frame needs to be stiffer/stronger too.

Arguably many disc brakes offer more readily accessible power than typical rim brakes, which may mean that the frame sees higher loadings with some riders who habitually use that power on a regular basis.

A well designed frameset that accepts rim brakes can offer a very pleasant, nicely resilient ride quality. I have very rarely felt that the ride quality was in the same league on a disc-braked bike.

cheers
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