Understanding Cycle Brakes

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aziraphale

Understanding Cycle Brakes

Postby aziraphale » 8 Mar 2007, 12:01am

Hi all,

I am an engineer by training and should really be able to work this our myself but I struggle. So please give me a helping hand.

After having cycles on various touring bikes for years I now have a reasonably average priced road bike.

unlike the other bikes I used to ride I have reached downhillpeads of more than 35mph without even trying hard. but I do not feel particularly safe - so I tend to break and try to slow it dow to no more than 25mph - especially in the approach to other junctions.

I do typically use the rear brake first and then at usually less than half the pressure slowly tighten the front brake slightly. If I only pull the rear brake or if the tension on the brake is too strong on the rear I have experience thgat my rear tyre tends to move sideways with a ffeling like the rear part of the bike was trying to overtake. Loosing the grip on the brake does help little curiously tightening the grip on the front does.

I do not understand this effect. It makes me feel insecure at high speeds.

I regularly get my bike serviced and also check very regulalry that the wheels are well centered between the brake pads.

Who can explain this phenomena?

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andrew_s
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Postby andrew_s » 8 Mar 2007, 1:14am

What's happening is that the back wheel is locking up and skidding.

When you brake, your weight gets transferred towards the front of the bike, and there isn't enough weight on the back wheel to keep it turning against the brake. If you brake hard enough, the back wheel comes right off the ground and you end up going over the handlebars unless you stop braking quick enough.
The steeper the downhill, the quicker this happens (ie with less braking force required).

As you've found, you have to ease off the back brake as you apply the front one harder.
If you want to stop as quickly as possible, you won't use the back brake at all (assuming the front brake is in good condition).

Tallis the Tortoise

Postby Tallis the Tortoise » 8 Mar 2007, 7:54am

I have had trouble with the back wheel locking and moving to the left as it skids when using only the rear brake in the wet. I tend to use both brakes together to varying degrees. Even without locking the rear wheel, one on its own just feels odd to me. Perhaps to do with pulling with only one had while braking at only one end of the bike.

I do generally have panniers on my bike all the time though, making pitch-over less likely and so allowing more confident use of the front brake. That may be more of an issue on a road bike with a shorter wheelbase than my tourer and the centre of gravity further forwards.

Andy :-)

aziraphale

Postby aziraphale » 8 Mar 2007, 8:22am

Tallis/Andrew

Thanks for your comments. I have considered locking to the proble myself, but wouldnt I expect to see slight skidmarks looking at the tyre right after this happens.

As you've found, you have to ease off the back brake as you apply the front one harder.
If you want to stop as quickly as possible, you won't use the back brake at all (assuming the front brake is in good condition).

I am a bit puzzeled by this. When I brake two things happen:
  • The rotational energy of the wheel is transferred to the bike. This momentum will cause the rear wheel to receive less load when using the front brake and the frontwheel to receive more load when using the rear brake. either of these effects will not apply the load on the wheel where the brake is applied as the rotation is around its centre and has no arm of a lever.
  • The total median of the weight will continue to push forward therefore transferring the weight towards the front wheel and off the rear wheel.

Now while I accept that the overall deceleration can in any case lead to me going over the handlebard when braking too hard the effect of the rear brake should counteract those forces while the front brake will support them.
Where am I wrong or what am I missing out on the logic of wanting to apply the front brake rather than the rear brake?

Jimbo

Postby Jimbo » 8 Mar 2007, 8:27am

I do more than 90% of my braking on the front only. I only use the rear in wet conditions.

There's a page about this here:
http://www.sheldonbrown.com/brakturn.html

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Mick F
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Postby Mick F » 8 Mar 2007, 11:04am

I have always used both brakes equally. Yes, the front is more powerful, and the back can skid. But 'feel' your way and 'modulate' the braking as you slow down.

Cornering in the wet when going down hill can be tricky, I often feel that by braking too hard on the front, it may make the front wheel loose grip and skid outwards. So I use both, there-by maximising the tyre braking area to grip the road.

I regularly exceed 45mph down Gunnislake Hill, and yesterday reached 42mph down into Merrivale on Dartmoor. Never a worry.

Mick F. Cornwall

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Postby fatboy » 8 Mar 2007, 4:06pm

I've read the Sheldon Brown article that you get better braking by just using the front and I just don't buy it. How can it be possible to get better braking by only using one when you've got two. Anyone able to convince me otherwise?
"Marriage is a wonderful invention; but then again so is the bicycle puncture repair kit." - Billy Connolly

Jimbo

Postby Jimbo » 8 Mar 2007, 6:16pm

fatboy wrote:I've read the Sheldon Brown article that you get better braking by just using the front and I just don't buy it. How can it be possible to get better braking by only using one when you've got two. Anyone able to convince me otherwise?


He only says that is the situation in dry conditions on road.

Basically, under hard braking, the rear tyre is very lightly loaded because all the weight has been transferred to the front. Therefore, it can't contribute much to stopping and if it tries it is liable to skid.

In my experience, most of the skill in braking is bracing your arms hard to push back on the handlebars. I find this easiest in the drops on drop handlebars because you really have something to lean on.

BTW, there might be an exception when using heavily loaded rear panniers - in that case there might still be enough weight on the front wheel. I'm only speculating on that point, though.

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Postby Mick F » 8 Mar 2007, 8:19pm

I repeat: Use Both Brakes.

That's what they're there for. Even if the back one doesn't have the grip, it's still has a contribution. The more grip on the road, the better.

Mick F. Cornwall

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andrew_s
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Postby andrew_s » 8 Mar 2007, 9:53pm

It's not grip on the road that's the problem (on clean roads), but the gravitational and braking forces acting at the rider's centre of gravity, and the position of that centre of gravity relative to the front tyre contact patch.

If the back brake is contributing anything at all to slowing you down, you aren't slowing as quickly as you might be. Maximum deceleration comes at the point where the back wheel is just starting to lift off the ground.

If you need to brake harder, you have to move your centre of gravity downwards and backwards. An interesting consequence is that it is possible to stop a loaded tourer quicker than a race bike, given good enough brakes.

Once grip on the road deteriorates to the point where it is possible to skid the front wheel before lifting the back wheel, then the back brake will contribute to an emergency stop. The problem is knowing how hard it's possible to brake on the front without skidding, since if you misjudge it and skid, you'll likely be off the bike.

Having said that a back brake contributes nothing if you are stopping as quickly as possible, I don't (unlike Sheldon) suggest you don't use it. It's good for speed control (as opposed to stopping), and I usually apply it very gently when braking hard so that the back wheel skidding will warn me the back wheel will lift off if I brake any harder.

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Postby Mick F » 9 Mar 2007, 8:00am

Sorry, Andrew, I don't agree.

It may be my riding style, or my bike's geometry, but I still swear by using both brakes, and my rear brake contributes a great deal to stopping quickly. I am not aware of EVER feeling that my rear wheel is lifting off.

Mick F. Cornwall

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Postby jb » 9 Mar 2007, 8:39am

The front brake does the stopping, but a small amount of back brake is necessary to stop the vehicle 'jack-knifing'. ie the back wheel overtaking the front.

Hence motor vehicles have powerful discs on the front and very weak drums or discs on the back. the heavier or longer the vehicle the more important this becomes.

For 'coasting' down long hills its best to use the back if only to conserve the front breaks for emergencies.
Cheers
J Bro

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andrew_s
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Postby andrew_s » 9 Mar 2007, 11:19pm

Mick F wrote:Sorry, Andrew, I don't agree.

It may be my riding style, or my bike's geometry, but I still swear by using both brakes, and my rear brake contributes a great deal to stopping quickly. I am not aware of EVER feeling that my rear wheel is lifting off.

Mick F. Cornwall

Fine, so long as you are aware that you aren't stopping as quickly as you might be.

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Postby Mick F » 10 Mar 2007, 9:11am

Sorry, again. But how can I stop any faster by only using one brake?

The bike has two.

If a wheel skids, it isn't efficiently stopping the bike. (Or any other vehicle, for that matter.) Brake on the rear as hard as you can, so long as the rear wheel doesn't skid. Brake as hard as you can on the front so long as the front wheel doesn't skid.

Obviously, the front brake supplies most of the braking force, but the rear helps too. Simple physics.

Mick F. Cornwall

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Postby Mick F » 10 Mar 2007, 3:46pm

Tell you what. I'm going to carry out an experiment in the next day or two.

I'd be grateful if a few others would do it too, to get to the bottom of this conundrum.

Freewheel down a slight incline at a 'reasonable' speed, then apply both brakes to carry out an emergency stop. Repeat with just the rear brake, and repeat again with just the front.

If you apply the brakes at the same place, and commencing your freewheel from the same start position, and with a chalk to mark record your stopped position, we should end up with some useful data.

Give it a go, and report back!

Mick F. Cornwall