Braking. Front or Rear? Or Both?

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Mick F
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Braking. Front or Rear? Or Both?

Postby Mick F » 11 Mar 2007, 10:42am

It has been discussed on this Forum, the pros and cons of braking 'styles'.

I've just carried out an experiment, as promised to help decide how to brake. I rolled down a gentle slope nearby, and applied the brake/s and repeated each test four times from exactly the same place, with no push off, and achieved a speed of around 10mph, noting my stopping position with a chalk line on the road.

The BEST results for each test were:
Rear Only - 280 inches
Front Only - 134 inches
Both - 125 inches
(I used inches, coz I didn't have my glasses with me, and couldn't read the mm's!)

Therefore the Front Only gives an increased stopping distance of plus 7.2% over Both, and the Rear Only a plus 124%.

I admit that the results were as I expected, but I must add that I was surprised that the Front Only was nearly as good as Both!

Having read Sheldon Brown's thoughts, and listened/read to other's, I was doubtful that using only one brake was sensible. But I feel sure that using both has the advantage that just before your rear wheel starts to skid/lift, it does provide some speed reduction. After all, brakes do not stop a wheel dead, they slow down before locking. Also, it takes a finite time for the centre of gravity to shift forward.

I've never done so many emergency stops!

Anyway, please discuss, try your own experiments, and report back.

Thanks.

Mick F. Cornwall

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DaveP
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Postby DaveP » 12 Mar 2007, 12:01am

I had intended to post last night to agree with your analysis of the situation, but fatigue took over!
As long as the back wheel is rolling rather than skidding the rear brake can do its job of converting your kinetic energy into heat, dust and noise. In other words it will contribute to the braking effort right up to the moment when apparent "weight transfer" reduces the contact pressure enough for the wheel to lock. Weight transfer starts instantly but increases with time as the brakes bite progressively
So, if you give it your best shot using the front brake alone, and then try again using both brakes, even if you grab the front as fast and squeeze exactly as hard as before, the rear brake will still contribute to the total braking effort, dissipate some energy, and reduce the stopping distance.
I'm actually surprised that the difference between front and both is so small. I wonder if this could reflect the difference between doing something which is familiar and safe, and doing something which is perhaps a bit more likely to create an 'out of control' situation?
I'm not having a go! Just thought it was worth pointing out that theres more then physics involved.
Nice expt. Sounds as if you had a good day!

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CJ
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Postby CJ » 12 Mar 2007, 3:05pm

Result!

I always tell people it'll take them more than twice as far to stop if all they've got is a rear brake, compared to both or front only. It's nice to have some practical evidence to underpin the maths.

Thanks.
Chris Juden
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dasy2k1
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Re: Braking. Front or Rear? Or Both?

Postby dasy2k1 » 28 Feb 2010, 9:17pm

indeed i find that i can stop much faster with just front than just back,

however in my case both is significantly better than just one brake
as i am riding with chromed steel rims and rubbish single pivot calipers the limiting aspect is the brake itself...
so i would imagine with better brakes and alu rims i would get somthing like your tests

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Re: Braking. Front or Rear? Or Both?

Postby Freddie » 28 Feb 2010, 10:28pm

What is it you're trying to find exactly?. Maybe in an emergency both brakes are faster to a dead halt, but then if you can controllably lift the rear wheel with the front brake...

When cycling 'as usual' I almost always use the front brake unless traction dictates otherwise, seems to allow more control when braking through corners and is more responsive to input in general.

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Re: Braking. Front or Rear? Or Both?

Postby patricktaylor » 28 Feb 2010, 10:39pm

Who dug this up? It's been almost three years.

FWIW, emergencies aside, I tend to favour the front brake to equalise wear on the rims.

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Re: Braking. Front or Rear? Or Both?

Postby Freddie » 28 Feb 2010, 10:44pm

Grave robbers...wondered why I hadn't seen this one before...

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[XAP]Bob
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Re: Braking. Front or Rear? Or Both?

Postby [XAP]Bob » 28 Feb 2010, 10:51pm

Major deja vu with regard to these figures...

With the grave robbing posts coming after I started this I should point out that this was probably the thread Mick pointed out when he reused the figures, saying noone had commented :)

Anyhow. The point is to dispute the common "Front brake alone is best" claim.

There are actually two questions that are commonly asked:
- How do you get maximum braking force?
- How do you stop in the least distance?

The answer to the first question is the easiest (assuming that you aren't traction limited on the front wheel). Maximum braking force is achieved with the front brake only, when the rear wheel is resting on the ground (i.e. no weight on the rear wheel, but that wheel not yet lifted)
- If you apply any more force then the rear wheel will lift and the rider will be unceremoniously dumped over the handlebars.
- There is no traction at the rear wheel to provide any braking force.

The answer to the second question is less easy - at least in part because it's a dynamic, rather than static, mechanics problem. Whilst the answer to the first question implies that the front only brake should be best it fails to account for the dynamics of brake application. Brakes do not apply at once, and weight transfer also takes time.
What this means is that en route to the maximum braking force (which is front only, given the above assumptions) there is traction available at the rear wheel, and this can be used to provide a braking force, which will reduce the required braking distance.

To minimise braking distance both brakes should be applied with maximal force, the rear brake will then need to be released gradually as weight transfer reduces the traction available at the rear wheel. Of course that requires quite alot of skill.

If there is limited traction (i.e. if you can skid the front wheel) then the first question has a different answer, and the logic follows to the second:
- Max braking would be maximum front braking (just before locking the wheel) + max rear braking *at that weight distribution*
A shortcut has to be a challenge, otherwise it would just be the way.
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Re: Braking. Front or Rear? Or Both?

Postby rjb » 28 Feb 2010, 11:35pm

Just out of curiosity was this with the chopper :?: Can you repeat this with your loaded trailer attached and post the results again.

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Re: Braking. Front or Rear? Or Both?

Postby andrew_s » 1 Mar 2010, 12:41am

I mostly agree with [XAP]Bob, but would like to make two further points

a) If you allow the back wheel to lock it will skid sideways, which will have you off the bike unless you bring the wheel back into line by letting go of the rear brake.

b) weight transfer isn't a physical movement that takes time to happen. It is a direct consequence of the deceleration, with no delay. Since you can't grab a fistful of back brake any faster than of the front brake, any braking it contributes in the interval before the front brake is on as hard as possible will be minimal. The chances of being able to pull the rear brake and release it again in the half second or so it would take to pull the front lever in are slight to nonexistent.

To stop in the shortest distance, assuming you are not limited by either traction or weak brakes, you should slide your backside as far off the back of the saddle as your arm length will allow, whilst simultaneously snatching at the front brake as hard as is possible without the rear wheel lifting.
This is pretty basic science.

Snatching the front brake as hard as possible without lifting the rear wheel obviously requires a considerable degree of skill and experience.
I for one have no intention of undertaking the practice and risk required to gain that experience. I pull the back brake gently, whilst progressively pulling the front brake harder until the rear wheel starts to skid (after maybe 1.5sec), at which point I let go of the back brake. I accept this means that I'm not stopping as quickly as is possible, but I don't claim that stopping quicker isn't possible.

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Mick F
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Re: Braking. Front or Rear? Or Both?

Postby Mick F » 1 Mar 2010, 7:41am

It wasn't me that resurrected this thread, it was dasy2k1 and it was a year or two before I was given Chopper!

I started this thread in the hope that it would spark a debate, it took 3 years, but at least it has started. I've referenced this thread on a few occasions, and it has steadfastly refused to spark off a debate, so I'm grateful to dasy2k1 - thank you!

I agree with [XAP]Bob's statements. It was they way I thought, and he's put it very well. However, I must add that on my Mercian, with me riding, it never seems to lift the rear. I've tried to explain this before, my brakes are good Campag Chorus and are keen yet progressive, but something to do with Mercian's geometry or my fat bum, I'm never in danger of doing a header. It could be my skill or riding style, as when I brake hard, I shift my weight rearward. This is simple on a Brooks of course due to the shiny leather, but I do it naturally. No doubt if there was a hyper-emergency situation, I wouldn't have time to shift my weight, so perhaps I could do a header.

Talking of weight distribution, anyone remember my Pot Hole Incident? My wheels were wrecked, the front had a "flat" on the rim, and the rear went "heart-shaped" showing that at speed and hitting a deep and wide pothole, my weight was over the back wheel whilst the front was lightly loaded. I was doing in excess of 30mph on the drops.

Any road up, please feel free to replicate my experiment, and report your findings.
Mick F. Cornwall

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Re: Braking. Front or Rear? Or Both?

Postby mw3230 » 1 Mar 2010, 7:53am

I accept everything posted above but cannot add to the debate because in an emergency stop situation I seem to grab all the brake levers I can get hold off and yank (or American?) as hard as possible. I appreciate the physics but I can't think and react at the same time. Instinct takes over.

I do remember on my motorbike driving test 40 years ago being asked the question about how to apply the brakes in an emergency and the correcct answer was both brakes but the front one slightly in advance of the rear - I passed
Retired and loving it

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Re: Braking. Front or Rear? Or Both?

Postby 531colin » 1 Mar 2010, 7:56am

Mick F wrote:
Talking of weight distribution, anyone remember my Pot Hole Incident? My wheels were wrecked, the front had a "flat" on the rim, and the rear went "heart-shaped" showing that at speed and hitting a deep and wide pothole, my weight was over the back wheel whilst the front was lightly loaded. I was doing in excess of 30mph on the drops.
.


Or does it show that a dished rear wheel is inherently weaker than an un-dished front?
Or that the spokes/ rim fatigue quicker in a dished wheel?
Of course your weight was on the rear wheel, where else could it possibly be unless you were doing handstands on the handlebar?
But what happened to the wheels proves nothing. We all know of the basic flaw in bike design which loads the weaker wheel more heavily!

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Mick F
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Re: Braking. Front or Rear? Or Both?

Postby Mick F » 1 Mar 2010, 8:51am

Not quite correct in your assumption about my wheels. I still have the rims BTW!

My wheels did not buckle, they remained true - both of them - just non-circular. If the inherent weakness in a dished rear wheel was the problem, the rear wheel would have buckled as well of going heart-shaped. I can assure you, the rear remained just about true.

My point about the weight distribution was that basically, my front wheel hit a vertical "wall" of about 6" and it tried to roll over it, denting itself on the way. The rear wheel OTOH dug in and destroyed itself, even cracking. This shows that by far the most of my weight was over the rear.

Compare this to a different bike. If the weight distribution was more equal - or less un-equal - there would have been more weight on the front, and the front wheel could therefore have dug in, and I may have done a header. I didn't do a header. My rear wheel grabbed the rear mudguard and jammed itself under the rear brake bridge and locked up. I slid to the left and glided down the road on my left hand side, still holding the 'bars and remaining clipped in. Horrific, but not as horrific had I done a header at 30mph plus. The front wheel was still free to rotate.

This shows that my bike with me on it doesn't have a propensity to headers.

None of this answers the basic question about best practice for braking hard. Sheldon brown states that the rear brake is a waste of time being applied in an emergency. I don't agree, because in the real world, some braking retardation takes place before the rear starts to skid.
Mick F. Cornwall

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Re: Braking. Front or Rear? Or Both?

Postby mw3230 » 1 Mar 2010, 9:12am

Just a thought, is there any merit in adjusting the rear brake so that it will retard but not stop the rear wheel. In that way presumably the maximum braking can be achieved but without the wheel locking up - a sort of primitive anti-lock braking ? Perhaps the fine tuning of adjustments would be beyond the engineering tolerances of the equipment (or rider)
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