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Taper Braking
(Why 75%/25% is incorrect)

By: Hoddy Hodson



Mr. Hoddy Hodson sent me this very informative e-mail message today (4/1/96) and I have included it here for all of you to read because it more than validates my earlier thoughts about how effective front brakes have become, but also because it speaks with authority about how this has come about. (JRD)

Many motorcycle instructors, from Part 1 up to Police Advanced, still quote the following old chestnut: you brake 75% Front and 25% Rear (on a dry road - 50%:50% in the wet).

This advice is also enshrined in Motorcycle Roadcraft and the IAM Group Handbook. So it's a pity that, nowadays, it is wrong.

Progress changes things.

The 75%:25% rule made its first appearance a LONG time ago. The early diagrams explaining it show drum braked Triumph Speed Twins, so it's not unfair to assume that the same 75%:25% rule has been around at least 25 or 30 years.

The old Speed Twins and their ilk had little in common with today's motorcycles. They had a twin leading shoe front brake of about 7" diameter [I can't find anyone who's old enough to be certain] operated by a bowden cable from a handbrake lever. The rear brake was single leading shoe, about 6" diameter, but operated by a sturdy 10" long footbrake lever, by a leg honed to muscular perfection by kick-starting the damn bike in the first place.

And the tires? They were no wider than the widest mountain bike tires of the 1990s, they were poorly designed even compared to the car tires of the day (some cars already had tubeless tires, but all motorcycle tires were high aspect ratio cross-plies). The usual tread pattern was ribbed front and block rear. And the all important contact patch was long and thin - not least because the wheels were 20" or more in diameter.

But above everything else, it's the design of motorcycle frames that has changed. The old Speed Twin and its like were TALL. The vertical engine, surmounted by a spine tube frame (with enough gap to allow daily tappet adjustment) meant a high riding position. The center of gravity of a bike (with rider aboard) in the '60s was probably a foot or more higher than it is on most 'bikes today.

And there were two sorts of front forks. Rock hard (race 'bikes and those carrying heavy Rickman fairings) and spongy soft - prone to dive to the fork bottoms under the lightest braking. The net result of either type was that, under braking, the front went almost rigid - like a pushbike's forks.

If you've got locked forks, a narrow (low grip) front tyre and a center of gravity that small planes have to detour around, it s not surprising that you're cautious of using your front brake. If that brake is a grabby drum brake (they "self-servo"ed so the braking effort was not proportional to how hard your hand squeezed the lever) you do as much as you can with the controllable rear brake. And, anyway, a locked rear was controllable even on a Speed Twin.

New Tricks Motorcycle design has moved on since the old dogs of the 1960s. In fact, it had already moved on far enough to make the 75%:25% rule questionable by the '80s.

The BIG sign that 75%:25% is wrong is that most 'bikes these days (since the RD350, at least) can do "stoppies" - and not crash immediately after. In a stoppie you push the front brake to its limits, you brake so hard that the rear leaves the ground. This is neither big nor clever; but it does prove to the most hide bound among us that that bike, at that instant, was stopping using 100% Front brake.

I'd now like you to think about proddy racing. Production racing because (apart from Owen's missing alternator - sorry, Mr. Scrutineer) the 'bikes used should be the same as those you meet on the road. Now, I understand that, in a race, most competitors are trying to ride as fast as they can - they are not there to put on a show of stunt riding to impress the crowd.

Yet, horror of horrors; they do NOT brake 75%:25%. Stand at the braking point at the end of the straight (do not stand on the bend at the end of the straight: it's where Reg Ford usually smashes into the crowd). You will see many of the rider lift their rear tire clear of the deck under maximum braking. They are doing stoppies - they are braking 100% Front and 0% Rear.

And they are not doing this deliberately, to show off (apart from Jamie Whitham at the end of a race!). They are doing it because, nowadays, it's the natural, instinctive way to brake as hard as the 'bike can possibly brake.

What's new, *****cat? Modern motorcycles (as above, this includes most road 'bikes designed since the RD350) are radically different from that old Speed Twin. The few that aren't, Retros like the Zephyr and trail 'bikes, probably still brake 75%:25%.

[Apparently, one of the first things Geraint Jones teaches on his Moto-X school is how to brake . You do this by learning to stop a motocrosser from 50mph, on mud, using only the front brake. So, even on the dirt, there's scope for more front brake use - if you have the cojones!]

Modern bikes are lower - by about a foot (compare a GPZ500 with a 750 Triumph - the GPZ is more powerful, too). Modern bikes tend to be shorter, by around 5 inches. We have smaller wheels these days - fronts are 16" to 19" they used to be 18" to 21". And wheel widths, and hence the contact patches, are at least twice as wide as they used to be. The modern tires are stickier - even in the wet. And they are radials (or bias belted) so they deform to grip the road far better. And the low sidewalls help the 'bike's center of gravity stay low.

And front suspension, even if you don't have upside down fork legs, is ten times better at absorbing ripples that might upset a tyre under braking.

Brakes

You'll notice I haven't mentioned the brakes. I think the grabbiness of 1960's brakes, and the need to stand on that big footbrake lever, is one of the root causes of the 75%:25% rule. That was how people found they had to brake, so they assumed it was the best way to brake.

Since then, Triumph have died; been reborn; died again and been reborn as a far better bike. Rules for braking written to suit the 1990s SpeedTriple would differ a lot from those written for the Speed Twin of the '60s.

But the masters of motorcycle design are the Japanese. Now, believe it or not, they tend to design things to do their job. Very occasionally they screw up, but most things they get right. Mudguards keep the mud off. Foot pegs don't bend under your weight. You can reach the levers and the switches at the same time. [The old Triumphs, sad to relate, didn't manage any of these things].

So we'd expect modern Japanese bikes to have brakes suited to their function - stopping the bike as quickly as possible. So, how do they set up their brakes?


Front Two 320mm disks, each gripped by 6-piston callipers.
Rear One 220mm disk, gripped by a 2-piston calliper.

(These specs are from the new Kawasaki 750, but just about any 1995 or '96 Superbike has a similar setup).
At a conservative estimate, the front brakes are 5 times as powerful as the rear (remember the diameter of the disk has a big effect). And I'd bet that the foot lever is now as short as the handbrake lever.

So why have the Japanese fitted brakes so out of line with the 75%:25% rule? Are they foolish? Is it some sort of "look at the size of my brakes, darling" fashion accessory? Or is the 75%:25% rule just plain wrong these days?

Answer: the 75%:25% rule *is* just plain wrong these days (for most modern 'bikes on most dry roads).

What's the truth? The truth is, there is NO truth. Any fixed apportionment of braking effort, front to rear is wrong. In cars, they teach taper braking - you bring the pressure up gently, to avoid a skid until the weight transfers forwards; at which point you can brake hard; and you let it off gently as you roll to a halt, to avoid a jerk when you stop.

'Bikers, too, need to learn taper braking. But as we have separate front and rear brakes, we also need to learn to taper the force from rear to front and back to rear again as we slow.

[Owners of Moto Guzzi and Honda linked brake systems can leave now - but remember, as you depart, that racing Guzzis always removed the linked brakes - they aren't quite as good as separate systems right at the limit.]

An ideal stop goes something like this:

  • You apply both brakes gradually and with almost equal force for the first phase of your braking.


  • The weight will transfer forwards as the front suspension compresses, and your arms bend.


  • There's now more weight on the front (up to 100% if you're braking at 1g - and modern road bikes can brake at up to 1.2g).


  • You now let off most - or all - of the rear brake and increase pressure on the front, which now has most or all of the grip. This middle phase of braking can be 100%:0% - if it is less than 85% Front, you probably aren't braking near your bike's limits.


  • The bike slows and the forces you are exerting through the brakes and tires diminish (the energy in the bike is proportional to the square of your speed).


  • The front begins to rise back up on its suspension.

[If it's an emergency, you now breath a sigh of relief and a small prayer of thanks].

  • You taper off on the front brake - to prevent a slow speed lock up - and increase the rear brake pressure once more.


  • Even stopping from 100mph, the last 5mph is slow riding, and you should only use the rear brake for slow riding. So you do the final phase of stopping 0% Front and 100% rear.

If you MUST quote a fixed apportionment of effort - I'd say it is 85%:15% - which is in line with the way Japanese 'bike designers set up the brakes.

But the truth is, situations will vary which is why we should forget 75%:25%. Motorcyclists need to learn to taper brake; to balance front and rear brakes in a sensitive, reactive fashion - not to follow an outdated mathematical tenet.

And, to slip in two quick plugs - you can learn how at the Nurburgring Perfektion Training courses or on London Advanced Motorcyclist's Machine Control Days.
_________________________________

Hoddy Hodson 29/3/96

This article expresses the views of the author. All care and due diligence has been take in its composition and I fully believe it to be correct, but then I think water is wet, so who am I to judge. Feel free to copy and circulate this article, but only with this disclaimer!

http://www.msgroup.org

(James R. Davis is a recognized expert witness in the fields of Motorcycle Safety/Dynamics.)

     
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