This advice is also enshrined in Motorcycle Roadcraft and the IAM Group
Handbook. So it's a pity that, nowadays, it is wrong.
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 tyres? They were no wider than the widest mountain bike tyres
of
the 1990s, they were poorly designed even compared to the car tyres
of
the day (some cars already had tubeless tyres, but all motorcycle tyres
were high aspect ratio crossplies). 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 centre 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 centre
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.
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 tyre 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.
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 tyres 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 centre 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.
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. Footpegs 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 a Kawasaki 750, but just about any 1995 or
'96 Superbike has a similar setup).
[Now look at an R6 Yamaha in '99!]
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).
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.]
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
tyres 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
dilligence 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!