revtothelimit - motorbike forum

Here's an explanation...

Firstly, MOMENTUM (which is the product of mass x velocity) always wants to go in a straight line. If you were able to throw a cricket ball in space, it would travel in a straight line from the moment you released it. It needs a FORCE to make it change speed or direction. On earth, the ball is subject to the force of gravity which pulls it back to the ground, plus the force generated by wind resistance which slows it down. The momentum of the bike is no different - it wants to go straight on, and the rider has to input a force to make the bike change direction.

Secondly, at anything over about 10 mph, road bikes are designed to be STRAIGHT LINE STABLE - remove your hands from the bars, and they'll carry on in a straight line with no input from the rider at all, even if the front wheel hits a bump - the bars will wobble then self-correct*. If you're leaning over when you let go of the bars, the bike will snap UPRIGHT with NO INPUT from the rider. You can see that from some rather odd track crashes - when the rider parts company with the bike, but the bike bounces back upright again... it'll carry on minus rider until the speed drops down to walking pace when it finally falls over. ONLY AT WALKING PACE does the rider need to actively steer in the mild weaves which Horse is talking about. It's easy to try. Get up to 20 mph (you'll need a big empty car park or deserted straight road with a mild camber) ideally in 3rd gear if the bike will do it so you don't get too much engine braking, then simply lift your hands just off the bars. The bike will roll straight until the speed drops down to about 10 mph, then the bike will start to wander, and finally (it's about 6-7 mph on my bikes), it'll threaten to fall over at which point you have to actively steer it. (* Which should tell you most of the instability in a bike travelling at any speed is induced by the rider.)

So the bike's stability in a straight line is a combination of its design and momentum.

Thirdly, momentum acts on the combined Centre of Mass (cCoM) of bike and rider. Very roughly speaking, it's usually about where the back of the tank meets the front of the seat - that means it is around 750 - 800 mm ABOVE the surface of the road.

But - fourthly - the force we apply to get a bike to CHANGE DIRECTION necessarily happens where the tyre touches the road SURFACE.

And this is where it gets interesting.

Moving in a straight line, the cCoM is directly above the line connecting the points where the tyres touch the surface - the bike's mass is balanced directly over the points of contact with the ground.

Imagine what would happen if you stood by a perfectly-balanced bike and pulled a rug out from under the front wheel. If you were standing to the right of the machine, the front wheel would be pulled to the right - and the bike would topple left.

When we steer the front wheel to point to the RIGHT, the force acts on the wheel to pull it to the right - and through it, the forks and the rest of the bike - works at ground level. The mass of the bike meanwhile - acting through the cCoM at the height of the seat - tries to go straight on.

You should be able to see what happens next. With the front wheel steering to the right, the cCoM is no longer directly above the line connecting the wheels, and the bike leans to the left.

Effectively, when we counter-steer, we're pulling the front wheel out from under the cCoM to the right, the moving bike actually topples to the LEFT.

You can try this without riding the bike. Try walking along by placing one foot directly in front of the other - this means you're balancing just like the bike and you should be able to stay upright with no big problem because your CoM is directly above your feet. Now take one more pace but place your left foot to the RIGHT of your other foot - you'll find you lose your balance and your body topples LEFT! It's analogous to the front wheel steering out from under the bike, with the bike toppling the other way.

That's enough to explain the INITIATION of counter-steering - there's actually a bit more to know about what's happening mid-corner and why the bike wants to straighten up again, but the info above is enough to get the bike to ROLL off the vertical and to generate a LEAN angle to get you round a bend.

Although people love to do demos with spinning bicycle wheels to 'prove' that the front wheel is a gyroscope (it is) and because you're forcing the gyroscope to lean it steers the bike (it doesn't - gyroscopic precession is insignificant and IIRC Tony Foale calculated it was around 5% of the forces operating on the front wheel), it's almost entirely down to straight-line momentum, and the steered wheel causing the bike to topple sideways.

Frankly speaking... that rear brake advice is awful. "Use the rear brake in addition to engine braking and you may never need the front until you decide to go quickly on the open road" is simply WRONG and on so many levels.

On basic training, we spend a lot of time teaching new riders NOT to be afraid of the front brake and to gain confidence in using it to stop the bike when they need to stop, and here are two 'advanced' trainers undoing all that work.

I've just dug out my sopy of Motorcycle Roadcraft and it says: "Reliance on the rear brake alone has serious disadvantages" and goes on to describe them. So how this serving police officer managed to achieve his Class 1 whist ignoring this is an interesting question.

That page alone would make me steer well clear of the book (using counter-steering obviously).

iansoady wrote: Tue Jan 26, 2021 10:30 am I've just dug out my sopy of Motorcycle Roadcraft and it says: "Reliance on the rear brake alone has serious disadvantages" and goes on to describe them. So how this serving police officer managed to achieve his Class 1 whist ignoring this is an interesting question.

The 'rear only in bends' was in the old blue, slim volume, Roadcraft, which was replaced by the 3x the size (it's grown even more since) version in 1997.

So it's possible that he did his police training then.

I saw a well-known trainer (ex police instructor) give a talk to an IAM group about his (then) new video. In the Q&A someone asked what he thought of the new version? "Not read it".

Blimey, didn't know he had his own manual:

I read that rear only braking thing and was seriously confused as its completely wrong. Reading further through the thread and the reference to the old slim pre 1997 version of roadcraft is the one i actually did my initial driving course from in 1994. I will be honest i dont remember it being taught then as i was in car learning mode for my courses that i had to pass at the time but seeing it being promoted now is worrying in an "advanced" manual.

Fortunately my students at the level i instruct will never buy that book and hopefully listen to the correct instruction

Bigyin wrote: Wed Feb 03, 2021 11:21 pm I read that rear only braking thing and was seriously confused as its completely wrong. Reading further through the thread and the reference to the old slim pre 1997 version of roadcraft is the one i actually did my initial driving course from in 1994.

This is the original text. I think 1965 was the first specific motorcycle edition. The information was current through to 1996 (?) when the 'new' format book was published.

That said, I somehow doubt whether any police instructors took it at seriously.

See the end of the second paragraph. Essentially, avoid the front brake, use rear and gearbox.

I've always wondered why the rear tyre can be expected to pull the bike up when the front tyre can't. Granted rear wheel slides are (sometimes) more controllable. The only plus I can see is that if you do lock the rear mid-lean, at least the rear swings out, which at least points the bike in the direction you want to go if you do manage to release it again. But aside from racers 'backing it in', is it likely that an average road rider could manage that with any degree of finesse?

And the implicit suggestion that slowing via rear contact patch will increase when using the brakes and gearbox.

Horse wrote: Thu Feb 04, 2021 2:48 pm And the implicit suggestion that slowing via rear contact patch will increase when using the brakes and gearbox.

A theory to be tested on:

a) a big single
b) a two-stroke twin

Actually, poor wording by me. I meant that the grip available would miraculously increase.

Horse wrote: Thu Feb 04, 2021 3:10 pm Actually, poor wording by me. I meant that the grip available would miraculously increase.

That's what I understood you to mean

If you're riding a two stroke virtually all the stopping power comes from the rear brake.

If you're on a big single and shut the throttle, there's so much engine braking you'll find there's relatively little the rear brake can do before it locks the wheel.

Meanwhile, there's that front tyre... not doing very much OK, in extreme conditions like ice and snow there's not a lot of grip but in the dry and even on a decently grippy wet surface like Shellgrip, there's plenty for the front tyre to do some decent work and you'll pull up much faster - or tighten the line if you maintain your lean and decelerate at the same time - using both brakes together.

Two years later, and wanting to find the 'books' thread, this popped up too.

The Spin Doctor wrote: Tue Jan 26, 2021 8:54 am ONLY AT WALKING PACE does the rider need to actively steer in the mild weaves which Horse is talking about.

Nope, he wasn't. I wasn't talking about the rider taking action, only about the bike doing it. As per other description (below).

The Spin Doctor wrote: Tue Jan 26, 2021 8:54 am
At anything over about 10 mph, road bikes are designed to be STRAIGHT LINE STABLE - remove your hands from the bars, and they'll carry on in a straight line with no input from the rider at all, even if the front wheel hits a bump - the bars will wobble then self-correct*. If you're leaning over when you let go of the bars, the bike will snap UPRIGHT with NO INPUT from the rider. You can see that from some rather odd track crashes - when the rider parts company with the bike, but the bike bounces back upright again... it'll carry on minus rider until the speed drops down to walking pace when it finally falls over.

Re slow riding, one of the exercises we did was getting riders to gradually reduce speed (at around 2-3mph-ish). Then when the bars start to 'flop' and the bike feels as if it wants to have a lie down (typically the point at which rider wants to put a foot down, quickly), apply a bit of power, just enough to pull the bike straight and upright.

Possibly a tad late as corrections go ...

I'd been reading the thread, thinking it was this year's...
One thing that struck me, reading the 'rear brake only' advice from the old Roadcraft, is that since 1965 we've had ABS and far better tyres. So if it was ever correct it's probably not any more.

1965 for ABS? Wasn't the K100 BMW pretty much the first? That would have been early 1980s. Even then, it was really only intended for upright and straight. 'Cornering' ABS was much later.

Tyres. Hmmm Better compounds from the later '70s. Tubeless on the CX500 and radials on the K1?


Whatever the technology, there will always have been situations where braking in corners was required - just more carefully?

Scootabout wrote: Tue Feb 28, 2023 10:47 pm I'd been reading the thread, thinking it was this year's...
One thing that struck me, reading the 'rear brake only' advice from the old Roadcraft, is that since 1965 we've had ABS and far better tyres. So if it was ever correct it's probably not any more.

On my dad's old bike only the rear brake was effective, if the front had worked the forks would probably have folded

Horse wrote: Wed Mar 01, 2023 4:12 am 1965 for ABS? Wasn't the K100 BMW pretty much the first? That would have been early 1980s.

I've just realised the ambiguity of my wording. What I meant was, since that was written in 1965, ABS has been invented and tyres have dramatically improved.