Invaluable info: by a REAL expert rider

[Oscar]

With the engine running at a constant speed (and thus the tyre turning at a constant speed) if you reduce the effective diameter of the tyre you will go fewer yards per minute. Same no of revolutions, but less distance per revolution.

So in order to maintain a constant bike speed you will have to increase the revolutions at the tyre which will usually mean accelerating.

But also the leaning tyre forms a conic section so it will roll left or right (which is actually what makes you go round the corner). That introduces a horizontal vector in addition to the original two (down and forward). That means you have to supply more energy to keep the same speed on the forward vector. (which I think is a fancy way of saying that it creates drag, as you said more simply. I just like using fancy words like vector.)

So at a constant throttle setting (and everything else being equal)which has a higher speed, a bike with a 17' rear or one with an 18'?

[Racey Rider]

If I snap the throttle open just after I turned in,
Id be on my ass on the road 'again'.

you forgot the word, again.

[Ixion]

So at a constant throttle setting (and everything else being equal)which has a higher speed, a bike with a 17' rear or one with an 18'?

Constant throttle is a bit uncertain. The 18" will want to go faster, but the engine at that throttle setting may not be developing enough power for the higher speed (ie fitting the 18" would make the revs drop)

At a constant RPM the 18" will go faster. So to make the 17" drive the bike at the same speed as the 18" you would have to turn it faster . It doesn't go as far down the road per rev.

Which is the point. In a turn the effective radius is less. So for the same RPM you will not go as many yards. You will be slower. So to maintain the same speed you have to increase the revs the tyre does.

Extrem and absurd example. Lay the bike on its side, with the tyre touching the ground, motor running , in gear. Bet you it doesn't move forward at all, it will just slowly go round and round . Start picking it up, as it comes upright it will start to gain forward momentum.

[Oscar]

Constant throttle is a bit uncertain. The 18" will want to go faster, but the engine at that throttle setting may not be developing enough power for the higher speed (ie fitting the 18" would make the revs drop)

At a constant RPM the 18" will go faster. So to make the 17" drive the bike at the same speed as the 18" you would have to turn it faster . It doesn't go as far down the road per rev.

Which is the point. In a turn the effective radius is less. So for the same RPM you will not go as many yards. You will be slower. So to maintain the same speed you have to increase the revs the tyre does.

Extrem and absurd example. Lay the bike on its side, with the tyre touching the ground, motor running , in gear. Bet you it doesn't move forward at all, it will just slowly go round and round . Start picking it up, as it comes upright it will start to gain forward momentum.

Very interesting, and why didn't I pay more attention in 7th Form physics (or in fact attend more classes).

However in practice the best way to increase my corner speed is to get to where I would normally brake and say "God save the queen", and THEN brake. The other thing I mentioned earlier was not to change down too many gears...

[Wolf]

I've always (tried) to brake on the approach to a corner, then hold a constant throttle on the entry, and accelerate from the apex out.

He recommends accelerating from the entry (ie throtle on all through the corner). I'm going to have to rethink my technique.

Any thoughts on this . What's safest, not what's fastest on the track. We're talking road riding here not racing.

Dad always told me to accelerate from the entry - "power through the corner". "Lose speed on the approach, gain it as you power around the corner."

One day I was giving a GT50 (no typo) absolute death trying to keep up with my uncle in his V8 on a twisty road. I had it at full throttle when I entered the corner, meaning I had nothing in reserve to give it to "power through" - Bloody terrifying, nearly ended in grief, one of the worst cornerings of my "riding career" - never made that stupid mistake again.

Had been through that same corner before a lot faster on a bigger bike with plenty of throttle in reserve - came out a damn-sight faster than I went in.

[Wolf]

A clarification:

When I corner, I roll the power on - gently at first and then more firmly. This counters the slowing caused by the change in diameter as the bike leans, feeds power to counter inertia and force a vector change (go round corner, not straight into paddock). I generally aim for as smooth a corner as possible, even if I'm riding fast - I don't yank the throttle open.

Dad's grasp of physics was not as advanced as Mr Ixion's - he waffled about countering "centrifugal force" - which is a layman's explanation of inertia and the conservation of momentum. His take was: decelerate, downshift if necessary, then feed power gently from the beginning of the turn to stop yourself from going wide and increase the power steadily so you accelerate out of the corner.

He also taught me to choose proper lines through a corner, closer to racing lines (but factoring in that you have on-coming traffic which you don't have on a race track so on right hand turns you daren't get too close to the centreline).

Dad and I had our disagreements and misunderstandings but when it came to learning roadcraft from him he was excellent (albeit a bloody hard master, critcal and hard to please) - he had a passion for vehicles (and all classes of licence except "trolley omnibus" (tram) to prove it) and I suppose that any expression of interest vehicles (especially from his "weird" bookish son) was greeted with enthusiasm.

He taught me to ride on our 1970's RV90 in the paddocks when I was a kid and later bought me my first bike (RX125) when I was 18 and swore like a trooper when I binned it going wide on a corner and hitting gravel. Used to tell great tales of riding and impart the lessons to be learned from them - observations skills, accident avoidance etc.

I'm glad I had him as a resource to learn from - I'm sure that if I had a less capable rider as my teacher I'd have found motorcycling a lot more painful and expensive than I did (which probably would have priced it out of my reach). I'm a firm believer that there should be proper, mandatory, instruction as part of the licensing requirements for all vehicles. I may have learned a few "bad habits" over the years, some from dad (experienced and skilled as he was, he wasn't perfect) some from failing to listen properly or coming up with my own solutions, but I could have learned a lot more bad habits than I did.

Thanks for the link, Ixion, looks like a great resource.

[Tricia1000]

Positive throttle throughout the bend will put the weight of the machine on the back tyre, with out overloading it. Which is when the tyre grip trade off comes in. Then as you reach the apex of the bend, and the limit point is racing away from you, then you can lay on the acceleration, but sensitively feeling it through the back tyre.

[Artifice]

the rear tyre has a larger contact patch than the front. weight transferance to the back tyre,(by apllying a little throttle) when going through a corner helps ensure the front doesn't slide out on you.