contact patches at full lean.................
 

someone will know this i'm sure...................

does a 190/55 on a 6 inch rim give more contact patch at full lean than a 180/55 on a 5.5 inch.

i think it does but not sure???

I reckon its the same!!!

If could actually notice the difference, you'd already know! :lol:

My two-pennorth..
Because of the greater radius of the 190/55, I reckon you'd have more area in contact with the track/road - but less 'ground pressure' per sq mm because the bike's weight is more widely spread. (so on a wet/slippery surface you'd actually have less grip).

I could well be wrong though! One for Shazzam I think..

yes i await an essay at some stage, although i'm kind of with your thinking.

My 996SPS came with a 5.5"/190 rear setup which was changed to a 180 for all the "advertised" advantages. I then changed to mags and had a 6" rear but maintained a 180 tyre.

I've now got a 190 (D208RR) for Catalunya in October, having been convinced by Dunlops Sales Manager that the benefits are noticeable.

Having just bought a bike with what looks like a 6in mag rear, fitted with 180 (BT0012ss (?))...why does the tyre in general look wider and flatter than my 748 fitted with standard rear wheel and a 180 (pirelli diablo)?

Does the slightly wider wheel flatten the profile of the 180 tyre, or is the difference I can visually see due to the diference in tyre profiles from make to make?

Does that make any sense at all?!?!?:(

pretty much answered your own question. it should have a 190 on if its a 6inch rim, the profile (as you correctly noticed) will be flatter as the tyre is being widened out. pop a 190/55 on and it should turn in better.

Well, thats what I thought...but then why do people stick a 180 tyre on a 6 inch rim to quicken the steering, if, as you suggest, it flattens out due to the wider rim???

standard rims on a ducati are 5.5 inch

You should use the rim width specified by the tyre manufacturer to get the proper contact patch.

Different designs have different suitable rim sizes. If you use a rim width outside the recommended size range you loose a lot of the engineering that goes into a tyre. Todays tyre designer develops a new design using finite element analysis to balance tyre carcass construction, profile, sidewall stiffness, stress, temperature, wear, spring rate, tread compound and pattern - as well as contact patch. At different lean angles and uneven road surfaces, all of these factors affect camber thrust (grip.) Size of the contact patch is important but the stability of the patch is equally important.

Profile design is just one of the ways to improve the tyre’s performance. It allows control over the shape of the contact patch and the degree to which it changes as the bike leans over. A longitudinal-oriented footprint gives acceleration and braking grip, a lateral footprint gives sideways grip. You need a bit of both, and a good profile will give it to you.

Of course you’re at liberty to mess with this equation, or take other rider’s suggestions, but with so many other variables such as suspension settings and even just tyre pressure, why not take the tyre manufacturer’s recommendation on rim width?

Shazzam

There seems to be 2 thoughts on how a tyre grips.

The CoF between tyre and road

or -

that the tyre and road interlock with each other.

(or is it a combination of both)

What's your take on this???
:)

Whatever happened to the good ol' Dunlop TT100? It was triangular in profile if I remember right. Had them on my 180 Jota in the early 18th century. The further you leaned the bike, the bigger the contact patch was - at least that was the idea. Seemed to work, 'cause I ground plenty of ally from the generator cover testing the theory. (Ahh! Those were the days..)

Shazaam! you are the font of all knowledge! One day I too hope to be able to write with such a well-informed opinion.

As an aside to the (well-documented) technological constraints, it's well known that different manfacturers use very different profiles, providing different shape contact patches dependent on angle of dangle. I loved Dragons, and now love Diablos, but never realised how different they were to, say, Dunlops until I saw them alongside each other on a rack. Seeing that made me deliberately go for a higher profile front to help the bike drop in better (it can sometimes be like wrestling Pablo Escobar's rottweiller...).

I also thoroughly agree that there's a million other variables involved at any given moment, from your suspension to whether you had Guiness or John Smiths the night before ('cos you've got have one or t'other:)).

In order to sort things out on the suspension front I'd recommend we persuade (with cash) a suspension guru to attend the next TA. We could split the cost between a bunch of us and get some meaningful improvements to our respective bike setups.

Cheers,

Ali

Steve,
The laws of physics regarding friction says that the frictional force only depends on the coefficient of friction between the two surfaces - and the force pushing them together. Surprisingly, it’s independent of contact area. So different hard rubber blocks (that weigh the same) on a steel plate will begin to slide at the same point, no matter how big the contact patch.

So, if that’s true then why do dragsters use such big tyres?

Well, as it turns out, a soft rubber tyre on a road surface behaves differently than say a hard rubber block on a steel plate. In fact, a dragsters acceleration is greater than what you would predict if the coefficient of friction is 1.0 between tyre and road - that is, no slipping whatsoever.

How is this possible?

The explanation is that the soft rubber molds itself to the small peaks and valleys of the road, interlocking with it. A dragster’s tyres will actually lock into the road, propel the dragster forward and JUMP to a new position, over and over, allowing it to appear to violate physics. Cool eh?

If you think about it, when you lay down a strip of rubber, what you’re really doing is shearing-off a layer of rubber. So the shear strength of the compound controls the level of forces generated, not the frictional coefficient.

[Edited on 9-29-2004 by Shazaam!]

That sounds pretty good to me.
But what about the very high corner speeds GP125 bikes can achieve with very small contact areas? Are the forces in cornering lower than accellaration forces to the point where a small area is enough to cope?

Thanks
Steve

WeeVeeTwin, I had a TT100 on the rear of my BSA Thunderbolt, good tyre, mind you I put a CR83(I think) triangular race tyre on the front-now that was what you might call a 'little nervous'-the front would just slip out from under you until the sidewall hit the deck and then it just gripped forever. Things we did in out youth eh.........:lol:

John

Tyre grip works in a similar way to brake pads/discs.
With brakes Kinetic energy has to be lost to slow the bike down, this is done by converting it to heat (caused by friction) The more heat you create (friction through pressure) the quicker you lose kinetic energy thus slowing the bike down.

Now looking at tyres the same rule applies, its not about the amount of contact area but how much pressure you can apply to the area, this will be affected by how the bike is set up - suspension, tyres, chassis, fork rake, rider experience etc = grip. GP bikes grip well because they are set up well.

Just look at the 125's wizzing round at 130 mph and gripping, stick to stock sizes and spend the extra on a well set up bike