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Riccal

Racing Tyre Sizes - Whys And Wherefores

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So, heres one that has been occupying my mind now and then for quite a while. I have tried googling the question but got nowhere.

Gp 125 racing motorcyles - they have a narrow rear tyre.

Now, the old 500 2 strokes and the Moto2 class as well as MotoGp have wide tyres. The simple answer is that the 125s are only tiddly little things. But they are lapping in times better than a good rider, or even a pro racer, could on a street race rep like a fireblade or GSXR and they are running 180/190s.

To my mind that seems to indicate that bigger tyres would improve things for the 125s but obviously that aint so cos if it was they would do it.

So, why aint it so. What is the science behind this decision. I can usually discourse happily about tyre strategies for the big boys and how treads in racing tyres aint for more grip in the rain but to allow more flex to build heat quicker in a cooler environment etc. etc. but this one has me stumped.

Cheers

Riccal

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Wider tyres have more rolling resistance and greate more drag. It's a trade off. Or, this is what the rules dictate.

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I assumed it would come down to a trade off between drag and any cornering advantage implied but these things are bldy quick and, whilst not producing the rear wheel

BHP of a 'blade or gixer which does run big tyres is probably quicker. Perhaps it wouldnt be if it had wide tyres but I would have thought the dynamics responsible for the wide tyre good on the bigger bike scenario would hold good on a smaller bike as well if it is quick.

So, whilst it obviously is a trade off does anybody know exactly what is being traded off to give what result?

Riccal

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A Fireblade weighs over 400lb. A 125 GP bike weighs about as much as a pair of socks. edit or 80 kilos

I expect that is what its going to come down to. A fuelled blade with rider about 280 kilos, a fuelled gp125 with rider about 160 kilos.

But im still looking for the science as to how and why the weights make a difference. A fully fuelled gp250 back in the day with rider would have been about 180 kilos. Not much difference really but they ran 180s on the rear so, not much difference in weight, fair bit more umph but only about 4/5 seconds somewhere like Jerez in lap time difference.

I know it aint a life changing thing but if anybody has the ansa would be grateful as it has been niggling. Thinking I know a fair bit about the sport and the kit that goes along with it but when my little girl (who now and again shows Daddy solidarity by watching stuff like this and rugby with me) asked me why the difference between tyres I realised I couldnt answer satisfactorily.

Cheers

Riccal

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I'm no expert on bikes but I'd expect the trade-off comes from the speed at which the rider can flick the bike over from one side to the other to set up before his turn. The difference in inertia (mass) and leverage (changing the centre of gravity over a thinner tyre) probably explains most of it. So a twisty circuit the advantage would be more noticeable.

Is this on the right track?

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Its partially to do with grip and the power a bike can put down on the road, a standard IL4 bike if you ride it quickly the front end will come up and some of the power is put into lifting the bike rather than moving it forward. The ZX10R was notorious for this.

A smaller bike doesn't have so much power so needs less traction to put down the power.

OTOH there is a 3rd confusing bit to it as car tyres are shaped like this [ bike tyres ( yet racing bike tyres are shaped like < but it depends on how twisty the track is and thus people end up with a compromise between < and ( My CBR tyres are { shaped with a harder middle bit however I use touring tyres which are grippy enough to get your shoulder down on the road when going round a round about.

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might be unrelated, but might not... ever looked at the gyroscopic effects of wheels? Bigger tyre = heavier mass on the edge so much more gyroscopic action. Light bike/rider combo presumably makes is harder to overcome gyro?

If you haven't tried it, get a bicycle wheel, hold the axle and spin the wheel - when its going you can balance it on 1 finger - even more fun if you sit in a swivel chair at the time.

http://www.youtube.com/watch?v=8H98BgRzpOM

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I'm no expert on bikes but I'd expect the trade-off comes from the speed at which the rider can flick the bike over from one side to the other to set up before his turn. The difference in inertia (mass) and leverage (changing the centre of gravity over a thinner tyre) probably explains most of it. So a twisty circuit the advantage would be more noticeable.

Is this on the right track?

Its partially to do with grip and the power a bike can put down on the road, a standard IL4 bike if you ride it quickly the front end will come up and some of the power is put into lifting the bike rather than moving it forward. The ZX10R was notorious for this.

A smaller bike doesn't have so much power so needs less traction to put down the power.

OTOH there is a 3rd confusing bit to it as car tyres are shaped like this [ bike tyres ( yet racing bike tyres are shaped like < but it depends on how twisty the track is and thus people end up with a compromise between < and ( My CBR tyres are { shaped with a harder middle bit however I use touring tyres which are grippy enough to get your shoulder down on the road when going round a round about.

Both those prob have an element of truth to them. It is obviously something to do with the equipment matching the power output of the bike which is obviously lower than the other classes but it confused me as the 125s not that much slower/less powerful than the 250s.

Still, bl..dy good fun tho innit however it is achieved. People dont believe the angles that can be achieved nowadays until you show em a photo. Knee down used to be ultimate but these guys are scraping their arms on the ground!!

Keep it up.

Riccal

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might be unrelated, but might not... ever looked at the gyroscopic effects of wheels? Bigger tyre = heavier mass on the edge so much more gyroscopic action. Light bike/rider combo presumably makes is harder to overcome gyro?

If you haven't tried it, get a bicycle wheel, hold the axle and spin the wheel - when its going you can balance it on 1 finger - even more fun if you sit in a swivel chair at the time.

http://www.youtube.com/watch?v=8H98BgRzpOM

Hmm - I quite like that. The bikes are so light (and the riders usually) that it may be something to do with making em more flickable.

Not a million miles away that.

Riccal

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A friend of mine's just done his Advanced Motorist bike course and they explained the physics of tyre traction at some point. As far as I remember from what he said, there's a lot less surface contact with a motorbike and considerably more power (to weight ratio) going through that small surface area. This is why bike tyres wear out so quickly - and more worryingly they "square off" as they wear, meaning that the rider feels a disconcerting bump as they lean into a turn.

That's all I know...apologies if it's not really relevant.

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A friend of mine's just done his Advanced Motorist bike course and they explained the physics of tyre traction at some point. As far as I remember from what he said, there's a lot less surface contact with a motorbike and considerably more power (to weight ratio) going through that small surface area. This is why bike tyres wear out so quickly - and more worryingly they "square off" as they wear, meaning that the rider feels a disconcerting bump as they lean into a turn.

That's all I know...apologies if it's not really relevant.

Yeah on the high profile racing tyres the contact patch is usually accurately described as being credit card size. Not quite as radical on tyres for the average user - even on high end sports stuff. Doing track days here in Spain would go through a set of tyres (especially the rears) in 1 weekend. The quick boys a cupla sets especially if they were fast enough for slicks. Gotta be quick on a slick cos if you aint it doesnt get hot enough to grip properly. We lost a guy here returning from the track on slicks - they never reach operating temperature in road conditions. I dont think the answer to our question is gonna be traction but maybe im wrong.

The bump a rider feels is a step as the tyre has gradually lost its clean line. Racing tyres with their higher profile dont suffer from this.

Riccal

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A friend of mine's just done his Advanced Motorist bike course and they explained the physics of tyre traction at some point. As far as I remember from what he said, there's a lot less surface contact with a motorbike and considerably more power (to weight ratio) going through that small surface area. This is why bike tyres wear out so quickly - and more worryingly they "square off" as they wear, meaning that the rider feels a disconcerting bump as they lean into a turn.

That's all I know...apologies if it's not really relevant.

its why you buy twin compound tyres BT45s BT21s and the new michilin Jellies are like this down the centre strip they are much harder than the edges, the edges are like pencil erasers it works quite well 10-20000 miles is possible.

On BT45s if you get a stanley knife you can cut an extra 3mm of tread into the tyre when you make it bald!

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Yeah on the high profile racing tyres the contact patch is usually accurately described as being credit card size. Not quite as radical on tyres for the average user - even on high end sports stuff. Doing track days here in Spain would go through a set of tyres (especially the rears) in 1 weekend. The quick boys a cupla sets especially if they were fast enough for slicks. Gotta be quick on a slick cos if you aint it doesnt get hot enough to grip properly. We lost a guy here returning from the track on slicks - they never reach operating temperature in road conditions. I dont think the answer to our question is gonna be traction but maybe im wrong.

The bump a rider feels is a step as the tyre has gradually lost its clean line. Racing tyres with their higher profile dont suffer from this.

Riccal

Race tyres are different in more than one way, race tyres can only go through a very limited number of heating and cooling cycles. That is before they start to delaminate.

Something which chagrins many riders is how once they have cleaned off the chicken strips the edges turn blue from curing compound contained in the tyre seeping out!

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A friend of mine's just done his Advanced Motorist bike course and they explained the physics of tyre traction at some point. As far as I remember from what he said, there's a lot less surface contact with a motorbike and considerably more power (to weight ratio) going through that small surface area. This is why bike tyres wear out so quickly - and more worryingly they "square off" as they wear, meaning that the rider feels a disconcerting bump as they lean into a turn.

That's all I know...apologies if it's not really relevant.

The squaring off would indeed be an issue, but only if the rider only ever accelerated hard whilst upright (i.e. a wuss) otherwise the wear should be relatively even. Granted most miles are done whilst the bike is vertical so I can how it would come about, but it shouldn't be that noticeable

As for the OP question: as someone above pointed out, it's to do with the size of contact patch required to put the power down. No point having a massive contact patch if the engine wasn't big enough to spin the rear end...

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I think it's all a trade-off. A wider tyre will grip better, but it will also have more drag which needs more power needed to accelerate the bike. Ideally you want as small a tyre as possible to reduce the drag, but that would have bad consequences for the cornering & acceleration :)

So, A smaller lighter bike doesn't need wide tyres - but if you put them on it would be slower to accelerate, harder to lean in to a corner (heavier tyre == more gyroscopic effect, making it harder to tilt) but would grip better.

The bigger bikes only need wider tyres for the extra grip when cornering and to be able to put more power down without lighting up.

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Do you know, I was kinda hoping there was gonna be a complex techchnical unifying solution but I reckon the various posts here have got it as a mixture of things - contact patch can be smaller as power is less and bike easier to pitch in when thin tyre on light frame.

I do think the odd thing tho is not much difference weight/power wise between the 125 and 250 gp bikes but its enough to put the 2 bikes into different categories where handling characteristics and equipment is concerned.

Thanks all answers everybody - I have to sign off the internet now for a few hours so forgive me if I am abandoning a discussion.

Cheers

Riccal

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