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Why Rear Tyres Are Bigger Than Front Tyres?


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#1 Snowman

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Posted 06 January 2012 - 05:14 PM

We probably take this for granted but what is the reason?

One reason is that front and rear tyres have different paths when the motorcycle is turning. I guess the rear tyre would probably run out of rubber while the front is getting the bike through.

Is this a valid reason? Are there others?

-Tony

#2 Dylan Code

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Posted 09 January 2012 - 04:15 AM

That's a very good question and I'd like to see if anyone with an engineering background can chime in and provide technical insight to this topic.

#3 phible

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Posted 16 January 2012 - 07:28 PM

No one knows?

I was thinking aerodynamics is also a factor.

#4 Snowman

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Posted 19 January 2012 - 02:07 PM

It's interesting to note that bicycles do not have wider rear tyres. So it probably has to do with the fact that there's an engine attached to the rear wheel. Thinking about it even the F1 cars have a lot wider rear tyres as these tyres have to transfer great power to the tarmac.

There's even a wikipedia article here http://en.wikipedia....Motorcycle_tyre but it does not say anything about tyres' width.

I hope Steve Brubaker from Dunlop will notice the topic and give some insight into this.

#5 Eirik

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Posted 19 January 2012 - 02:31 PM

It's interesting to note that bicycles do not have wider rear tyres. So it probably has to do with the fact that there's an engine attached to the rear wheel. Thinking about it even the F1 cars have a lot wider rear tyres as these tyres have to transfer great power to the tarmac.

There's even a wikipedia article here http://en.wikipedia....Motorcycle_tyre but it does not say anything about tyres' width.

I hope Steve Brubaker from Dunlop will notice the topic and give some insight into this.


You will probably need powersteering to control a 180-200 mm front tyre under racing conditions. The effort required to turn the motorcycle would be massive with a big tyre. Also, the steering influence from a wide tyre when leaned over would be very hard to deal with when riding on the edge; the rider could in an extreme situation see the handlebars ripped out of his hands. Hitting the brakes mid-corner with such a wide tyre would probably be impossible. Also, it is likely that the higher weight would upset the delicate front suspension more than the rear since the fork also must deal with steering.

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#6 Snowman

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Posted 19 January 2012 - 04:09 PM

You will probably need powersteering to control a 180-200 mm front tyre under racing conditions. The effort required to turn the motorcycle would be massive with a big tyre. Also, the steering influence from a wide tyre when leaned over would be very hard to deal with when riding on the edge; the rider could in an extreme situation see the handlebars ripped out of his hands. Hitting the brakes mid-corner with such a wide tyre would probably be impossible. Also, it is likely that the higher weight would upset the delicate front suspension more than the rear since the fork also must deal with steering.


I agree but I'd rather argue why the rear tyre is not as slim as the front one. Older bikes actually do have narrower rear tyres. What do you think?

#7 Eirik

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Posted 19 January 2012 - 07:37 PM

It has to do with load and grip, something we've discussed to some length under another topic. Other than style, the ideal tyre has zero width. You could say that tyres are as narrow as possible and as wide as required. It's a compromise between grip, logevity and handling. It's actually amazing how well tyre makers together with chassis designers have been able to make modern bikes handle with such wide tyres.

Older bikes had generally less power, so they could do with narrower tyres. But when power and weight crept up radically, starting in the late 60s, so did tyre wear. A 100 hp old style motorycle with a 4-inch rear tyre would typically wear it flat in 2000 miles or less if ridden with a bit of vigor. And that's with a touring style rubber.

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#8 Hotfoot

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Posted 19 January 2012 - 11:44 PM

It has to do with load and grip, something we've discussed to some length under another topic. Other than style, the ideal tyre has zero width. You could say that tyres are as narrow as possible and as wide as required. It's a compromise between grip, logevity and handling. It's actually amazing how well tyre makers together with chassis designers have been able to make modern bikes handle with such wide tyres.

Older bikes had generally less power, so they could do with narrower tyres. But when power and weight crept up radically, starting in the late 60s, so did tyre wear. A 100 hp old style motorycle with a 4-inch rear tyre would typically wear it flat in 2000 miles or less if ridden with a bit of vigor. And that's with a touring style rubber.



Yup, I agree with all of the above - I think when you have a high horsepower bike you need that larger contact patch so you don't just rip the rubber right off the tire. My little 40 hp Moriwaki has a skinny rear tire, as does my little 200cc (4 stroke) dirt bike, neither has a lot of horsepower and they don't wear out the rear tire like a high horsepower bike would. In fact, in my last race weekend, I realized my FRONT tire wore out (on the Moriwaki) before the rear tire. That never happened on my 600, and my husband goes through 2-3 rear tires on his 1000cc before changing the front.
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#9 Sean G

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Posted 20 January 2012 - 10:30 PM

Well, on some bikes the rear is not bigger.

Several vintage (race) bikes run equal tire widths, and last year a lot of 250 Ninjas either ran 110/120 or 120/120 combinations. Kinda fun when you can rotate your tires front to back and side to side.

As others have said, lower horsepower reduces the need for large contact patch to reduce tearing. As for cornering loads, they seemed to carry the riders to the top of the podium with little trouble.

-Sean

#10 DUNLOP-RTS

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Posted 01 March 2012 - 03:55 PM

We probably take this for granted but what is the reason?

One reason is that front and rear tyres have different paths when the motorcycle is turning. I guess the rear tyre would probably run out of rubber while the front is getting the bike through.

Is this a valid reason? Are there others?

-Tony


The biggest and most compelling reason the rear is larger than the front on current sports bikes...

You get better lap times with that configuration.

Try swapping them and see if you go faster, try making the front the same size as the rear and see if you go faster. If you do, then we will start talking more. Until then, I think its best to stick with what works.

For this subject its more productive to spend some laps working on your riding, rather than a physics debate. Nothing to be gained here.
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#11 Snowman

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Posted 20 September 2012 - 08:48 PM

Thanks for the informative replies.

Steve, it sounds like you don't know the answer to this question :)

Regards,
Tony




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