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post #1 of 9 (permalink) Old 03-08-2003, 07:09 AM Thread Starter
 
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Tire size

My Gixxer came stock with a 190 rear tire. Everyone suggested get a 180 and will make the bike handle better (it did). One guy said you will loose traction when accelerating.

My trivia question is... does a bigger tire provide more traction? Remember this question is relating to street use. I'm not drag racing.

Three possible answers:

More traction (explain why)
Same traction (explain why)
Less traction (explain why)
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post #2 of 9 (permalink) Old 03-08-2003, 11:14 AM
Dad
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Traction should be about the same because the footprint actually engaging the road will be about the same. That is set by air pressure and can be nominally increased and decreased (within reason) by air pressure changes. The same holds true for cornering with the added advantage of not having to "climb up" on the extra width. You stay closer to C/L which makes the steering effort less. Tire design can have an effect on this as well, but all else being equal, these principles hold true.

You buy that?

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post #3 of 9 (permalink) Old 03-08-2003, 02:14 PM Thread Starter
 
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Hi Dad,

Lets say the tire presssure is equal and say the footprint of the bigger tire is larger.

Would the bigger tire have more traction?
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post #4 of 9 (permalink) Old 03-08-2003, 08:07 PM
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Quote:
Originally posted by Andy
Hi Dad,

Lets say the tire presssure is equal and say the footprint of the bigger tire is larger.
"Why say it if it ain't so", he replies in a totally smart ass tone.

Just kidding. I think we're heading in a similar direction with this in that, if fewer square inches were engaged with the road, and those same fewer inches were still carrying the same weight, the pounds per square inch loading would be higher, resulting in more friction per square inch, netting out to approximately the same gross friction as the larger footprint at the lower PSI loading. Is that the theory?

When I said you can play with air pressures, that is basically what I was getting at. It has the effect outlined above. Although, experience playing with air pressures suggests that the relationship between surface area, a fixed load, and the resulting friction is not a wash. Within reason, decreasing air pressure increases the footprint and traction, increasing air pressure reduces the footprint and traction. Another factor involved shows in heat in the tire and the resulting effect on the friction but I don't think you were going into that detail... were you?

What I was getting at originally was that due to the arc design of the tire and a minimal change in o'all width (approx. 1/4" per side), either tire will flex as much as the air pressure in it will allow, resulting in the same square inches of tire engaging the pavement to support the equal load. A minor allowance could be made for differences in carcass stiffness but it would be miniscul. The "real world" difference is in lifting the bike up on that wider edge at extreme lean angles but there is virtually no difference in the amount of rubber meeting the road. That's my story and I'm sticking to it.

Your turn... show me the error in my ways.

Keeping the "Hap" in "Happy Holidays"!

Regime change begins at home.

Blind patriotism is worse than no patriotism.

Last edited by Dad; 03-08-2003 at 08:15 PM.
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post #5 of 9 (permalink) Old 03-09-2003, 01:51 AM
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Quote:
Originally posted by Andy
Hi Dad,

Lets say the tire presssure is equal and say the footprint of the bigger tire is larger.

Would the bigger tire have more traction?
I think it depends more on the coefficient of friction of the road surface.
As Dad said the pressure will be increased with the 180 tire, so even with a larger patch you will end up with the same overall friction.
In other words, I think you'll have the same acceleration with the 180 except if you are on ice, in which case the 180 will help you accelerate better.

Aris
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post #6 of 9 (permalink) Old 03-09-2003, 06:58 AM Thread Starter
 
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Dad & Aris,

That was the answer I was looking for. The larger footprint by itself does not add any more traction.

So lets take it to the extreme. What happens if I put a tire that has only half the surface area touching the ground as my current tire does.

Dad has already eluded to the answer. Heat becomes a problem. The small tire would heat up so hot the rubber would go beyond the point where it gets more traction and will start to loose traction.

So a slightly smaller tire like my 180 will heat up a little faster and will actually provide more traction unless I heat it beyond the point that the rubber looses traction.
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post #7 of 9 (permalink) Old 03-11-2003, 05:15 AM Thread Starter
 
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After thinking about this problem overnight. I have come to the conclusion that even if the contact area is the same, the overall mass of the tire (180 being smaller then 190) would make the 180 heat up sooner and a little hotter so it should give better traction to a point. Certainly for street riding I'm going to over heat it.
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post #8 of 9 (permalink) Old 03-11-2003, 07:17 AM
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Quote:
Originally posted by Andy
After thinking about this problem overnight. I have come to the conclusion that even if the contact area is the same, the overall mass of the tire (180 being smaller then 190) would make the 180 heat up sooner and a little hotter so it should give better traction to a point. Certainly for street riding I'm going to over heat it.
I'm following your theory here, that the lesser mass with the same energy input will result in a higher temp. I'm not sure that actually works that way due to the poor heat conductivity of the rubber. Last year I pitted at a track day with a guy who had a pyrometer. It was a clockwise track, so there are always 360 deg. more rights than lefts and we measured tire temps across the tread. Right side was about 20 degrees hotter than left, measured about halfway from center to each side. With that difference occurring only several inches apart, I doubt the small mass difference would have much bearing. It seems that the bulk of the heat stays to a large part where it was created. I'm not sure any additional mass would be able to draw that heat away.

With a more dramatic change in size, some of these things might have some bearing, but even then, your air pressure will dictate the tire's distortion/contact patch to the extreme of a smaller tire eventually appearing flat. I think the smaller tire's problems would lie more in the action of the sidewall and it's ability to be an active part of the suspension than in the contact patch alone. These are my own educated guesses. It would be interesting to hear from an actual tire engineer.

Another interesting thing is that all race rubber is maxed at a 180 rear. They don't even make a 190. I'm guessing that has to do with an optimum package of contact patch to carcass characteristics. Stop it, you're making me think too much.

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post #9 of 9 (permalink) Old 03-11-2003, 08:06 AM
 
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Quote:
Originally posted by Dad


Another interesting thing is that all race rubber is maxed at a 180 rear. They don't even make a 190. I'm guessing that has to do with an optimum package of contact patch to carcass characteristics. Stop it, you're making me think too much.

I think 190 rear race tires are starting to come around. I believe John Reynolds raced a 190 or 195 rear in BSB on his GSXR1K.

Pirelli, Metzler and Bridgestone are selling 190/55-17 DOT Race tires, so the trend is to get wider while still keeping the characteristics of a 180/55.

BUT, this is getting off topic. The guy who posted the question said it was for street use, so he won't be anywhere near the limits of a 180 or a 190 rear.

I currently use a 180/55 rear for the street and Pilot Race 180/55 on the track. It all comes down to personal preference and riding style.
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