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Old 04-05-2014, 10:26 AM   #16
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Originally Posted by Brian McGreevy View Post
Very good questions. The only time where you would want to have smaller contact patch is if you don't have enough mass and/or downforce to properly load the tire and get it to its operating temperature. Otherwise, as long as you can get the tire to operating temp, larger contact patch will always produce more grip. The friction between the surface and the tire increases with increasing contact area, all other variables held constant (tire temp, pressure for a pneumatic tire, tire construction).

I knew the answer was something like that, I just didn't know how much it would increase/decrease performance.

With the answers that you have given, maybe it is time that we do look at wider tires. As long as the cars can generate the grip, why not?
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Old 04-05-2014, 10:47 AM   #17
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I knew the answer was something like that, I just didn't know how much it would increase/decrease performance.

With the answers that you have given, maybe it is time that we do look at wider tires. As long as the cars can generate the grip, why not?
Well, the downside is that you'd increase cornering speeds even more. They're already very high. In spec classes, the increase in grip would put more emphasis on horsepower - everyone's least favorite thing. That and the tires would be a bit more expensive due to having more material. The upsides are that you would almost certainly have a larger setup window and tires would last longer.
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Old 04-05-2014, 11:02 AM   #18
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To me, speeds are fine. If anything we need to be going slower in the stock/spec classes. If you look at the older videos, even the last of the bushes days, our 17.5 classes are way faster than those days. Not even close.

I agree with your points in the advantages and disadvantages. Looking at it that way, tires will last a couple of weeks longer if we go wider but the cars will be faster, this more prone to breaking in the crashes. Kind of a trade off, especially when you introduce the added expense of the new tires. They still need to build new molds, as well as use more material.

Maybe the technology has improved everywhere in the industry with the tires being the one thing holding us down to "reality" if you will. Maybe it is a good thing to stick with what we have.

Definite pros and cons to both sides.
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Old 04-05-2014, 11:28 AM   #19
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Agreed on many things. 17.5 is quicker than 19T used to be. I'm not the only one with the opinion that 21.5 is as quick as stock needs to be. Half the reason USGT is so popular is because of this. Even this is quicker than the last years of brushed 27T.

A tire size change would likely have to be in concert with a change to the power pack for spec racing. I don't know if it would help more than it hurts but I would be very curious to try and see. Heck, that's what was done with USVTA and USGT.
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Old 04-05-2014, 12:37 PM   #20
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There is a couple of ways of thinking about it. I'm not a scientist or an engineer, nor will I pretend to be.

What is the difference in gains between PSI of contact area and friction/psi of the tire contact patch? I'm just going to pull numbers and they are totally unrealistic numbers so don't pick them apart. Lets say you have a 24mm, tire width that we have now and when it's on the car, ready to o on the start line it spreads out and gives 96 square mm of contact patch, but it creates 3 psi of weight on the tires.
Now you have a 28mm width tire, it will create 112 square mm of contact area. Now you've reduced the weight on the tires contact patch to maybe 2psi. Are the friction gains, assuming the same surface on both tires, going to make up for the lack of pressure on the tires? I know the cars will weigh the same and have the same wight pushing down on the tires but it comes down to surface area as well.

I hope this makes sense to people. If not I can try and explain it better.

Again, I am no engineer or scientist or anything. Hopefully someone will have better answers.
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Very good questions. The only time where you would want to have smaller contact patch is if you don't have enough mass and/or downforce to properly load the tire and get it to its operating temperature. Otherwise, as long as you can get the tire to operating temp, larger contact patch will always produce more grip. The friction between the surface and the tire increases with increasing contact area, all other variables held constant (tire temp, pressure for a pneumatic tire, tire construction).
It has been a while since my full size car racing days... and a while since I spent time reading any of Carroll Smith's work.

But I recall from "Tune to Win", that the gist of the tire width/contact patch section was that, tire pressure aside, contact patch is solely a function of vertical load on the tire. A wider tire section width will not result in a larger contact patch... It will result in a contact patch of the same area, but one whose shape is wider and shorter... providing, of course, that the tire pressure is constant in our comparison. Thus(pulling numbers out of thin air here... and assuming that the contact patch is rectangular instead of elliptical) a standard 24mm touring car tire may have a contact patch that is 24x4mm(resulting in 96sqmm of tire contact with the racing surface)... And a wider 28mm touring car tire may have a contact patch that is 28x3.4mm(again, resulting in 96sqmm).

The benefit of a wider tire comes from the fact that each discrete patch makes contact with the racing surface for less time over a given revolution of the tire... making the tire more thermally efficient. And that a wider/flatter contact patch develops its peak traction capacity at a smaller slip angle.
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Old 04-05-2014, 01:10 PM   #21
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The benefit of a wider tire comes from the fact that each discrete patch makes contact with the racing surface for less time over a given revolution of the tire... making the tire more thermally efficient. And that a wider/flatter contact patch develops its peak traction capacity at a smaller slip angle.
Engineer to Win and Tune to Win are quite good. Good catch on this.

Your last sentence outlines the root of where my argument was coming from, as most racing tires tend to produce peak friction coefficients (and lateral grip) at relatively low slip angles compared with say, road tires. Additionally, higher longitudinal grip will be on offer on corner exit due to the decrease in slip angle needed mid-corner for peak lateral grip.

The thermal efficiency aspect of this is where I think modified racing would benefit highly.
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Old 04-05-2014, 01:18 PM   #22
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Rotating mass??
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Old 04-05-2014, 01:51 PM   #23
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While rotating mass is always a concern, in this case, I think it would be marginal. Look at the extra mass of the VTA tires and the higher turn 25.5's run them without issue. I think that the 17.5's or mod motors wont even notice the difference.
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Old 04-05-2014, 02:59 PM   #24
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Well if you are considering a wider wheel because the full sized cars run a wider wheel...then you should look at diameter as well. The current RC rim diameter of 64mm scaled up equates to a 25.2" wheel! GT and DTP cars run an 18" rim. The closest I could find to a 25" rim are LMP which run a 28" rim in LMP1 and 23" rim in LMP2. So realistically speaking TC rims should be more like 46mm in diameter.
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Old 04-05-2014, 03:35 PM   #25
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I see, and I suppose you could have a larger rim and lower profile tires, and even end up with a lower rotating mass. Or near the same. I am not the engineer.
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Old 04-05-2014, 05:47 PM   #26
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The cars I mentioned already run a fairly low profile tire...the point being is the scale of the rim and wheel is way off.
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Old 04-05-2014, 06:15 PM   #27
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The unsprung mass increase would have more of an impact than the rotating mass. Particularly on less than very smooth surfaces or tracks with lots of quick transitions. That's not to say the potential advantages wouldn't outweigh this.
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Old 04-05-2014, 07:03 PM   #28
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No reason to change sizes. Tire problems at the warm up are a manufacturer specific problem.
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Old 04-05-2014, 07:09 PM   #29
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Sounds like the tires are blowing out at high speed right? The rotating mass is the problem if that is the case, not the tire size.
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Old 04-05-2014, 07:17 PM   #30
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Sounds like the tires are blowing out at high speed right? The rotating mass is the problem if that is the case, not the tire size.
They are blowing out because the belt is too thin - thinner than what is used by other manufacturers.
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