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rws262  
#1 Posted : Wednesday, August 23, 2017 3:21:16 PM(UTC)
rws262

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Apologize if this has been discussed already, but I could not find anywhere a discussion on the difference in chassis effects when using the narrow vs wide RR? Also, same question goes for the 8" vs 10" wide LR tire.

Thanks!
Mike Dicely  
#2 Posted : Thursday, August 24, 2017 6:43:04 AM(UTC)
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There is more traction per square inch on a narrow RR. However, the tire efficiency principle says that the traction will go up per square inch as more weight is applied, but it does not go up linearly (in a straight line). The tires efficiency goes down as more weight is applied. There is a graph of this concept in my Rethink Dirt Paper in the tech department on this website.http://www.hyperracing.com/pages/tech_department/chassis_setup/rethink_dirtadvanced_theory.aspx

Knowing this we conclude that a wider tire, which has more surface area will have more traction than a narrow tire. Thats the easy part and is defiantly true, you can not argue this point.

But there are other factors to consider. When a narrow tire is put on the RR, it will have the effect of moving the RR in. Moving the RR in causes a jacking effect on the chassis, the car raises up when in the turn. What happens when the car raises up? It gets more forward drive making the car tighter, especially in the second half of the turn.

Interestingly, the west coast has bought into this theory of the narrow tire getting more traction. What does the west coast do different than the rest of the country (for the most part)? They use z-link rear suspension. The Z-link rear has the effect of squatting the rear under acceleration (less anti-squat than a wish bone). This squatting lowers the chassis killing forward drive (anytime we lower CGH, there is less weight transfer to teh rear under acceleration). That is why you see the narrow tire on the cars out west. You can use your imagination to find other ways to solve this problem rather than running a narrow tire.

Another consideration is that because a narrow tire does have more PSI, it will be able to work into the dirt more providing what they call mechanical grip. This is the result of the tire actually getting down into the dirt, like the paddle of a boat will be able to provide more thrust if plunged deeper into the water. But when is the surface of a dirt track soft enough for the tire to actually be able to get down into the surface? Maybe warm ups? Other than that it will help when you are running the cushion where the dirt is soft, but usually you are trying to keep the car free up there, aren't you? But this is something to think about.

8" vs 10" LR is a little more straightforward. 10" gets more traction except on wet dirt or dirt that the tire can get down into for mechanical grip. But if you are trying to free the car up, then use an 8" LR. In practice, I have run both back to back and really can't tell any difference.

But one other consideration is that 8" tires can offer different stagger combinations, stagger of course is very important.

Edited by user Thursday, August 24, 2017 6:50:14 AM(UTC)  | Reason: Not specified

thanks 1 user thanked Mike Dicely for this useful post.
rws262 on 8/24/2017(UTC)
Phil Meisner Jr.  
#3 Posted : Thursday, August 24, 2017 7:13:38 AM(UTC)
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What about the 8" LR vs 10" LR for wingless? If you race at a track with a width rule this would allow you to have your RR out two inches further with a 8" LR. Also, I would think this would allow the car to turn easier and have less lateral weight transfer?
Mike Dicely  
#4 Posted : Thursday, August 24, 2017 7:15:27 AM(UTC)
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All True.
rws262  
#5 Posted : Thursday, August 24, 2017 4:03:36 PM(UTC)
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It seems to me that Z links are inferior then?
Mike Dicely  
#6 Posted : Thursday, August 24, 2017 5:09:43 PM(UTC)
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They are not inferior. It just depends on what you want. In many applications we try to limit weight transfer. Like when we run a tight cushion, or if the chassis has a high static CGH, like a midget or full size sprint car.
Even with 600's you have to be careful how much weight transfer you get, too much causes more problems than not enough.
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