Tire Physics Question...which is quicker?
Thread Starter
Melting Slicks
Joined: Jun 2002
Posts: 3,070
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From: Charleston South Carolina
Tire Physics Question...which is quicker?
Running on Hoosiers in SS with a coupe I noticed they have a 285/30/18 size. I've been using a 275/35/18 but I'm calculating a sidewall of 96.25cm vs. 85.5cm on the on the wider tire. This wider tire with the shorter side wall should make the car quicker and the gearing shorter. Is my thinking correct here??? I'm not concerned with top end speed in autocross, but quicker in accelleration is a major plus, I can't change the stock rim sizes and stay in SS.
1) What's the equivalent rear end change for a 1 inch tire-diameter reduction? Assume a 3.42 rear to start.
2) Lowering only the rear by 1/2 inch will transfer approximately how much weight to the rear? Will the 51%/49% weight distribution be equalized???
OK you physics geeks...go at it!!
:steering:
1) What's the equivalent rear end change for a 1 inch tire-diameter reduction? Assume a 3.42 rear to start.
2) Lowering only the rear by 1/2 inch will transfer approximately how much weight to the rear? Will the 51%/49% weight distribution be equalized???
OK you physics geeks...go at it!!
:steering:
Melting Slicks
Joined: Aug 2000
Posts: 2,112
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From: Sindelfingen Germany
Re: Tire Physics Question...which is quicker? (TedDBere)
Answers.
1. A 3.57 rear should do the same effect as a 1 inch smaller diameter
2. statically the difference may not be measurable, dynamically I have the following in mind: lowering a car makes the center of gravity go down, on the other hand, raising it, will put the center higher. When accelerating, a higher center of gravity will put a higher moment on the rear tires. Imagine a motor cyclist accelerating: if he sits upright, the front tire will raise when accelerating, so the complete load is on the rear tire. I think he only bends down to avoid a turnaround. So I think for accelerating only, one should not lower as long as the front tires still have contact to the trac to be able to steeer the car. For driving curves you need lowereing toi keep all tires on the ground.
OK, I know answer #2 is worth a big discussion. Maybe I´m wrong with this. But answer # 1 must be pretty good. :D
Frederik
1. A 3.57 rear should do the same effect as a 1 inch smaller diameter
2. statically the difference may not be measurable, dynamically I have the following in mind: lowering a car makes the center of gravity go down, on the other hand, raising it, will put the center higher. When accelerating, a higher center of gravity will put a higher moment on the rear tires. Imagine a motor cyclist accelerating: if he sits upright, the front tire will raise when accelerating, so the complete load is on the rear tire. I think he only bends down to avoid a turnaround. So I think for accelerating only, one should not lower as long as the front tires still have contact to the trac to be able to steeer the car. For driving curves you need lowereing toi keep all tires on the ground.
OK, I know answer #2 is worth a big discussion. Maybe I´m wrong with this. But answer # 1 must be pretty good. :D
Frederik
Thread Starter
Melting Slicks
Joined: Jun 2002
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From: Charleston South Carolina
Re: Tire Physics Question...which is quicker? (C5 Frederik)
1) The guys I run against usually use a 26.8 inch diameter rear tire and this one would be a 24.6 inch diameter. Based on what you've said this would make it seem that they were running a 3.42 rear and mine would run like a 3.72 in comparison.
2) I would be dropping the rear only, not the front, with this tire. Would this transfer weight to the rear and how much? Would this be better for accelleration and worse for cornering?
:steering:
2) I would be dropping the rear only, not the front, with this tire. Would this transfer weight to the rear and how much? Would this be better for accelleration and worse for cornering?
:steering:
Team Owner
Joined: Aug 2003
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From: VA
St. Jude Donor '03-'11
Re: Tire Physics Question...which is quicker? (TedDBere)
1) The guys I run against usually use a 26.8 inch diameter rear tire and this one would be a 24.6 inch diameter. Based on what you've said this would make it seem that they were running a 3.42 rear and mine would run like a 3.72 in comparison.
2) I would be dropping the rear only, not the front, with this tire. Would this transfer weight to the rear and how much? Would this be better for accelleration and worse for cornering?
:steering:
2) I would be dropping the rear only, not the front, with this tire. Would this transfer weight to the rear and how much? Would this be better for accelleration and worse for cornering?
:steering:
on #2, I think C5 Frederik's answer still applies. Since you're dropping the rear only, the CG only moves down 1/2 the distance (b/c the weight dist is near 50/50). But the basic idea is the same
As for impact on handling, I think the biggest impact will the change in sidewall stiffness. If you're only changing the rears, they'll be stiffer. It's been a long time since I played with that kind of stuff, but I think stiffening the rear makes the car understeer more (could be backwards)
Pro
Joined: Apr 2002
Posts: 658
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From: North York Ontario
Re: Tire Physics Question...which is quicker? (TedDBere)
To answer this question several assumption must be made some are:
Consider Straight line acceloration only.
lowering the car at the rear will raise the CG above the rear wheel center.
only dynamic forces will be affected.
If we lower the rear by 0.5inches The increase of the moment arm will be about 0.25 inches.
Assume that SG is 10in above the wheel center
Therefore the increase in dynamic wheel load is .25/10x100=2.5%
That sounds significant, however this increased load will only be effective as long as the power can create dynamic friction (wheel spin) (less than 2 second for a stock C5). I would say 0.05sec decrease in time to get to 30mph. It would be neat if someone had real world experience on this.
but then again I could be wrong.
Consider Straight line acceloration only.
lowering the car at the rear will raise the CG above the rear wheel center.
only dynamic forces will be affected.
If we lower the rear by 0.5inches The increase of the moment arm will be about 0.25 inches.
Assume that SG is 10in above the wheel center
Therefore the increase in dynamic wheel load is .25/10x100=2.5%
That sounds significant, however this increased load will only be effective as long as the power can create dynamic friction (wheel spin) (less than 2 second for a stock C5). I would say 0.05sec decrease in time to get to 30mph. It would be neat if someone had real world experience on this.
but then again I could be wrong.
Thread Starter
Melting Slicks
Joined: Jun 2002
Posts: 3,070
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From: Charleston South Carolina
Re: Tire Physics Question...which is quicker? (auctiondepot)
So your saying that not only will the car be quicker due to the smaller diameter tires but will also be quicker due to the lowered rear?? This is too good to be true. In autocross races are won and lost on 0.001 of second. In a run I can have 5-6 bursts up to and over 30-50mph. If I can gain 0.05 seconds each time I could shave 0.3 seconds due to the weight transfer alone. I must be missing something here???
:steering:
:steering:
Pro
Joined: Apr 2002
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From: North York Ontario
Re: Tire Physics Question...which is quicker? (TedDBere)
I am not speaking from real experience, but it seams to me that in autocross you have to consider cornering, which involve balance, and with a lighter front end you may not come out as fast (understeer).
Further my assumption about moment arm being above the center of the wheel is wrong oops, should be above the contact patch, which halves everthing! so the gain should be .025 second to 30MPH (44ft/sec.).
Further my assumption about moment arm being above the center of the wheel is wrong oops, should be above the contact patch, which halves everthing! so the gain should be .025 second to 30MPH (44ft/sec.).
