Any Newton fans?
12-03-2002, 03:55 PM
#16
Burning Brakes
[quote]Originally posted by Russ Murphy:
<strong>Erick,
So you're saying that what you lose in accelerative capacity by going to the bigger gear you can gain back by choosing a smaller gear? Careful here, I'm wading into the deep end of the pool
but I think that's correct.
F= (engine torque) (gear ratio) / R tire radius</strong><hr></blockquote>
Actually, I'm already over my head, so I don't really know what I'm saying. I'm just theorizing by thinking about the extremes:
A (theoretical) 1" diameter wheel would accelerate like crazy up until its ridiculously low top speed (velocity).
A (theoretical) 50" diameter wheel would have a very high potential top speed, but would have VERY limited acceleration potential because of the torque required to turn this huge wheel.
I just reviewed a video of me on the (long) front straightaway at a local track. It was 24 seconds of acceleration - topping out at about 140mph.
I can't help but wonder what the effect of using a 16", 17", or 18" wheel would be for this very long straight section. Based on your calculations, it seems that it may be a huge difference. But that is why I asked if you had a theory on what the "optimal" tire/wheel diameter would be be given a certain situation. Hence, my thinking about the "extremes".
Fun stuff!
Erick
<strong>Erick,
So you're saying that what you lose in accelerative capacity by going to the bigger gear you can gain back by choosing a smaller gear? Careful here, I'm wading into the deep end of the pool
but I think that's correct.F= (engine torque) (gear ratio) / R tire radius</strong><hr></blockquote>
Actually, I'm already over my head, so I don't really know what I'm saying. I'm just theorizing by thinking about the extremes:
A (theoretical) 1" diameter wheel would accelerate like crazy up until its ridiculously low top speed (velocity).
A (theoretical) 50" diameter wheel would have a very high potential top speed, but would have VERY limited acceleration potential because of the torque required to turn this huge wheel.
I just reviewed a video of me on the (long) front straightaway at a local track. It was 24 seconds of acceleration - topping out at about 140mph.
I can't help but wonder what the effect of using a 16", 17", or 18" wheel would be for this very long straight section. Based on your calculations, it seems that it may be a huge difference. But that is why I asked if you had a theory on what the "optimal" tire/wheel diameter would be be given a certain situation. Hence, my thinking about the "extremes".
Fun stuff!
Erick
12-03-2002, 04:17 PM
#17
Race Director
Well,
A smaller tire will accelerate better since it has less rotational mass and because of a small change in gearing. That is what Russ Calcuated. Rememeber also that if you had a 1" diameter wheel vs a 50" diameter you can easily get to the same gearing by changing final drive ratios to match properly. You can't however eimilate the weight of the 50" diameter vs 1" assuming the same wheel and tire material and construction.
In general the smaller the wheel diameter the better unless you reach top speed in top gear.
However this does not 100% translate to the track since a change in tire diameter can impact gear changes in corners. For example you may be inbetween gears with smaller tire and then be slow since you need more gear changes or are not in you power band.
Also the impact of tire width needs to be figure in. You may be faster down the straight with 255/40 vs 275/40, but you may give up that speed in the corners. My guess is that this is more important for the front tires. Also the narrower tire may limit how much power you can put to the ground in the rear. This will then limit top speed because the only power that counts is power going from the tire to the ground.
Phil,
A rough calculation may be meaningless for a race car since there are only two things that matter in racing. Lap times and out braking your opponent after the big straight. It may very well be that any acceleration that you lose from the taller and heavier 275/40 tire may be made-up in the corners or on corner exit speed on the long straight.
Probably better to run laps both ways and find out assuming you can run laps consistantly enough to even see the difference.
A smaller tire will accelerate better since it has less rotational mass and because of a small change in gearing. That is what Russ Calcuated. Rememeber also that if you had a 1" diameter wheel vs a 50" diameter you can easily get to the same gearing by changing final drive ratios to match properly. You can't however eimilate the weight of the 50" diameter vs 1" assuming the same wheel and tire material and construction.
In general the smaller the wheel diameter the better unless you reach top speed in top gear.
However this does not 100% translate to the track since a change in tire diameter can impact gear changes in corners. For example you may be inbetween gears with smaller tire and then be slow since you need more gear changes or are not in you power band.
Also the impact of tire width needs to be figure in. You may be faster down the straight with 255/40 vs 275/40, but you may give up that speed in the corners. My guess is that this is more important for the front tires. Also the narrower tire may limit how much power you can put to the ground in the rear. This will then limit top speed because the only power that counts is power going from the tire to the ground.
Phil,
A rough calculation may be meaningless for a race car since there are only two things that matter in racing. Lap times and out braking your opponent after the big straight. It may very well be that any acceleration that you lose from the taller and heavier 275/40 tire may be made-up in the corners or on corner exit speed on the long straight.
Probably better to run laps both ways and find out assuming you can run laps consistantly enough to even see the difference.
12-03-2002, 04:30 PM
#18
Race Director
You cannot neglect the aerodynamic effect of a wider tire: that will increase your wind resistance and drag. And adding wider wheel wheels, if need be, will add more weight and more drag. Also: due to a wider contact surface, your roll resistance will also be greater by a miniscule amount, but since we're talking theory here, we might as well recognize this too, right?
Question:
Is the tire available as a 275/35/17 or 275/30/17 ? Then you'd be closer to the original diameter, thus best of both worlds: same acceleration, better cornering grip. (But you still have more drag and roll resistance)
Of course, a larger diameter will net you a higher top speed, provided you have enough room to get there!
Just my $0.42.
-Zoltan.
PS: I like Fig Newtons too! Mmmmm Fig Newtons...
Question:
Is the tire available as a 275/35/17 or 275/30/17 ? Then you'd be closer to the original diameter, thus best of both worlds: same acceleration, better cornering grip. (But you still have more drag and roll resistance)
Of course, a larger diameter will net you a higher top speed, provided you have enough room to get there!
Just my $0.42.
-Zoltan.
PS: I like Fig Newtons too! Mmmmm Fig Newtons...
12-03-2002, 06:35 PM
#19
Race Director
Well if we are going to be so particular... 
Any change in sidewall height will effectivly change your spring rates. With high racing spring rates the tires, even race tires, play a large roll in effective spring rate. A change from 225/40 to 275/40 will lower your effective spring rate and thus you may need new springs to counter act.
Also the change in ride height can effect overall aerodynamic efficiency and that will impact top speed....
Heck all of these effects from at tire change can change you lap times by 0.001 of sec....
<img src="graemlins/xyxwave.gif" border="0" alt="[bigbye]" />
Any change in sidewall height will effectivly change your spring rates. With high racing spring rates the tires, even race tires, play a large roll in effective spring rate. A change from 225/40 to 275/40 will lower your effective spring rate and thus you may need new springs to counter act.
Also the change in ride height can effect overall aerodynamic efficiency and that will impact top speed....
Heck all of these effects from at tire change can change you lap times by 0.001 of sec....
<img src="graemlins/xyxwave.gif" border="0" alt="[bigbye]" />
