How do wheel/tire size affect speed
06-17-2009, 09:22 PM
#16
aero tire drag is more about churn through the air than it is frontal area presented to the wind. Where the tire touches the ground the wheel isn't moving so there's little aero drag near the surface of the pavement. . At the top of the tire it's moving twice the speed of the car which at 130 mph vehicle speed is a ton of drag. A wider tire has a lot more drag becuase the entire width increase of the tire is churning throuhg the wind, not just the little bit that you see exposed when you look at the front of the car.
06-18-2009, 02:58 AM
#17
Rennlist Member
Not really. I was hitting the same rev limiter blip at the exact same exit birm spot in 2nd gear. the time for the wider tires was .3 seconds faster a lap. 14 corners, so the difference is pretty small. the biggest gain was the feel of the car, stabilty in traffic, and reduction of push.
The 130 at the end of the straight both times could be misleading. As you say, the cornering is better so one would expect your V-min in the mid corner and corner exit is a bit higher. So you may be seeing a hit from the extra weight and width but not realizing it.
06-18-2009, 03:04 AM
#18
Rennlist Member
The top of the tire is held completely covered by the fenders. We are not talkng about paddle wheels here. 
The top of the tire doesnt see any air flow drag, its all about the air moving around the body. The lower tire area is flat plate drag and its a major problem for open wheel cars, but not so much for sports car covered wheel vehicles. As far as drag says, it is the tires' increased frontal area that you can see that makes the difference. The fact that the tire is rotating has no bearing on drag of the tire. this is the reason manufactures can get a drag figure from their designs. It doesnt change with speed. (meaning the spinning tire going faster doesnt change anything) Drag coefficients are fixed and total drag depends on speed of the vehicle, square frontal area, and drag coeffficient. The wider tire effect is mainly rolling friction and whatever is increased as far as frontal area.
The top of the tire doesnt see any air flow drag, its all about the air moving around the body. The lower tire area is flat plate drag and its a major problem for open wheel cars, but not so much for sports car covered wheel vehicles. As far as drag says, it is the tires' increased frontal area that you can see that makes the difference. The fact that the tire is rotating has no bearing on drag of the tire. this is the reason manufactures can get a drag figure from their designs. It doesnt change with speed. (meaning the spinning tire going faster doesnt change anything) Drag coefficients are fixed and total drag depends on speed of the vehicle, square frontal area, and drag coeffficient. The wider tire effect is mainly rolling friction and whatever is increased as far as frontal area.aero tire drag is more about churn through the air than it is frontal area presented to the wind. Where the tire touches the ground the wheel isn't moving so there's little aero drag near the surface of the pavement. . At the top of the tire it's moving twice the speed of the car which at 130 mph vehicle speed is a ton of drag. A wider tire has a lot more drag becuase the entire width increase of the tire is churning throuhg the wind, not just the little bit that you see exposed when you look at the front of the car.
Last edited by mark kibort; 06-18-2009 at 01:03 PM.
06-18-2009, 03:20 AM
#19
Rennlist Member
a couple of factoids here.
First of all, I incorporated and did the math for you with the 2x factor. If you go through the calculations, at 17" on the diameter the differnce would be about 1.4x as if that weight was sitting in the car. at the outer diameter, its more like 2x. So yes, you are right, as you have that weight on the outer areas of the rolling mass, its effect is more. 2x is about what it would be if the weight is in the 21 to 26" area. so, 2lbs a tire is 8lbs overall. because its rolling its effect is like that weight being double in the car as far as acceleration forces, or 16lbs. we are talking about 2hp or less . Clearly not enough to measure any acceleration differnces.
Now, tire friction. again, how much friction can a sticky slick produce? having done dyno runs with new hoosiers. rolling friction TOTAL for the rear wheels was 20hp at 150mph rolling down to 10hp at 80mphs. Thats all the friction. including bearings, and some gearing. I even did some experiementing with low pressures in the tires and couldnt get more than a hp change on the dyno run which is just noise.
dont know the friction of greater than 1/16" toe toe, but I dont think we are talking enough to make the differences we are looking for here.
mk
First of all, I incorporated and did the math for you with the 2x factor. If you go through the calculations, at 17" on the diameter the differnce would be about 1.4x as if that weight was sitting in the car. at the outer diameter, its more like 2x. So yes, you are right, as you have that weight on the outer areas of the rolling mass, its effect is more. 2x is about what it would be if the weight is in the 21 to 26" area. so, 2lbs a tire is 8lbs overall. because its rolling its effect is like that weight being double in the car as far as acceleration forces, or 16lbs. we are talking about 2hp or less . Clearly not enough to measure any acceleration differnces.
Now, tire friction. again, how much friction can a sticky slick produce? having done dyno runs with new hoosiers. rolling friction TOTAL for the rear wheels was 20hp at 150mph rolling down to 10hp at 80mphs. Thats all the friction. including bearings, and some gearing. I even did some experiementing with low pressures in the tires and couldnt get more than a hp change on the dyno run which is just noise.
dont know the friction of greater than 1/16" toe toe, but I dont think we are talking enough to make the differences we are looking for here.
mk
It could be a combination of factors that add up to the loss of acceleration that you are noticing. Mark's experience not withstanding (as his car is heavier with more power and he only changed the front tires) there certainly will be more rolling resistance with the wider tires, especially in the back. How much toe-in do you run? Anything more than about 1/16 total is going to cause significantly more friction as your tires get wider.
Also, 10 lbs may not be much, but if it is all around the circumference of the wheels (which it probably is - and it is farther out with the 18s), it could add quiet a bit to the inertial forces that you must overcome while accelerating.
Finally, though you were running the same tires, how many heat cycles were on the 17s vs the 18s? Older tires, even though still quite grippy could be a little harder and so have a little less rolling resistance. All these factors, though each one small, could add up to the bigger differences that you see.
Also, 10 lbs may not be much, but if it is all around the circumference of the wheels (which it probably is - and it is farther out with the 18s), it could add quiet a bit to the inertial forces that you must overcome while accelerating.
Finally, though you were running the same tires, how many heat cycles were on the 17s vs the 18s? Older tires, even though still quite grippy could be a little harder and so have a little less rolling resistance. All these factors, though each one small, could add up to the bigger differences that you see.
06-18-2009, 03:28 AM
#20
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I aready addressed the diameter issue as far as the location of the mass on the rotating wheel and tire. about 2x the effect as if it was in the car and being accelerated.
As we talked about in other discussions, drag has to be pretty substantial to effect accleration forces and times. power=force x speed
I like the example of the wing and downforce effects on drag.
at 100mph if you had 250lbs of downforce, it might only be near 25lbs of drag.
25lbs of drag, working on a 24" diameter tire and near a 4.5:1 gear ratio, is only about 5ft-lbs of torque at the engine. Now, if you are a miata, its going to be more of an effect with acceleration vs a 500ft-lb corvette.
As we talked about in other discussions, drag has to be pretty substantial to effect accleration forces and times. power=force x speed
I like the example of the wing and downforce effects on drag.
at 100mph if you had 250lbs of downforce, it might only be near 25lbs of drag.
25lbs of drag, working on a 24" diameter tire and near a 4.5:1 gear ratio, is only about 5ft-lbs of torque at the engine. Now, if you are a miata, its going to be more of an effect with acceleration vs a 500ft-lb corvette.
+100
Sit in office chair. Hold heavy weight out with straight arms from body.
Start to spin.
Pull weight towards body.
You spin lots faster. (good old physics experiment)
With 18" wheels, heavy (wider) tires and heavier rim mass away from rotational center is like spinning with weight further from body.
Did the 18" wheels and wider rubber net you faster lap times? (just curious. sorry if I missed it. Just read it was retarding terminal speeds in the straights)
Sit in office chair. Hold heavy weight out with straight arms from body.
Start to spin.
Pull weight towards body.
You spin lots faster. (good old physics experiment)
With 18" wheels, heavy (wider) tires and heavier rim mass away from rotational center is like spinning with weight further from body.
Did the 18" wheels and wider rubber net you faster lap times? (just curious. sorry if I missed it. Just read it was retarding terminal speeds in the straights)
06-18-2009, 09:50 AM
#22
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I'm not an expert in physics, so I'm having a hard time rationalizing some of this to my casual understanding. Please help:
If acceleration is proportional to the (HP divided by the mass of the car), the effect of the higher tire/wheel weight should be calculated using the total mass of the car + wheels, not just the wheels. So the effect of 1.2 hp on your car may be significantly less than it would be on a much lighter car, where the weight of the wheels contribute a much higher percentage of the total mass (i.e. proportional increase in mass is greater, so there's a proportionately greater effect on acceleration). Perhaps it's still not enough to be noticeable??
I thought that inertia is calculated at the center of gravity of the entire car, not at the center of a given wheel. In other words, the rotational inertia is applicable only if considering a single wheel in isolation, but on the car, it is already accounted for in the inertia calculations for the entire car. What am I missing? Are the required calculations just more complicated than the macro level view? Thanks.
Just going on my casual understanding, I would have looked at the forces that act in the opposite direction of hp, namely, mass and friction. Of these two, I would think that the extra friction of the larger contact patch is the largest contributor to the reduction in speed, especially if amplified by tow-in as identified by Larry. To clarify, isn't the diameter of the wheel inconsequential compared to the width of the tire? Help!
Just as dynos are calibrated, you can use the rate of change of speed (acceleration ) and mass (weight of the tire), to determine an exact hp difference.
Doing the work for you, it works out to be about 2x the weight as if it was in the car. so you can see if you have a 6lb rolling weight increase, it will effect net HP available by 12lbs as if it was in the car. that might only be 1.2hp and I dont think you can notice this kind of hp/weight change.
Doing the work for you, it works out to be about 2x the weight as if it was in the car. so you can see if you have a 6lb rolling weight increase, it will effect net HP available by 12lbs as if it was in the car. that might only be 1.2hp and I dont think you can notice this kind of hp/weight change.
Just going on my casual understanding, I would have looked at the forces that act in the opposite direction of hp, namely, mass and friction. Of these two, I would think that the extra friction of the larger contact patch is the largest contributor to the reduction in speed, especially if amplified by tow-in as identified by Larry. To clarify, isn't the diameter of the wheel inconsequential compared to the width of the tire? Help!
06-18-2009, 10:01 AM
#23
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Mark,
Do you have the data to show all of the numbers you've been using? You say you're shifting at the same spot, but is it the exact same? Are the exit speed numbers the same? Are the top speed numbers the same?
If you're off by 1mph in any of those it equates to tenths of a second in real world time. So if you're just going off what you see and feel and not off hard data then all of what you are saying isn't a constructive addition to the discussion here.
Do you have the data to show all of the numbers you've been using? You say you're shifting at the same spot, but is it the exact same? Are the exit speed numbers the same? Are the top speed numbers the same?
If you're off by 1mph in any of those it equates to tenths of a second in real world time. So if you're just going off what you see and feel and not off hard data then all of what you are saying isn't a constructive addition to the discussion here.
06-18-2009, 11:48 AM
#24
Three Wheelin'
It could be a combination of factors that add up to the loss of acceleration that you are noticing. Mark's experience not withstanding (as his car is heavier with more power and he only changed the front tires) there certainly will be more rolling resistance with the wider tires, especially in the back. How much toe-in do you run? Anything more than about 1/16 total is going to cause significantly more friction as your tires get wider.
Also, 10 lbs may not be much, but if it is all around the circumference of the wheels (which it probably is - and it is farther out with the 18s), it could add quiet a bit to the inertial forces that you must overcome while accelerating.
Finally, though you were running the same tires, how many heat cycles were on the 17s vs the 18s? Older tires, even though still quite grippy could be a little harder and so have a little less rolling resistance. All these factors, though each one small, could add up to the bigger differences that you see.
Also, 10 lbs may not be much, but if it is all around the circumference of the wheels (which it probably is - and it is farther out with the 18s), it could add quiet a bit to the inertial forces that you must overcome while accelerating.
Finally, though you were running the same tires, how many heat cycles were on the 17s vs the 18s? Older tires, even though still quite grippy could be a little harder and so have a little less rolling resistance. All these factors, though each one small, could add up to the bigger differences that you see.
I've got to agree with Larry that its probably a combination of factors.
Could you post the data as well, a screenshot of the preceding corner and straight. You could be at WOT at different points in the corner despite the same apex speed you're actually accelerating earlier with the 17s. It could be that the dynamic rolling radius of the tires is different. Static radius doesn't really matter. It gives a good reference point but the radius can and does change. Larry pointed out that rotational inertia most likely increased. That will affect it as well as well as other points mentioned already.
06-18-2009, 12:17 PM
#25
Rennlist Member
Yes, your paragraph below is correct. The weight factor I used is correct, (i.e 2x or whatever it ends up based on the increased weight on a given diameter).
I only used the ball park ounder 2 hp to cover a range of weighted cars/hp to weight cars.
So, to your point, if we are talking about a 3000lb car and it has a 10:1 weight to HP ratio, 10lbs is going to have the effect of near 1hp. If the car is 3000lbs and has a HP to weight ratio of 20:1 (half the HP) then the effect is even less)To your point, if a lighter car with the same HP to weigtht ratio of 10:1 had the same wheel weight change (10lbs effectively), the effect would be identical.
weight lbs /HP= weight/hp ratio
3000/300 = 10:1
3010/301 = 10:1
1500/150 = 10:1
1510/151 = 10:1
This shows that 10lbs effective increase in weight, for a same 10:1 weight/hp race car will accelerate the same. (all other things being equal or unchanged).
If a car has a worse HP to weight ratio, the effect will actually be less.
3000/150 = 20:1
3010/150.5 = 20:1
In this case, same weight change only effected the hp ratio by the equivilance of .5HP.
lastly, a lighter car with 2x the weight to hp ratio
1500/75 = 20:1
1510/75.5 = 20:1
This shows that even a lighter car with the same wheel weight change and 1/2 the hp to weight abilities, is effectived less. the 10lbs really only equates to near .5hp as well.
Its kind of counter intuitive, thats why I broke down the values to show the effects on weight to HP ratios which determine acceleration at any vehicle speed.
Now, for your second question. we are talking the effects of a rotating mass. the entire mass of the car can be measured. Inertia is used for rotating mass. the acceleration power requirements for the rotating mass will be higher. If we increae the rotating mass, all we need to know is the diameter and distribution of the rotating mass to determine its equivilant effect on the entire mass of the car. this factor is about 1.4x on a 17" diameter and more like 2x on a 20-25" diameter. (ball park)
To your last point, the rolling friction is a force that works against acceleration. Ive measured the rear wheels on the rollers on the dyno, which probably is pretty close to what it would be on the road. the differnce of a 1" tire width on a 8-10" tire, is around 10% more width. the contact patch would grow by near the same amount. whatever the friction is of the rolling tire, it might go up the same. However this value is almost insignificant. at relatively high speeds, aero drag is the dominant factor, greatly over shadowing rolling friction. again, I measured 20hp at 150mph for rear wheel forces acting against acceleration and 10hp at 80mph. (for 2 305 sized tires) most of this is internal bearing and gear friction of the rear end. Lets say its half. 10hp at 80mph, thats 5hp for rolling friction and 5hp for bearing and gear. If you increase the contact patch by 10%, thats a .5HP change. (1hp for all 4 tires in theory). aero on the other hand could be near 50hp at that same speed.
So many other factors could determine why a car is slower or faster at the end of a given straight on a race course.
In the end, the OP is seeing 6mph down the straight difference. This is a HUGE variance. There is always a possiblity of the engine having an issue reducing power because the weight weight differnce effect, barring all other issues, is only a net 2hp difference effectively and that in NO way can cause a 6mph straight line speed difference down a given straight.
6mph difference and an before equal car is now driving around you easily. one size of tire change doesnt make this kind of change one way or another. When I went from hoosier R3 to toyo RA1s, the weight difference was near 5lbs per tire. 25lbs . I saw very little difference in performance besides controlling a more "slidey" tire in the RA1.
I only used the ball park ounder 2 hp to cover a range of weighted cars/hp to weight cars.
So, to your point, if we are talking about a 3000lb car and it has a 10:1 weight to HP ratio, 10lbs is going to have the effect of near 1hp. If the car is 3000lbs and has a HP to weight ratio of 20:1 (half the HP) then the effect is even less)To your point, if a lighter car with the same HP to weigtht ratio of 10:1 had the same wheel weight change (10lbs effectively), the effect would be identical.
weight lbs /HP= weight/hp ratio
3000/300 = 10:1
3010/301 = 10:1
1500/150 = 10:1
1510/151 = 10:1
This shows that 10lbs effective increase in weight, for a same 10:1 weight/hp race car will accelerate the same. (all other things being equal or unchanged).
If a car has a worse HP to weight ratio, the effect will actually be less.
3000/150 = 20:1
3010/150.5 = 20:1
In this case, same weight change only effected the hp ratio by the equivilance of .5HP.
lastly, a lighter car with 2x the weight to hp ratio
1500/75 = 20:1
1510/75.5 = 20:1
This shows that even a lighter car with the same wheel weight change and 1/2 the hp to weight abilities, is effectived less. the 10lbs really only equates to near .5hp as well.
Its kind of counter intuitive, thats why I broke down the values to show the effects on weight to HP ratios which determine acceleration at any vehicle speed.
Now, for your second question. we are talking the effects of a rotating mass. the entire mass of the car can be measured. Inertia is used for rotating mass. the acceleration power requirements for the rotating mass will be higher. If we increae the rotating mass, all we need to know is the diameter and distribution of the rotating mass to determine its equivilant effect on the entire mass of the car. this factor is about 1.4x on a 17" diameter and more like 2x on a 20-25" diameter. (ball park)
To your last point, the rolling friction is a force that works against acceleration. Ive measured the rear wheels on the rollers on the dyno, which probably is pretty close to what it would be on the road. the differnce of a 1" tire width on a 8-10" tire, is around 10% more width. the contact patch would grow by near the same amount. whatever the friction is of the rolling tire, it might go up the same. However this value is almost insignificant. at relatively high speeds, aero drag is the dominant factor, greatly over shadowing rolling friction. again, I measured 20hp at 150mph for rear wheel forces acting against acceleration and 10hp at 80mph. (for 2 305 sized tires) most of this is internal bearing and gear friction of the rear end. Lets say its half. 10hp at 80mph, thats 5hp for rolling friction and 5hp for bearing and gear. If you increase the contact patch by 10%, thats a .5HP change. (1hp for all 4 tires in theory). aero on the other hand could be near 50hp at that same speed.
So many other factors could determine why a car is slower or faster at the end of a given straight on a race course.
In the end, the OP is seeing 6mph down the straight difference. This is a HUGE variance. There is always a possiblity of the engine having an issue reducing power because the weight weight differnce effect, barring all other issues, is only a net 2hp difference effectively and that in NO way can cause a 6mph straight line speed difference down a given straight.
6mph difference and an before equal car is now driving around you easily. one size of tire change doesnt make this kind of change one way or another. When I went from hoosier R3 to toyo RA1s, the weight difference was near 5lbs per tire. 25lbs . I saw very little difference in performance besides controlling a more "slidey" tire in the RA1.
I'm not an expert in physics, so I'm having a hard time rationalizing some of this to my casual understanding. Please help:
If acceleration is proportional to the (HP divided by the mass of the car), the effect of the higher tire/wheel weight should be calculated using the total mass of the car + wheels, not just the wheels. So the effect of 1.2 hp on your car may be significantly less than it would be on a much lighter car, where the weight of the wheels contribute a much higher percentage of the total mass (i.e. proportional increase in mass is greater, so there's a proportionately greater effect on acceleration). Perhaps it's still not enough to be noticeable??
I thought that inertia is calculated at the center of gravity of the entire car, not at the center of a given wheel. In other words, the rotational inertia is applicable only if considering a single wheel in isolation, but on the car, it is already accounted for in the inertia calculations for the entire car. What am I missing? Are the required calculations just more complicated than the macro level view? Thanks.
Just going on my casual understanding, I would have looked at the forces that act in the opposite direction of hp, namely, mass and friction. Of these two, I would think that the extra friction of the larger contact patch is the largest contributor to the reduction in speed, especially if amplified by tow-in as identified by Larry. To clarify, isn't the diameter of the wheel inconsequential compared to the width of the tire? Help!
If acceleration is proportional to the (HP divided by the mass of the car), the effect of the higher tire/wheel weight should be calculated using the total mass of the car + wheels, not just the wheels. So the effect of 1.2 hp on your car may be significantly less than it would be on a much lighter car, where the weight of the wheels contribute a much higher percentage of the total mass (i.e. proportional increase in mass is greater, so there's a proportionately greater effect on acceleration). Perhaps it's still not enough to be noticeable??
I thought that inertia is calculated at the center of gravity of the entire car, not at the center of a given wheel. In other words, the rotational inertia is applicable only if considering a single wheel in isolation, but on the car, it is already accounted for in the inertia calculations for the entire car. What am I missing? Are the required calculations just more complicated than the macro level view? Thanks.
Just going on my casual understanding, I would have looked at the forces that act in the opposite direction of hp, namely, mass and friction. Of these two, I would think that the extra friction of the larger contact patch is the largest contributor to the reduction in speed, especially if amplified by tow-in as identified by Larry. To clarify, isn't the diameter of the wheel inconsequential compared to the width of the tire? Help!
Last edited by mark kibort; 06-18-2009 at 12:54 PM.
06-18-2009, 12:30 PM
#26
Rennlist Member
There is no question that I could be off by 1mph. Im right at the needle pointed at 130mph at the braking zone for both the heavy and wider tires vs the ligher ones. I have video, but no telemetry data. Only repetable lap times with in 1/10s of a second on clean laps. The point is, It is REALLY straight forward simple physics to determine the power effects of a given weight change of a wheel or tire weight change. what Larry doesnt realize is that the formula was used and applied to the weight increase on the wheel and its effect on acceleration. What I can say is the effect is less than 2hp. If 2hp can generate the changes in speed off the corner and down the straight, that would be quite amazing. generally the trade offs are that a larger tire will help cornering speed, and subsequent straight line speed and lower laps times. He is not seeing this, and you should look othe places to see why, because the wheel weight effects are very easy to predict and determine.
The equation is this:
1/2Iw^2 rotational, which is related to straight line kinetic energy of
1/2M(w x R)^2
I = intertia
w = rotational speed
R= radius
If you solve for M, you get the rule of thumb of :
M=I/R^2
So, if you just look at rim weights,
1 lb added to a 15", 16" 17" rims is like adding 1.37lbs, 1.41lbs and 1.47lbs.(assuming the weight is .5" from the outer edge of the rim)
Now, you talk tires, it works out to be more like 2lbs for every 1lb added to the tire. (i.e. 1lb added to the tire is like it was 2lbs sitting in the car)
Now, there is no question the larger tires are faster. I could be slightly faster down the main straight because I am coming off the prior turn faster, there is a negligible power cost for the extra rolling weight, and im able to go deeper on braking because I had a more confidence inspiring braking feel with the larger front tires. In the end, .3-.4 second lap times were achieved with better control along the lap, in more varied car attitudes through traffic as well.
The equation is this:
1/2Iw^2 rotational, which is related to straight line kinetic energy of
1/2M(w x R)^2
I = intertia
w = rotational speed
R= radius
If you solve for M, you get the rule of thumb of :
M=I/R^2
So, if you just look at rim weights,
1 lb added to a 15", 16" 17" rims is like adding 1.37lbs, 1.41lbs and 1.47lbs.(assuming the weight is .5" from the outer edge of the rim)
Now, you talk tires, it works out to be more like 2lbs for every 1lb added to the tire. (i.e. 1lb added to the tire is like it was 2lbs sitting in the car)
Now, there is no question the larger tires are faster. I could be slightly faster down the main straight because I am coming off the prior turn faster, there is a negligible power cost for the extra rolling weight, and im able to go deeper on braking because I had a more confidence inspiring braking feel with the larger front tires. In the end, .3-.4 second lap times were achieved with better control along the lap, in more varied car attitudes through traffic as well.
Mark,
Do you have the data to show all of the numbers you've been using? You say you're shifting at the same spot, but is it the exact same? Are the exit speed numbers the same? Are the top speed numbers the same?
If you're off by 1mph in any of those it equates to tenths of a second in real world time. So if you're just going off what you see and feel and not off hard data then all of what you are saying isn't a constructive addition to the discussion here.
Do you have the data to show all of the numbers you've been using? You say you're shifting at the same spot, but is it the exact same? Are the exit speed numbers the same? Are the top speed numbers the same?
If you're off by 1mph in any of those it equates to tenths of a second in real world time. So if you're just going off what you see and feel and not off hard data then all of what you are saying isn't a constructive addition to the discussion here.
06-18-2009, 02:49 PM
#27
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Originally Posted by mark kibort
Now, tire friction. again, how much friction can a sticky slick produce? having done dyno runs with new hoosiers. rolling friction TOTAL for the rear wheels was 20hp at 150mph rolling down to 10hp at 80mphs. Thats all the friction. including bearings, and some gearing. I even did some experiementing with low pressures in the tires and couldnt get more than a hp change on the dyno run which is just noise.
dont know the friction of greater than 1/16" toe toe, but I dont think we are talking enough to make the differences we are looking for here.
mk
dont know the friction of greater than 1/16" toe toe, but I dont think we are talking enough to make the differences we are looking for here.
mk
You state that there is a total of 20 hp in friction at 150 mph. That is on the dyno, and there probably is a much lower cf between the tires and the roller than on a real road. And from the tire wear that I have seen on cars with too much rear toe-in, I know that there is a significant source of friction there. I can't quantify it, but I know it's there.
So if you think that I am wrong on all counts here, why is Brinkley seeing this "phenomenon"?
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Larry Herman
2016 Ford Transit Connect Titanium LWB
2018 Tesla Model 3 - Electricity can be fun!
Retired Club Racer & National PCA Instructor
Past Flames:
1994 RS America Club Racer
2004 GT3 Track Car
1984 911 Carrera Club Racer
1974 914/4 2.0 Track Car
CLICK HERE to see some of my ancient racing videos.
06-18-2009, 02:53 PM
#28
Race Director
Tire weight.
I can tell the differece between a fresh tire and worn tire when my little 134 whp car tires to accelerate. The car gets up to speed faster on old tires. However the overall lap times are still with in my margin of driving error. So while acceleration may be reduced if you can corner faster with the wider tires your top speed could increase because you accelrate from a higher speed.
I can tell the differece between a fresh tire and worn tire when my little 134 whp car tires to accelerate. The car gets up to speed faster on old tires. However the overall lap times are still with in my margin of driving error. So while acceleration may be reduced if you can corner faster with the wider tires your top speed could increase because you accelrate from a higher speed.
06-18-2009, 03:19 PM
#29
Rennlist Member
Larry,
I only stated the obvious. Simple facts. I go on real world data as well, and use physics to find solutions for problems so I dont go chasing my tail by changing tires to find a 50hp problem.
The wheel weight will only account for a maximum of near 2hp (figure out the ftlbs of engine torque if you like) so it is a rounding error compared to the 6 mph difference in straight line speed on that particular straight.
Yes, I agree, road friction will probably be more than the dyno rollers, but keep in mind, 20hp at 150mph was all factors included, with it going down to 10hp at 80mph.
Absolutely toe can be a breaking force. to see its effects, toe in your car and try and push it. we all know its easier to push at near 0 toe vs .5" toe out or in.
I have no idea of its value, but I think in this case, the toe didnt change. we are talking about the rolling friction of a 1-1.5" wider tire. all other things being equal, its a rounding error at best. Remember I have times over 10 years with all sorts of sizes of tires and weights. Havent seen much differnce in acceleration, only lap times if the tires were worn, or were street tires.
We are talking a HUGE difference in straight line speed here. 6mph is massively different and a one size different tire size wouldnt make such a difference.
Ive given you the value of the intertial component. You can either except that or try and discredit that fact. Im taking into account your intuitive feeling of the extra weighs effect and providing actual values.
The only variables remaining, is driver inputs, engine performance or set up changes. We know total forces needed to get a car at a .36 Cd to 100mph and its around 50-60hp. Most all of that is aero drag. all you need to do to verify this, is to find a top speed of some low HP car and then work out the aero drag based on frontal area, Cd and speed. the difference will be friction.
We are talking some pretty small numbers here.
Ill also put 6mph in perspective. At thunderhill, i have run for over 15 years there and recently, had a 60hp bump in Hp. WITH NO OTHER CHANGES, i went from 122mph to 130mph. about 8mph. This is backed up by the telemetry out puts of simular time cars in our races that have that data.
Do, you are talking of an effect of near 50hp to make a difference of 6mph down a straight. (1/3mile or so long). Longer straights it will take less. You get the idea.
Now, why is he seeing 6mph difference? Lets post the data and take a look.
mk
I only stated the obvious. Simple facts. I go on real world data as well, and use physics to find solutions for problems so I dont go chasing my tail by changing tires to find a 50hp problem.
The wheel weight will only account for a maximum of near 2hp (figure out the ftlbs of engine torque if you like) so it is a rounding error compared to the 6 mph difference in straight line speed on that particular straight.
Yes, I agree, road friction will probably be more than the dyno rollers, but keep in mind, 20hp at 150mph was all factors included, with it going down to 10hp at 80mph.
Absolutely toe can be a breaking force. to see its effects, toe in your car and try and push it. we all know its easier to push at near 0 toe vs .5" toe out or in.
I have no idea of its value, but I think in this case, the toe didnt change. we are talking about the rolling friction of a 1-1.5" wider tire. all other things being equal, its a rounding error at best. Remember I have times over 10 years with all sorts of sizes of tires and weights. Havent seen much differnce in acceleration, only lap times if the tires were worn, or were street tires.
We are talking a HUGE difference in straight line speed here. 6mph is massively different and a one size different tire size wouldnt make such a difference.
Ive given you the value of the intertial component. You can either except that or try and discredit that fact. Im taking into account your intuitive feeling of the extra weighs effect and providing actual values.
The only variables remaining, is driver inputs, engine performance or set up changes. We know total forces needed to get a car at a .36 Cd to 100mph and its around 50-60hp. Most all of that is aero drag. all you need to do to verify this, is to find a top speed of some low HP car and then work out the aero drag based on frontal area, Cd and speed. the difference will be friction.
We are talking some pretty small numbers here.
Ill also put 6mph in perspective. At thunderhill, i have run for over 15 years there and recently, had a 60hp bump in Hp. WITH NO OTHER CHANGES, i went from 122mph to 130mph. about 8mph. This is backed up by the telemetry out puts of simular time cars in our races that have that data.
Do, you are talking of an effect of near 50hp to make a difference of 6mph down a straight. (1/3mile or so long). Longer straights it will take less. You get the idea.
Now, why is he seeing 6mph difference? Lets post the data and take a look.
mk
Mark, I do not have the knowledge of physics to either prove or disprove your calculations. What I do have is 36 years of automotive competition experience to help me evaluate things in a "real world" sort of way. Something has to be causing the effect because I believe in Brinkley's data.
You state that there is a total of 20 hp in friction at 150 mph. That is on the dyno, and there probably is a much lower cf between the tires and the roller than on a real road. And from the tire wear that I have seen on cars with too much rear toe-in, I know that there is a significant source of friction there. I can't quantify it, but I know it's there.
So if you think that I am wrong on all counts here, why is Brinkley seeing this "phenomenon"?
You state that there is a total of 20 hp in friction at 150 mph. That is on the dyno, and there probably is a much lower cf between the tires and the roller than on a real road. And from the tire wear that I have seen on cars with too much rear toe-in, I know that there is a significant source of friction there. I can't quantify it, but I know it's there.
So if you think that I am wrong on all counts here, why is Brinkley seeing this "phenomenon"?
06-18-2009, 03:23 PM
#30
Rennlist Member
Are you saying that the worn tire is lighter and thus provides faster acceleration?
The weight of a worn tire vs a new tire is also negligible. now, if it is cooked and lost stick, maybe it has a lower coefficient of friction, but ive dynoed new and bald tires and have never seen much differnce in the dyno runs .
So, we all hopefully know the effect of weight on the tire. the only variable here is the Cf (coefficient of friction) and driver/car power as well as exit speed beginnng the acceleration distance.
The weight of a worn tire vs a new tire is also negligible. now, if it is cooked and lost stick, maybe it has a lower coefficient of friction, but ive dynoed new and bald tires and have never seen much differnce in the dyno runs .
So, we all hopefully know the effect of weight on the tire. the only variable here is the Cf (coefficient of friction) and driver/car power as well as exit speed beginnng the acceleration distance.
Tire weight.
I can tell the differece between a fresh tire and worn tire when my little 134 whp car tires to accelerate. The car gets up to speed faster on old tires. However the overall lap times are still with in my margin of driving error. So while acceleration may be reduced if you can corner faster with the wider tires your top speed could increase because you accelrate from a higher speed.
I can tell the differece between a fresh tire and worn tire when my little 134 whp car tires to accelerate. The car gets up to speed faster on old tires. However the overall lap times are still with in my margin of driving error. So while acceleration may be reduced if you can corner faster with the wider tires your top speed could increase because you accelrate from a higher speed.