AT what point would the wheel & tire size decrease torque
05-22-2003, 08:19 PM
#1
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Rennlist Member
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Thread Starter
AT what point would the wheel & tire size decrease torque
I have been told by someone I trust, that for me to move to an 18 wheel would actually decrease my torque. I have an 88 911 with 17 X 8 & 17 X 9's.
Any insight, he could give an explaination.
Any insight, he could give an explaination.
05-22-2003, 11:14 PM
#2
Any increase in the rolling radius of a tire will decrease the effective torque. Any decrease will have the opposite effect.
Tires on 18"rims are not necessarily going to be taller than tires for a 17" rim are.
You need to get the physical tire dimensions from the manufacturers(or measure them yourself) to compare what effect a tire will have.
Tires on 18"rims are not necessarily going to be taller than tires for a 17" rim are.
You need to get the physical tire dimensions from the manufacturers(or measure them yourself) to compare what effect a tire will have.
05-23-2003, 03:14 PM
#3
RL Technical Advisor
Sleder:
Bill V. is right on target!
You need the tire dimension information and revs/mile to make any accurate predictions on tire sizes and acceleration/speed.
Bill V. is right on target!
You need the tire dimension information and revs/mile to make any accurate predictions on tire sizes and acceleration/speed.
05-24-2003, 03:38 PM
#4
Changing the overall diameter of the tire/wheel assembly, like differential or gearbox gears, affects the torque at certain vehicle speeds. Using a larger tire OD is similar to using 5th gear instead of 4th at the same vehicle speed. For example, 5th gear will provide less torque at 40 mph and engine rpm will be less compared with 4th gear which will provide more torque at 40mph (engine rpm will be higher and closer to when max. torque develops). However, at the same engine speed, 5th will provide higher vehicle speed.
The net effect is that the available engine torque doesn't change, but the available engine torque range shifts to a higher vehicle speed (larger tire) or lower vehicle speed (smaller tire).
Hope this is not too confusing.
Sherwood Lee
<a href="http://members.rennlist.org/911pcars" target="_blank">http://members.rennlist.org/911pcars</a>
The net effect is that the available engine torque doesn't change, but the available engine torque range shifts to a higher vehicle speed (larger tire) or lower vehicle speed (smaller tire).
Hope this is not too confusing.
Sherwood Lee
<a href="http://members.rennlist.org/911pcars" target="_blank">http://members.rennlist.org/911pcars</a>
05-24-2003, 04:36 PM
#5
Sherwood, not quite true. The radius of the tire is a lever arm, the forward thrust is at the short end of an imaginary lever arm and the engine torque is at the long end. Visualize a lever used to lift a heavy object, for maximum lifting force the fulcrum needs to be as close as possible to the object being moved.
The math is as follow
Forward thrust = (T*G*F*E*12)/r
T - Engine torque @ a given rpm in ft-lbs
G - Gear ratio
F - Final drive Ratio
E - efficincy of the drivtrain
r - rolling radius of the tire in inches
Forward thrust in pounds
If you run some #s as a sim you will see that net thrust is reduced. but as you noted max speed if not aero or power limited is increased.
I have a really nifty ss(developed by some other Rennlisters) showing the effects if you are interested.
The math is as follow
Forward thrust = (T*G*F*E*12)/r
T - Engine torque @ a given rpm in ft-lbs
G - Gear ratio
F - Final drive Ratio
E - efficincy of the drivtrain
r - rolling radius of the tire in inches
Forward thrust in pounds
If you run some #s as a sim you will see that net thrust is reduced. but as you noted max speed if not aero or power limited is increased.
I have a really nifty ss(developed by some other Rennlisters) showing the effects if you are interested.
06-20-2003, 05:18 PM
#6
Rennlist Member
Sherwood is actually more on track than your post Bil.
no, no change in ave torque to the ground or applied Hp will occure with the change of a wheel size over the operating range. will you gain or loose torque in any gear? yes, but at a particular speed, you could loose torque just as easily.
actually, most wheel changes are mated with tires with proportionate changes in profile, so the rolling diameter remains constant. (ie an 18" rim with at 275 35 is the same diameter as a 17" rim with a 275 x 40)
even if you did a radical change in diameter, the only change is the effective gear ratios in each gear . (ie change the diameter 1" on a 25" diameter tire, and you have a 4% change in gear ratio)
Bill your formula and analogy is not correct either. HP is torque time speed. gear box ratio changes do not create or destroy HP. they change the profiles. And, over a desinged use of the HP over an application (ie road racing at a specific track or drag racing) you can either gain or loose EFFECTIVE HP by changine gear ratios either higher or lower. The larger diameter wheel just " gears up " the effective gear ratio. gear ratios multipy torque, and reduced speed. since we all have multiple gear ratios, you have to look at the loss in one gear and the gain in another! again, gears and gear ratios do not effect or change HP or torque.
In a nutshell, there is no way to say whether a larger diameter tire will gain or loose effective hp. for every gain, ill show you were you would loose. And, its not about top end speed. its about efficient application of HP, or said differently, used area under the HP curve of a given engine and drivetrain and application.
example: 300hp 300ftlbs of torque 6000rpm max rpm
5speed. shift points 40,70,100,145,180mph vs 44,77,110,160mph
10% taller tires vs normal tires (extreme example)
1st gear car with smaller tires gets the edge on torque (10%), until 40mph, now the tall tire car has an advantage until 44mph (35% torque advantage)(however long it took to get from 40-44mph, the taller tired car had the 35%advantage, vs the smaller tire 10% advantage from 0-40mph.)
same thing happens in 2nd
now, it comes to 3rd. smaller tired car had the advantage from 77mph to 100mph, but now has to shift to 4th, and the taller geared car has a 35% advantage from 100 to 110 staying in 3rd. KEY THING, since the top speed of this car will be around 110 in the 1/4 mile clicking off a 12ish second time, the time spent in 3rd gear is longest, and the 100mph to 110mph time is the longest period as well. the taller geared car actually has a 35% torque advantage over this period of time.
You can see in any application there could be cases for taller or lower gears or larger or smaller tires. Since the example is 10%, that is a 2" diameter diff. in real life circumstances, 4% is more of a big deal, and 2% is common.
Hope this helps
Mark
no, no change in ave torque to the ground or applied Hp will occure with the change of a wheel size over the operating range. will you gain or loose torque in any gear? yes, but at a particular speed, you could loose torque just as easily.
actually, most wheel changes are mated with tires with proportionate changes in profile, so the rolling diameter remains constant. (ie an 18" rim with at 275 35 is the same diameter as a 17" rim with a 275 x 40)
even if you did a radical change in diameter, the only change is the effective gear ratios in each gear . (ie change the diameter 1" on a 25" diameter tire, and you have a 4% change in gear ratio)
Bill your formula and analogy is not correct either. HP is torque time speed. gear box ratio changes do not create or destroy HP. they change the profiles. And, over a desinged use of the HP over an application (ie road racing at a specific track or drag racing) you can either gain or loose EFFECTIVE HP by changine gear ratios either higher or lower. The larger diameter wheel just " gears up " the effective gear ratio. gear ratios multipy torque, and reduced speed. since we all have multiple gear ratios, you have to look at the loss in one gear and the gain in another! again, gears and gear ratios do not effect or change HP or torque.
In a nutshell, there is no way to say whether a larger diameter tire will gain or loose effective hp. for every gain, ill show you were you would loose. And, its not about top end speed. its about efficient application of HP, or said differently, used area under the HP curve of a given engine and drivetrain and application.
example: 300hp 300ftlbs of torque 6000rpm max rpm
5speed. shift points 40,70,100,145,180mph vs 44,77,110,160mph
10% taller tires vs normal tires (extreme example)
1st gear car with smaller tires gets the edge on torque (10%), until 40mph, now the tall tire car has an advantage until 44mph (35% torque advantage)(however long it took to get from 40-44mph, the taller tired car had the 35%advantage, vs the smaller tire 10% advantage from 0-40mph.)
same thing happens in 2nd
now, it comes to 3rd. smaller tired car had the advantage from 77mph to 100mph, but now has to shift to 4th, and the taller geared car has a 35% advantage from 100 to 110 staying in 3rd. KEY THING, since the top speed of this car will be around 110 in the 1/4 mile clicking off a 12ish second time, the time spent in 3rd gear is longest, and the 100mph to 110mph time is the longest period as well. the taller geared car actually has a 35% torque advantage over this period of time.
You can see in any application there could be cases for taller or lower gears or larger or smaller tires. Since the example is 10%, that is a 2" diameter diff. in real life circumstances, 4% is more of a big deal, and 2% is common.
Hope this helps
Mark
06-20-2003, 05:50 PM
#7
Mark if you bothered to read and think about the post I never mentioned HP
The formula is the correct formula for FORWARD THRUST at a given output torque which will obviously change with any change in RPM
If you plot the torque @ different values(oh say for instance torque @ each 200RPM) the resulting graph will show you the forward thrust @ that point of the rpm curve with that gear and tire.
the result will look like this
<img src="http://forums.pelicanparts.com/uploads/Thrustcurve.jpg" alt=" - " />
and if the tire height is changed(or any of the other input parameters) the curves will correspondingly change
The formula is the correct formula for FORWARD THRUST at a given output torque which will obviously change with any change in RPM
If you plot the torque @ different values(oh say for instance torque @ each 200RPM) the resulting graph will show you the forward thrust @ that point of the rpm curve with that gear and tire.
the result will look like this
<img src="http://forums.pelicanparts.com/uploads/Thrustcurve.jpg" alt=" - " />
and if the tire height is changed(or any of the other input parameters) the curves will correspondingly change
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06-23-2003, 07:52 PM
#8
Rennlist Member
Bill, re-read, very carefully, my post.
It really is all about HP. Torque discussions will just confuse anyone trying to understand the complete picture. Your torque in any gear will go up in proportion to the amount you have reduced the overall gear ratio (reducing tire diameter). However what you really whant to kow, is the average torque or HP put to the ground over the operating range. That said, by changing
the tire diameter, the torque will go up, but your speed (MPH) on your curve will proportionally go down for every gear. so, your comment is correct for torque produced at a given (2000rpm example) rpm. However, you have to match the MPHs as you have on your graph, and they would go down proportionately.
The net result is ave torque to the wheels over the operating range, or better said, area under the Hp curve over the operating range. (ave works fine for our discussion.)
Now, if you had an identical car to a guy you were going to race , and there were no gears, (ie lets race in 1st gear from point a to point b) you may have a point. However, since we all have gears and we use them, changing final drive ratios or tire sizes, do very little to change in making a car acclerate faster , do to the trade offs in shift points and the LESS torque that the next gear would be developing after an earlier shift where there was a brief advantage. The most anyone can hope for is a slight gain in efficiency of the HP applied to the wheels. the best way to increase HP applied to the wheels is with a closer ratio gear box. this often yields a 8% area under the curve advantage for racing applications.
Hope that helps.
Mark
It really is all about HP. Torque discussions will just confuse anyone trying to understand the complete picture. Your torque in any gear will go up in proportion to the amount you have reduced the overall gear ratio (reducing tire diameter). However what you really whant to kow, is the average torque or HP put to the ground over the operating range. That said, by changing
the tire diameter, the torque will go up, but your speed (MPH) on your curve will proportionally go down for every gear. so, your comment is correct for torque produced at a given (2000rpm example) rpm. However, you have to match the MPHs as you have on your graph, and they would go down proportionately.
The net result is ave torque to the wheels over the operating range, or better said, area under the Hp curve over the operating range. (ave works fine for our discussion.)
Now, if you had an identical car to a guy you were going to race , and there were no gears, (ie lets race in 1st gear from point a to point b) you may have a point. However, since we all have gears and we use them, changing final drive ratios or tire sizes, do very little to change in making a car acclerate faster , do to the trade offs in shift points and the LESS torque that the next gear would be developing after an earlier shift where there was a brief advantage. The most anyone can hope for is a slight gain in efficiency of the HP applied to the wheels. the best way to increase HP applied to the wheels is with a closer ratio gear box. this often yields a 8% area under the curve advantage for racing applications.
Hope that helps.
Mark
06-23-2003, 08:01 PM
#9
Rennlist Member
Bill, so your graph is correct, but it can be a little misleading if the part about the increase in torque isnt footnoted with " at a proportionate lower or higher speed" . This way, folks that are wondering if their cars will be faster or slower, wont be mislead into thinking their acceleration will be hampered by a larger diameter wheel and tire. for this answer, we need to know the race . Drag race length, hp curves, gear ratios, etc. And for a road course, the same things will applie. Otherwise, racers would just use the lowest final drive gears and the smallest tires they could get a hold of!!
Mk
Mk
06-23-2003, 08:10 PM
#10
Mark, reread my posts carefully!! I never mentioned torque either(other than that measured on a dyno at various rpm). Forward thrust is the # of pounds pushing you linearly forward. That is the only thing you care about!!
06-23-2003, 08:21 PM
#11
Rennlist Member
No, that's not all you care about. You have to know the speed at which that torque or "thrust" is happening. Work, or accelerating a mass over a distance over time, requires HP and is HP by definition. Torque means nothing if speed is not attahced to it. If I can personally drive a car with a long enough lever to over 1,000,000lbs of thrust. (ie less than one hp) . once I attach a speed to it, we then have Work and HP!!! NOW, this is ALL we care about. again, more accurately, ave hp applied to the ground over time. or ave torque over time potted agaist distance. once you only look at HP, most of the confusing work is already done for you. (ie speed factored in) and then you dont have to worry about it.
Mk
</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Originally posted by Bill Verburg:
<strong>Mark, reread my posts carefully!! I never mentioned torque either. Forward thrust is the # of pounds pushing you forward. That is the only thing you care about!!</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">
Mk
</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Originally posted by Bill Verburg:
<strong>Mark, reread my posts carefully!! I never mentioned torque either. Forward thrust is the # of pounds pushing you forward. That is the only thing you care about!!</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">
06-24-2003, 02:10 PM
#13
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Hey guys, I don't want to be pedantic...but I will. Tyre diameter has no effect on torque - as teh torque supplied to rear axle is determined only by engine torque and gearing. However, as Bill & others may suggest, changing the total diameter of wheel & tyre will effect how that torque is converted in forward motion of hte vehicle.......thrust is a good term, hey Bill.
Sleder's initial question I think refers to the conversion of torque into thrust and how it may change with tyre diameter. The discussion of how the tyre diameter relates to the torque curve of the engine is probably more info than he was asking for in the first place.
Sleder's initial question I think refers to the conversion of torque into thrust and how it may change with tyre diameter. The discussion of how the tyre diameter relates to the torque curve of the engine is probably more info than he was asking for in the first place.
06-24-2003, 04:30 PM
#15
Rennlist Member
Simply put, it depends!! (on speed at which you will be acclerating for a given gear box, engine, and Tire diameters) Thrust changes too.
You give me a particular speed, and Ill show you two identical cars where the thrust can go DOWN for an decreased tire diameter or increased final drive ratio. this is my key point.
Again, its HP or better said, area under the HP curve (need a little calculus here, but averages can work for our discussion) that is put to the ground over the operating range! Thats part of the "All" you care about.
to answer the question correctly, of will a larger diameter tire hurt my torque? If the question is asked in the spirit of racing, or performance, then the answer is "It depends" and more than not, it wont mater measureably in the area of performance.
Mk
You give me a particular speed, and Ill show you two identical cars where the thrust can go DOWN for an decreased tire diameter or increased final drive ratio. this is my key point.
Again, its HP or better said, area under the HP curve (need a little calculus here, but averages can work for our discussion) that is put to the ground over the operating range! Thats part of the "All" you care about.
to answer the question correctly, of will a larger diameter tire hurt my torque? If the question is asked in the spirit of racing, or performance, then the answer is "It depends" and more than not, it wont mater measureably in the area of performance.
Mk