M758

Mark is right about all this. No mater what diameter rear tires you have the output of the engine Hp & Torque do not change.

I think we all know that engine perfomance is often measured at the flywheel. Changes to tires and gearing do not affect that. The only thing a drive train a gearbox does is to reduce power through various frictional losses.

Change the gears on a race car and you have ZERO impact on the power of the engine. Obvious of course. Change gears and you will have ZERO impact on power and torque as measured on at the wheels (assuming similar friction losses which is fair).

All a tire diameter change does is impact the final drive ratio a small amount. This has no impact on power to the groud.

What IS impacted is the gearing of the car. This will change the level of power at and given road speed. The reason is that you will be at a different point on your cars power band a the same speed. This can be very important.

For example.
Lets assume a well prepared and well driven car hits the apex point of a corner at 75.0MPH. This speed is governed purely by conering power of the car. Lets also say that this speed is 5000 RPM in 3rd gear. A quick check of the chassis dyno chart shows the car makes 190.5 wheel hp and 200 ft-lbs torque at 5000 RPM. Lets assume he is running tire diameter we will call A

So at this point with tire A, the driver has 190.5 whp and 200 ft-lbs of torque at his disposal to accelerate out of the corner.

Now lets suppose he changes to tire B which is 10% shorter than tire A. What changes?
Well speed at the apex would not since the 60 mph is governed by cornering power vs engine power.
The engine wheel hp and torque cruves do not change. What does change is his engine speed. Instead of 5000 RPM the engine is now 10% faster than before and is at 5500 RPM. Lets say now you go to the wheel dyno chart and look for the output and you get 209 hp and the same 200 ft-lbs of torque.

So therefore the running with tire B has more hp at his disposal than with tire A. Therefore he will be exiting the corner faster. Getting to the next corner fast really depends on how long and how many gear changes are required. Tire A may require a shift to 4th, but tire B may require a shift in to 5th before the next braking zone.
But as Mark said this not really more HP. All you did was move to a different part of the engine's power band. Also what would happen if your car had 5600 rpm red line? What is better Tire A or B. Now it really does depend.

Bottomline is tire diamter (in strict terms)only impacts gearing. What gearing is right depends on the track, your cornering speeds and your engine power profile.

In the real work smaller tire dimeters are often better for street cars convert to track use. Why? This is because the gear is often biased toward street driving where fuel economy and quite driving are important. A shorter tire means all the gears will be effetively closer together as a fucntion of road speed. So while you may no longer be geard to 200 mph, you can make better use of the power of the engine in speeds from 0-150 mph.
So in a way yes. Going to 18" wheels with an increase in diamter may give you less Horsepower to the ground at and given point in the track.
Example would be a friend's 924S. Going around the AMLS track at Las Vegas he was 3-4 seconds faster on 225/50R15 tires vs 225/50R16 tires. The 1" difference in tire diameter made a difference. He simply was out of his power band in many more corners. Of course other factors like wheel tire WEIGHT also make a difference. A heavier wheel & tire is bad for cornering and will increase the rotating inertia of the wheel and therefore make it slower to accelerate.

Bill Verburg

M758, I do believe that is what I said. Perhaps it is my fault for assuming too much but I really suspect that some folks do not read others posts. This is the last comment that I will make due to this failure to read.
Here is the forward thrust(what you feel in yoiur seat)(not engine torque)(the actual amount of force available to push the car forward, engine torque figures were obtained on a dyno) available from a 2.7 RS w/ stock gearing and 11.9" static loaded radius tires;
<img src="http://forums.pelicanparts.com/uploads/225_50.jpg" alt=" - " />
this one is the same car with 12.3" static loaded radius tires
<img src="http://forums.pelicanparts.com/uploads/245_50.jpg" alt=" - " />

M758

Bill I have read your posts. While everything you state is true you do neglect that two things are really changing in your graphs.

See your formula below

</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>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
</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">1) r Tire radius - Obvious
2) T Engine Torque - This is because the engine is running at a different speed. Match the RPM and the differene gets smaller.

Note also the LARGE effect changing from 1st to 2nd Gear has! This is HUGE. Of course we all know that we can run in first gear ever where. Sure a 1.7L 914 can have more Thrust than a 9ll Turbo, but how does that help him win beat the Turbo down the straight.

What you are missing is the concept of power. The car engine is basicly a generator. It provides a varible level of power output. It does this interms we call Hp and Torque. All the gearbox, final drive gear and tires do is two things 1) TRANSFER this power and 2) USE UP this power in friction. They cannot make more energy than what was created by the engine. You can gear a car in an way you want, but the power output will not change. The wheel forces change, but the total avalible power to move the car does not. To accelrate at a maximum speed you need to be at full power 100% of the time. Jet engines on airplanes for example provice a measure of thrust. That thrust is provided at a peak engine speed. To accelerate all you do is adjust the engines to provide that peak thrust and off you go. Acceleration is (neglecting air & other frictions) constant.

Now the internal combustion is very different in a car is very different. It provides a peak power level, but this can only be achieved at 1 particular ground speed for any set of gearing (including gear selected, final drive gear, and tires). Any acceleration immediatly takes you off of that peak power point. This we have gearboxes that can change gears to return the engine to its peak power point when ever possible.

So a tire diameter change just moves the peak power point on the engine relative to ground speed.

kahuna

I'm w/ Bill and Steve on this one. A tire acts like a simple machine, a shorter tire gives more mechanical advantage, than a taller tire. The result is more push form the shorter tire because overall gearing is numerically higher. It is also true that this will occur at a lower speed which may be what the others are implying.

mark kibort

Not necessarily. AGAIN, it depends. it depends on what speed you are starting and ending at.

go back to my last post. for every advantage in torque (and yes, bill and you are right about having a larger mechanical advantage at a GIVEN speed. however, DEPENDING on the speed, a smaller tire can produce LOWER final torque if ONLY the tire is changed on a two cars being compared with identical gear boxes.

Ideally, we would all love to have infinitely variable gear boxes. Then, it would not matter what tire size you used. NO advantage at ANY time would be gained by changing tire size then!!
Unfortunately, we have gear boxes and this is why it takes some thinking to choose the exact combination of gears, and tires for the job at hand. drag racing 1/4 or 1/8mile, 0-100mph, or road racing, will all require a certain gear box with certain ratios for a given cars inherent characteristics.
ALSO, going to a smaller tire doesn't make the gears closer together. the gear changes from one to the next is fixed. most street boxes are around a 134% change from gear to gear close ratio gear boxes and 6 speeds , this improves to 125% or better. Tire changes dont change this.

Hope that helps.

mark

</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Originally posted by kahuna:
<strong>I'm w/ Bill and Steve on this one. A tire acts like a simple machine, a shorter tire gives more mechanical advantage, than a taller tire. The result is more push form the shorter tire because overall gearing is numerically higher. It is also true that this will occur at a lower speed which may be what the others are implying.</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">

strudel

I recently switched from 245/16 to 275/17's on the rear of my '73 911 track/street car. The 17's are 10.3" tread width,25.3" high vs. the 245's which are 9.2" tread width, 24.5" high, all Hoosiers. My question is even though I may be getting better grip through the corners it could at the expense of the engine being out of it's ideal rev range and the added weight and rolling resistance of the 275's? The above posts are great and a wealth of info thanks for taking the time to reply.

ebaker

When comparing the relative "gearing" of different tires use the diameter or circumference or revs/mile. Do not use the "rolling radius" specification, that is the distance from the axle centerline to the ground on a loaded tire on a car.

mark kibort

You have just changed your final drive gearing by ( 24.5/25.3) or around 3%. to answer your question , I have to point you to my posts above. the information is there. It DEPENDS on car, turn speed, etc. could help, and it could hurt.
as far as increased weight, probably negligable, as the rim is usually lighter than the tire and sometimes going to a larger rim and shorter tire decreases the weight in the outer portion of the package. rolling resistance is also probably negligble.

mk

</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Originally posted by strudel:
<strong>I recently switched from 245/16 to 275/17's on the rear of my '73 911 track/street car. The 17's are 10.3" tread width,25.3" high vs. the 245's which are 9.2" tread width, 24.5" high, all Hoosiers. My question is even though I may be getting better grip through the corners it could at the expense of the engine being out of it's ideal rev range and the added weight and rolling resistance of the 275's? The above posts are great and a wealth of info thanks for taking the time to reply.</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">

911pcars

A visual clue as to the effects of increasing tire OD is represented by the rear tires on top fuel dragsters (thanks for allowing those words on this forum). These race slicks are designed to dramatically grow in OD with increasing wheel speed. The effect is to change the overall gear ratio (trans. ratio X rear axle ratio X ratio between OD @ 0 mph and + 0 mph.

As vehicle speed increases, the reduction in the overall gear ratio (numerically smaller by increasing tire OD in this case) allows the vehicle to attain a higher speed at the same engine speed. This is at the expense of torque (which isn't really needed as much at 200 mph). This is a one method of having a variable ratio gear box.

I don't know if this contributed anything significant other than describing the effects of tire OD in another manner.

Have a nice day,
Sherwood