calculating mph from tire circumference?

[entropy]

I'd like to start a new thread on this topic because my other discussion got diverted a bit.

The issue:
TFA needs to bring the correct alternate gearing (quick change) to WOS to get Don & my B licenses.
Our lakester has about 350 RWHP (Dynojet chassis dyno) and will start with our tall gearing (2.19), then switch to more aggressive gearing (e.g. 2.56) if we have trouble averaging the required B speeds (175-200mph) on the short course.

With Excel, we can quickly calculate vehicle mph:
- free standing tire circumference = 91.25" (Goodyear Front Runners)
- final gear ratio, currently = 2.19,
- max hp at engine rpm = 6100rpm
     theoretical mph @ 6100rpm = 241mph

BUT, BUT, BUT...
My asphalt bike experience (many passes) taught me I needed to subtract 3.8-4.2% off the measured tire circumference to get the calculated mph to match trap/GPS mph.  With that adjustment factor i could predict actual mph to about 1.0mph from tire circumference.

My salt bike's events (only 2) showed about 6% adjustment was required to get calculated mph to measure trap/on board GPS mph.

Why the adjustment is needed is a mystery to me (slip?, effective diameter decrease? gremlins?), but no matter, I just want to accurately predict trap/GPS mph so TFA can bring the appropriate alternate gearing to WOS.

My questions:
1. have i completely confused everyone?
2. has anyone else experienced the need to add an "adjustment factor" when predicting actual mph on salt?

I am a total rookie on 4 wheels, so thanks for any help you can provide,
Karl
TFA Racing

[hotrod]

Why the adjustment is needed is a mystery to me (slip?, effective diameter decrease? gremlins?), but no matter, I just want to accurately predict trap/GPS mph so TFA can bring the appropriate alternate gearing to WOS.

How are you measuring the tire circumference? Are you calculating it or measuring the actual roll out of the tire. The effective tire size is actually the rolling diameter (axle to tire contact patch) the way to measure roll out is to put a mark on the tire and then push the car on a flat surface so that the tire rolls several full resolutions and then measure that "roll out" distance for that number of revolutions. That will give you the actual effective circumference of the tire at low speed. Then you have to account for two other effects. Tire growth at speed due to centrifugal force, change in air pressure as the tire heats up and what ever changes in axle loading occur due to lift and aerodynamic forces.
Last you have to account for slip which is probably most of your "fudge factor".

Unfortunately I know of no easy way to measure the tire growth and he wheel slip.

You can get an idea of wheel slip if you can measure the difference between the rpm of one of the front wheels and the powered wheels at speed. Tire growth is much more difficult to measure without fancy equipment like high speed cameras. You might be able to get an idea about tire growth by careful observation on the dyno at some reasonable wheel speed.

Best bet is trial and error, make passes and keep good records of all the involved factors and then work out a formula that works for your car.
A trip to a drag strip and careful data collection during a run could be used to get real world info on tire growth at some reasonable speed.

[bbarn]

The correction that you are making is because of tire growth. I never new that tires grew so dramatically until I saw it first hand.

When comparing the wheel speeds on our liner (Front to Rear) I noticed an issue right off. The front wheels are solid aluminum and the rears are MT 30" B'ville tires. With ZERO slip and an increasing speed, the rear wheels actually slow down compared to the front wheels (rpm). This is because the rear tires with rubber on them grow thereby changing the gear ratio. (Video to prove it: http://youtu.be/CBhiJNbB6KA?list=UUudh2K7DaZfmUIiOXH-DYzw)

What we do with our calculations is estimate the tire growth for a particular speed. You can do a test similar to ours and extrapolate what the diameter of the wheel will be at a given speed and create an offset column in your spreadsheet for calculations. For licensing, you have a pretty broad window, I would use your guestimate numbers or do a max speed static test and measure the diameter. It is a fairly linear curve from static to full RPM growth. With that you can estimate the diameter at a particular speed more accurately.

[MAZDA1807]

Are you comparing your calculated MPH to what your time slip says? If so it might be different due to the time slip mph is over a measured distance and not at an instant spot.

[jl222]

  I've used the Bonneville Pro computer program for many yrs, were you can input tire circumference and gearing plus hp- cd-frontal area ECT and calculates your speed ,time and distance on track and rpm.

 Several yrs ago when we made it to the exit speed traps the rpm we recorded didn't match the speed, until I added 11/2 inches to circumference on 28x15x10 tires.

                JL222

[manta22]

Has anyone compared bias-ply tire growth with radial-ply tires?

Regards, Neil  Tucson, AZ

[tortoise]

The correction that you are making is because of tire growth.

The correction he is making is in spite of tire growth.

[entropy]

The correction that you are making is because of tire growth.

The correction he is making is in spite of tire growth.

Thanks for all yr thoughts, but tortoise put his finger on it.
The correction is negative...  And exceedingly consistent on asphalt, not much info on salt.
This why i call it "slip", not slip. 
As some of you have seen, tires sure grow with rotation speed.  Easy see it on the dyno if you look closely.
It is likely that the discrepancy is a combination of factors

Measuring rollout rather than free standing circumference with a soft tire could show a smaller radius/circumference. 
7-8 years ago I did tests both ways and didn't see much difference and free standing circumference is much easier to measure reliably.

Trial & error is indeed how i found out about this negative factor. 
I am hoping to find a car person who carefully measures their tires and associates the calculated speed with GPS or trap speed.

[jl222]

 I've used the Bonneville Pro computer program for many yrs, were you can input tire circumference and gearing plus hp- cd-frontal area ECT and calculates your speed ,time and distance on track and rpm.

 Several yrs ago when we made it to the exit speed traps the rpm we recorded didn't match the speed, until I added 11/2 inches to circumference on 28x15x10 tires.

                JL222

  OPPS that 11/2 inches was added to the tire diameter, not circumference, the older Bville Pro asked for diameter, the newer program ask for circumference.

          28'' verses 291/2'' or 4.73'' added to circumference.

    We just input for 29'' or 1'' of tire growth 91.12'' in Bville program as not sure of growth until time slip and rpm thru 5 mile exit trap.

                       JL222

     

[Hot Rod Lincoln]

http://www.wallaceracing.com/calcmph.php

I found this calculator from Wallace Racing to be very helpful in 2012. Two different cars were within 2 mph from what the calculator estimated.

[entropy]

Guys,
Thanks much for your input.
It seems that no one has experienced the need to put a negative factor on the gear/mph calculations to make them match the actual GPS/trap speeds.  Maybe car tires respond different motorcycle tires?
We will bring extra gear sets to bracket possibilities

Our new lakester has Racepak GPS + front wheel(powered) & rear wheel (passive) sensors. 
On early passes we'll calibrate the sensor output to match GPS measurement, so we will hopefully get good data.

[Stainless1]

Karl, on our under-powered Lakester we usually figure that tire growth will equal slip within a per cent or less. 
Do you think you were seeing clutch slip on the top end causing you to need negative correction?  That is the first place we look when RPM and trap speed don't match.
We have an excel sheet we have used for years... it is bike based but if you send me your info I will put it in a copy and send it to you... works for cars when you use 1 for primary gearing.

[fredvance]

I have, and still do, used Karls corrected spreadsheet for years and it is spot on!!

[Sumner]

Karl, on our under-powered Lakester we usually figure that tire growth will equal slip within a per cent or less...

That has probably been the case with us as well as our spreadsheet seems to be within a mph or so of our data rpm from under 150 to 250 using the same tire diameter.

How critical is this anyway?  They give you a 25 mph range for each license pass.  Shoot for the middle as you don't get points for running say 174 on the 150-175 license but you will not like the results if you error and go over the 175.  I ran fairly close to the upper end of the range last year since we had a lot of data from all the times Hooley ran and I trusted it.

Karl I think the one item for you guys that will be the bigger unknown is the automatic transmission and running a torque converter if I remember correctly that that is what you are running.  The converter slippage could be a bigger factor than any tire growth.  Until you know what that is I'd look for an rpm about 6-10 mph under the max for that license.

Also I'd look for a speed that has an easy to read number on the tack.  If say 165 is 5344 rpm then I'd look at what 5300 or 5400 was. 

Also to move to the long course you have to be 175 at the 2 1/4 so you might not be running in high.  I'll bet for you that second will be the gear to probably get it done in.  I think you are using a 3 speed 350 transmission??  I didn't use 4th until my A license run so doubt you will be running 3rd until over 200. 

If you need to figure your speeds in 1st and 2nd the spreadsheets here let you do that easily...

http://purplesagetradingpost.com/sumner/bvillecar/bville-spreadsheet-index.html

... and they will quickly show you speeds at easy to read tach numbers,

Sum

[entropy]

Karl, on our under-powered Lakester we usually figure that tire growth will equal slip within a per cent or less. 
Do you think you were seeing clutch slip on the top end causing you to need negative correction?  That is the first place we look when RPM and trap speed don't match.
We have an excel sheet we have used for years... it is bike based but if you send me your info I will put it in a copy and send it to you... works for cars when you use 1 for primary gearing.

Stainless, thanks for the offer of yr spreadsheet, but i'm thing i'm Ok in that respect.  I always used stout lockup clutches so i doubt that clutch slip was the source of the "slip".  The correction factor was so consistent that i gave up looking for a source and just used the derived factor.
Last night both Tom Burkland and Sumner pointed out the likelihood of tq convertor slip, maybe in the 10% range.  DumbAzero, bike guy me never thought of that, and a 10% adjustment makes our recent dyno data make sense (202mph at 5700rpm).
Karl