Help coming up with MPH... Sprint Car

[loudandproud]

Hey guys...

I just started wrenching on a local 358 sprint car team and learning the ropes. They were a top five team last year with a couple wins under their belts.
One of the things I noticed is alot of scattered notes. Im helping the guys use excel to come up and organize alot of different information.

Right now Im working on a gearing chart/dyno chart crossreference.

One variable i need to come up with is how many inchs the car travels per revolution of the rear axle.
Would be super simple with a non-staggered rear tire... but how do you do it with a staggered rear car?

Say we are running a 94" lr tire and a 104" rr tire... how would you calculate the MPH?

I would guess the distance per revolution (tire that is) would be (94+104)/2 = 99 in/rev.
But then I thought about how the car loads the right rear super heavy and thought the car might be driven more by its larger diameter...

So then i though of something like this, as simple expression that trys to take this in effect... x1(rr tire)=104 x2(lr tire)=94 y=(x1-x2)*.75 ... then rev/min would = x2+y == 101.5 in/rev.

There is no way to be super accurate with this, but im just trying to come up with a baseline. A 3" jump in rr tire diameter is not uncommon from the heat to the feature so I want to be able to see how much its going to affect our gear...

[af2]

But then I thought about how the car loads the right rear super heavy and thought the car might be driven more by its larger diameter...

Yep that is what I looked at when running stagger. The outside tire will also grow. How much is a guess.

[Unkl Ian]

Radar gun at the end of the straight ????

[af2]

Radar gun at the end of the straight ????

Dammit Uncle we did not have that in the 80's.

[Barbapapa]

Where do you even get a radar gun?

In road racing we have things like Traqmate that use accelerometers and GPS to give you nice data reports of what the car did with speeds, cornering and braking forces etc. You can pour over the graphs in between sessions.
There are even iphone apps that do something similar but with presumebly less accuracy.

[Unkl Ian]

Where do you even get a radar gun?

Rent one: http://www.radargunrentals.com/

Or buy one: http://www.radarguns.com/

[barnym17]

Use laptimes to get avg mph.Or ask the normally present deputies there for security if they will use their handheld gun on your car.

[Bubstr]

I could go along with a radar gun if you really wanted to know what your speed is. Application of this knowledge is where I may differ with some. I would be looking at exit speed in the corner. This is where the money is made. There is an old adage in dirt circle track racing, "Corner speed is King". What your corner speed is and traction determine your gear selection to take advantage of it. Every engine has a working horsepower curve. You want to match your exit speed and traction to the correct part of your curve and you let it eat at the end of the strait. There is always a sweet spot where your gear raises or lowers torque at the tire, at exit to help you maintain traction and it is dependent on corner speed and traction. Building an engine to help along this purpose is sometimes confusing also. One with usable horsepower is usually faster than one with maximum horsepower if it creates a sharp rise in wheel torque in the part of the curve you use getting off the corner. This does not mean you can turn the gas pedal into a on / off button, it's just to help the driver. Usually when your driver comes back and says "I can't get in the gas like the other guys", it means you didn't go around the corner as fast and has little to do with gear. It's like drag racing against someone with a rolling start. You start off in the hole and end up in the hole.

[MadBill]

There're plenty of cheap, accurate GPS-based speed measurement devices available, as well as free or nearly so phone apps, e.g.: https://play.google.com/store/apps/deta ... free&hl=en. Propping one up alongside the tachometer and aiming a video camera them it will tell you a ton about handling, gearing, RPM range, etc. Most of the phone apps will even do a speed trace... Add a light that come on at WOT; who could ask for more?

[governor]

mount a GPS in for a couple of runs, it will give you peak and ave MPH.

Gov