crazy idea for 180mph speedo in any 240.... i think it would work too...

[nismofly]

its still going to be horribly inaccurate

you might as well do the calculations with tire size, gearing, rpms, etc...to figure out speed @ rpm in gear X

[JimmyMethod]

Think about it this way, you can get away with speeding really bad if you know it's reading really low... at least for that first ticket.

[drifter_for_life06]

only if the cop that stops you is....not a cop.....they see how fast your going on the radar gun or whatever that particular cop is using

[masticatingcow]

Okay, I'll be honest. 2BN, you suggested that the change in suspension geometry would contribute to an inaccurate speedometer. I'm trying to piece that together in my head... and no, I didn't search. Shoot me. LOL.

During high speed passes, the car experiences lift. The lift is what causes the suspension to change. As the car rises, the wheels move in their relation to the ground. Still, the speed sensor is in the TRANSMISSION. It reads off the driveshaft, not the rear wheels.

I can understand how lift would change the wheels' aspect to the ground, but that shift in aspect does not change the distance each wheel travels by any significant degree. Yes, under lift, the wheel diameter will be closer to perfectly round, but we're talking about MAYBE half an inch. Probably less, since most of the lift is/should be absorbed by the suspension. And that's on a 15 inch wheel with meaty tires and thick sidewall. On a lower profile wheel, the difference is even more miniscule.

So where does suspension geometry so radically affect the speedo reading? It sounds like you're saying that at a given speed, the speedometer will SUDDENLY read off because of a sudden change in the suspension geometry. I don't think that's the case. I think you're right in considering the tiny changes the suspension makes at speed, but I think these changes and imperfections exist all the time, and that the amount by which the speedo will be off will increase linearly as speed increases. If this is the case, it can be calculated and compensated for, which will provide a more accurate reading at higher speeds.

[PantherRacer]

Racing Excuse #29: I was testing out my new speedometer...

[drifter_for_life06]

wonderfuly said masticatingcow

thats such a horrible excuse panther....but im sure people do use it

[Dattebayo]

A half an inch makes a big difference in how accurate your speedo is, interestingly enough. Its sort of like the theory is exponential, and I will use a parrallel idea that you are familiar with: changing tire size.

When you change your tire size two things happen. First off you gain some amount of castor. The castor effects how much traction you have going in a straight line and when experiencing lift, your tire may actually slip a bit with respect to how much lift the car is experiencing. There fore you can gain a little measured distance in the VSS by losing some of your friction.

Second, your wheel is larger, therefore the ratio of [number of rotations] to [time] has been altered. This is the equation for speed, figured out by your VSS. The VSS couns rotations by one full turn of the driveshaft = one rotaion of the rear wheel in this case. Now you can see that when you add it all up: the faster you go, the farther off you are from the actual speed. Its pretty simple when you think about it...

[Dattebayo]

Those two ideas put together can make it a real problem to measure your accurate speed without an independant point of reference. A device like GPS, radar (not so relaible), laser, a echo-location device of somekind, you get the idea.

Now when the actual lift comes into play with a stock sized wheel, the tire is lifted up a bit, adding a little bit of size to it's rotaion and a little loss of friction, the two basic elements in the equation used for measuring speed. Every mile an hour measured gains a percentage of error that adds to the next measured mile an hour.

Points? ANything I missed? Ideas?

[masticatingcow]

Well put. I see what you're saying and completely agree. But these factors, such as the shift in caster, increase in wheel diameter, etc, seem to occur predictably... how hard would it be to calculate these changes and make the speedo read more accurately?

[Dattebayo]

Im sure with your own team of crack scientists and a world of test equipment anything is possible.

AN idea would be to have some kind of sensor keep an eye on the pavement, but you would need reference marks... But just so you know, 180 mph on a dyno is the closest thing to 180 you would see to the speedo working accurately.

the tire is lifted up a bit, adding a little bit of size to it's rotaion and a little loss of friction, the two basic elements in the equation used for measuring speed.

I would like to change this as I have made a mistake here. Friction is not part of the equation for speed, it is distance traveled. The loss of friction can affect the distance traveled; this is what I meant.