Cal Club Autocross Forums

My son Max has a big physics project due in May. I was thinking he could test how tire pressure affects adhesion. Basically we'd have to set up a skidpad, find a kart for him to drive, locate a G meter and measure the maximum Gs at five different inflation rates (two overinflated, two underinflated and the correct inflation). To make sure the measurements were accurate, we'd do each measurement three times, for a total of 15 runs. I figure it'd take 30-45 minutes. I was thinking we might do the measurements at one of our events, during lunch or something. Is this something we'd even be allowed to do? If so, would I just try and work it out with the event master? Enquiring minds want to know.

On a practice day we ought to have enough room to setup a skid pad. We'd need to borrow an SSS and a youth steward to supervise, but it wouldn't take long.

I think the MaxQdata units are pretty easy to transfer. So, just find somebody willing to loan for an hour. We might even have one in CASOC you can use.

does he have a hypotethis?

wondering what physics he used to come up with it......

and I know I can't spell....

One of the problems is that the world is complicated....

The results would only be applicable to that style kart.

Some karts, like Roberts have so little power but lots of grip. They have a spec gearing, so more airs means greater rear tire diameter and more top speed.
Karts are fixed axle, so at some speeds more tire pressure might lower grip and allow the rear to free up and turn better. One tire has to slide in order to turn so grip can hurt.
You could actually go faster too in a kart if you could increase the outer tires diameter (and decrease the inner) in a skidpad test with biased air pressure.

Here's an interesting one... Weight. We could ballast the kart to different weights and test the effect on skidpad G's.

No hypothesis yet. Before we get to that point, I'm going to have to sit down with him and talk through what might happen to a tire if it was over or under inflated and how that might affect grip. I just came up with the idea about two hours ago, so we're sort of doing it bass ackwards. I just wanted to figure out if it was even feasible. Sounds like with a little help from my friends (apologies to Lennon and McCartney) it's do-able.

Steve Ekstrand wrote:One of the problems is that the world is complicated....

The results would only be applicable to that style kart.

Some karts, like Roberts have so little power but lots of grip. They have a spec gearing, so more airs means greater rear tire diameter and more top speed.
Karts are fixed axle, so at some speeds more tire pressure might lower grip and allow the rear to free up and turn better. One tire has to slide in order to turn so grip can hurt.
You could actually go faster too in a kart if you could increase the outer tires diameter (and decrease the inner) in a skidpad test with biased air pressure.

Here's an interesting one... Weight. We could ballast the kart to different weights and test the effect on skidpad G's.

I like that one. Plus then I'm not screwing with someone else's tires.

I wish my physics projects were this cool. Well we did have that day with the potato gun...

Ethan West wrote:I wish my physics projects were this cool. Well we did have that day with the potato gun...

That's why I want to do something like this...to show Max that physics is the most fun of the sciences (apologies to the chemists, biologists and earth scientists out there) because it's real-world. It's everything we do every day.

the main problem is the change in grip from tire pressure adjustments is going to be incredibly difficult to measure. like less then .1g different. well within any noise that will be present on an accelerometer signal, and probably well within the driver consistency noise if you were to use a timer and calculate average acceleration on a ~100ft skid pad.

it would be very challenging to get decent data.

Marshall Grice wrote:the main problem is the change in grip from tire pressure adjustments is going to be incredibly difficult to measure. like less then .1g different. well within any noise that will be present on an accelerometer signal, and probably well within the driver consistency noise if you were to use a timer and calculate average acceleration on a ~100ft skid pad.

it would be very challenging to get decent data.

You're right, especially with a noob driver, whose driving will be less consistent. I actually like Steve's suggestion of using different amounts of ballast and measuring the effect of weight. If you can think of something that might be a more clear measurement, by all means chime in!

This is actually pretty good. With a kart, you could do a tilt test rather than a skid pad. Takes a whole lot of variation out of it.

I would think a good grade will come from a well written report, identifying the controled and uncontrolled varibles in the system and well documented control and experimental data, not so much on the actual outcome. I wish a lot of lab guys here were graded by a high school teacher, then I might get some sensible reporting.

Jay W

Kurt Rahn wrote:

Marshall Grice wrote:the main problem is the change in grip from tire pressure adjustments is going to be incredibly difficult to measure. like less then .1g different. well within any noise that will be present on an accelerometer signal, and probably well within the driver consistency noise if you were to use a timer and calculate average acceleration on a ~100ft skid pad.

it would be very challenging to get decent data.

You're right, especially with a noob driver, whose driving will be less consistent. I actually like Steve's suggestion of using different amounts of ballast and measuring the effect of weight. If you can think of something that might be a more clear measurement, by all means chime in!

It would be definitely a fun project even though I'm not sure how easy you can measure the results - and if you will have consistency.

Another thing that you might want to try is to measure the difference between static friction versus sliding friction.
You can use any car with an ABS and you can try braking from 40-50mph with ABS on and without ABS (but should lock the wheels).
You son can use these results and he can also try a quick experiment at the class with a box on a inclined surface - if the angle it's right he should be able to to have the box staying still and then after a light tap having the box sliding all the way down (proving that static friction is larger than sliding friction).

Chris