In my last blog I talked about frontal area, and how a small frontal area seems to be one of the key ingredients in making a gravity racer that can go really fast. Part of my reasoning for this was that the Bodrodz guys in the USA were the quickest out there, and this was surely to do with the incredibly small frontal area of their cars even though they used kart tyres, which must offer a lot of grip due to large contact patch, and the corresponding amount of rolling resistance (or at least I thought at the time). Fast Donnie was kind enough to leave a comment, and brought up the idea that grip has nothing to do with contact patch – only the material the tyre is made from, and the amount of weight placed on it.
That’s interesting, I thought.
So I did some reading and what do you know – he was bang on right. Contrary to popular belief, a wider, fatter tyre does not offer more grip because of increased contact area, or for any other reason either. All that matters is the material that the wheel is made from, and the force that is applied to it, regardless of contact patch size.
As a follower of F1 I have been pleased to see the wider tyres in 2017, and the 3-5sec faster lap times associated with them. I, like many, assumed this was because of more grip offered by the wider contact patch on the fatter tyres. This, however, is incorrect.
If this all sounds unbelievable, there’s a little experiment that people have done that proves it. Take a brick. A brick has three different size faces – the large flat top and bottom face, the smaller side faces, and the much smaller end faces. Sit the brick on any surface and loop some string around it and tie a knot to make a loose “belt”. Into this, hook a spring type weighing scale and start to pull the brick sideways. You will be able to read off the weight on the scale when the brick starts to slide on the surface. Repeat this for all three faces. You’ll find that the amount of force (weight) required to make the brick slide in all three cases is the same, even though the contact patches are all different. This is because the materials (the brick itself and the surface on which it sits) and the weight of the brick remain the same.
Great, more in depth article HERE
So Why Are “Sport” Tyres Fatter?
Yes – sport tyres are often more grippy. This is simply because they are made from a softer rubber. As previously stated, the only thing that matters is the materials and the force applied. However, softer rubber wears out quicker, so tyres made from softer compounds are made bigger to “share the wear”. If a tyre was made that was twice as soft/grippy in the same size as a regular tyre it would only last half as long, so making them bigger helps to keep its durability in an acceptable range. The new F1 tyres for 2017 are simply softer.
The second reason is that of strength. Sports tyres have to cope with a lot of power – be it from acceleration, braking or cornering forces. This force basically attempts to rip the tyre from the rim, so having a meatier, stronger construction helps the tyre to keep its shape. Temperature is an issue too – extreme forces generate a lot of heat, and a larger tyre is more able to dissipate heat rather than melting and falling to bits.
How Does This Relate to Gravity Racing?
Well, first up, gravity racers don’t have an engine, so acceleration forces through the tyre are not an issue. Also because braking is often minimal (especially if you’re doing a high speed run) neither are braking forces. The only real area that you need grip for is cornering, and the amount of grip required for the corners on the track that you’re racing down depend on how fast you’ll be going and how much your car weighs. More speed and more weight will require more grip to corner without having to slow down first.
So really, as with many things in gravity racing, the optimal amount of grip required depends very much on your car, and the race it is being designed to take part in.
Well, now I’m wondering if there is a direct correlation between grip and rolling resistance. Is there a “best” way to have just the amount of grip you need, with the minimal amount of rolling resistance, and this will be the subject of my next blog, after some more research… 🙂
Please comment below!