Are we finally going to end the “It’s raining lies” series? Yes, yes we are.
Screamer vs. Big Bang
Before we begin, let’s take a brief tour through a seemingly unrelated topic in the motorcycle world: big bang vs. screamer engines. A big bang engine is one where all the pistons fire at the same time (or very close together). A screamer engine spaces out the ignition pulses as much as possible. From an engineering standpoint, it shouldn’t matter much, but the screamer is a little more powerful because it vibrates less. However, from the rider’s perspective, the firing order makes a big difference. Bikes with screamer engines tend to send their riders off the high side. How the heck does piston firing order affect the rider?
In a big bang configuration, the tire gets a big kick in the ass every 720 degrees of rotation. But it also gets a long rest period before the next kick. In a screamer, the tire is getting kicked every 180 degrees (assuming a 4 cylinder motor). Apparently the downtime in the big bang configuration gives the rider more time to sense the level of grip and adjust accordingly. In a word, the big bang gives compliance.
Before getting to the objective stuff, let’s be subjective and talk about how driving in the rain makes us feel.
- How does a car feel on a wet track? Unpredictable.
- What are we afraid of? Crashing the car.
- How does that make you drive? With a large margin for error.
It’s fine if you don’t want to admit it, but I will. Racing in the rain scares me a little. The tires don’t make the same sound. The steering wheel doesn’t have the same tug. The throttle pedal feels like an on/off switch. When things go wrong, it seems they go wrong suddenly and without warning. That said, I actually really like driving in the rain. The extra stress makes it extra fun.
The reason why we soften the suspension in the rain is to slow down weight transfer. A car with a stiff suspension is sort of like a bike with screamer engine. It is theoretically the faster configuration. Stiff suspension leads to less weight transfer which leads to more grip. Lap times should be lower with stiffer suspensions. This is true regardless of the wetness of the track. However, there is also the human element to consider. The weight transfer in a car with stiff suspension is much more abrupt than a car with soft suspension. A human driver needs time to make adjustments to grip, and a suspension that is too stiff does not give the driver enough time to sense and react to changes in traction. So what are the physics underlying this phenomenon?
Basics of Friction
The coefficient of friction (CoF, or µ), is a ratio of the downward force of gravity divided by the frictional force. In the old days it was thought that you couldn’t get more than 1G of frictional force, and that the CoF was limited to 1.0 (this was due to blindly following Coulomb’s Law, which doesn’t really apply to viscoelastic compounds like rubber). Racing tires can generate over 1.0G, and much more with downforce.
Tire grip comes from the interaction of the rubber with the road. These interactions occur at a variety of scales from invisible molecules to stuff the size of tires themselves.
There are two separate properties that account for tire friction: adhesion and hysteresis.
- Adhesion – Microscopic contacts between the tire and surface. This is also called mechanical keying.
- Hysteresis – Macroscopic contacts that deform the rubber. The energy used to deform the rubber creates grip.
Adhesion and hysteresis sometimes compete with each other. As a tire gets hotter, it increases its adhesive properties but loses hysteresis. Adhesion likes a smooth surface while hysteresis likes a rough surface. The optimal operating temperature of a tire is therefore a complex function that depends on the properties of the rubber and both the microscopic and macroscopic texture of the surface.
To simplify matters, one usually talks about the optimal friction and relates this as the CoF. The CoF of a steel plate doesn’t change, so it’s a convenient simplification to think of the CoF as a single value. But the CoF of rubber actually changes and therefore can take a variety of values depending on the situation.
Load is sub-linear
It is well known that friction increases with load. But the grip of tires with respect to load is sub-linear. That is, if you increase the load on a tire by 2-fold, it gives less than 2-fold more grip. As a result, all things being equal, a lighter car will have higher corner speeds than a heavier car. One reason for this may be that there are physical limits to hysteresis. Colloquially, once a tire has been sufficiently mashed into a surface, it can’t be mashed any further.
Whenever a tire is asked to do anything other than roll freely, it will have some slip. We’re not talking about slip angle here. Imagine braking instead. There is a continuum from freely rolling to fully locked. At 0% slip, the tire has a CoF of nearly zero (there is some rolling resistance). At 100% slip the tire is locked into some amount of grip, but that grip isn’t optimal. The peak friction occurs at a relatively mild amount of slip.
Speed affects grip
A tire that is moving across a surface a high speed cannot press into the surface as well as it can at low speed. This means that tires have less grip at higher speeds.
The optimal slip ratio also changes with speed. The faster you go, the lower the optimal slip ratio. We often think of the CoF as a fixed value, but it isn’t. Given that you have less grip and a lower optimal slip ratio, it’s not just self-preservation that should make you drive more reservedly at high speed.
Water affects grip
Water affects grip by getting between the tire and both the microtexture and macrotexture. It can therefore reduce adhesion and hysteresis. Grooves or other kinds of texture in both tire and surface can help evacuate water.
The amount of water on the surface is really critical. If the water film is thin, slick tires grip better than grooved tires. But if there is too much water to be evacuated by the macrotexture, the grip of a slick tire becomes terrible.
Under certain conditions, a tire may hydroplane. In the figure below, the dashed line represents a constant CoF while the solid line represents a variable CoF. The actual stopping distances are given in the inset, which match the variable CoF. The take-home message here is that the grip of wet tires depends on speed. Presumably that’s because of hydroplaning.
Water interferes with microtexture and macrotexture. It can also cause hydroplaning. As a result, the coefficient of friction of a wet tire is anything but constant. A dry tire is easy to drive because it has a very broad band of traction in which the CoF doesn’t change much. You can over-drive the hell out of it and it will still perform okay. This is not true of a wet tire, whose CoF depends on the amount of water, the grooves in the tire, and the speed of the tire. Push a wet tire too far and suddenly, you’re spinning.
The reason why one softens the suspension in the rain is because the coefficient of friction of a wet tire is variable and volatile. By slowing down weight transfer, we give the driver time to adapt to an unpredictable CoF.
Let’s finish off this series of posts with a few key points about driving in the rain.
- The reason why traction loss feels sudden in the rain is because it actually is. So be careful out there.
- You may not notice much difference in braking in wet vs. dry but it is substantial.
- Be extra careful at higher speeds where hysteresis and hydroplaning effects seek to rob you of traction.
- When applying throttle, make sure you do so gradually because once a tire starts spinning, the loss of traction is catastrophic.
- Grip in corners is pretty good as long as you don’t upset the traction with too much throttle, too much brake, or jerky inputs.
- The more water there is, the bigger the tire grooves need to be. If you don’t have grooved tires, pump them up so they have a crowned profile. If you do have grooves, decrease tire pressure.