Why More Unsprung Mass Won't Give You Amazing Grip

Everyone wants more grip in the wet. Its scary. Slippy. And the tyres give you less time to react. You want those tyres to be pressed as firmly into the race track as possible. So, despite commonly heard paddock wisdom, you might be considering adding more unsprung mass to give you better grip in the wet?

Well the answer is no don’t do it - but here is why:

If you need to measure your unsprung mass for calculations, here is a practical method for measuring unsprung mass on a race car.

Reader question - More Unsprung Mass?

Never content to over-examine every aspect of racing car dynamics, I thought I’d ask your input on rain tyres.  My theory, with absolutely nothing to back it up, is that for running in the wet, you should run the highest unsprung weight possible to reduce changes in the tire patch to the fullest extent possible i.e. heavy wheel and heavy tyres.

We run in the rain a fair amount, so it’s not an academic question, but I have no idea how to approach analysis of this.

John

Hi John

Great to hear from you. 

Yes I can see your logic here I think.

So the more unsprung mass you have, the more the tyre (tire!) will be pressed into the ground. Like pressing an eraser into the desk with more force.

In the wet this is even more important because you want the tyre to “key” into the track surface as much as possible - largely because the other mechanism for tyre grip “chemical adhesion” is not as strong because the tyre doesn’t heat up. 

Hopefully I understood you correctly?

Why More Unsprung Mass Is Bad

So for me there is some merit in this thinking. What it misses is that the suspension is a dynamic system of springs.

Their is a concept called “contact patch load variation” 

Whenever you are driving, even on a flat smooth road, there are oscillations at the tyre contact patch.

This is because the tyre is squished into the road when each part comes in contact with the road, and then springs back when you roll forward it is no longer in contact with the road. 

Take a watch of this video if you want to get a feel for this.

The result is that no matter what you do, your contact patch vibrates. 

A Tyre Not In Contact With The Road Gives No Grip

For more grip in the wet or dry you want to minimise the amount it vibrates. Not on the down side but when it is springing back up - a tyre that is not in contact with the road can give you no grip. 

The primary tool for managing this is the damper or shock. The thing is this is not attached to the contact patch, it is attached to your suspension - your unsprung mass. 

So before the damper can help with the tyre, first it needs to control the oscillations of the unsprung mass too.

More Unsprung Mass Gives Your Dampers A Tough Time

The vibrations from the tyre will be causing this unsprung mass to oscillate too. 

The more unsprung mass you have, the more work the damper has to do in controlling this and the harder it makes the job of controlling your tyres contract patch as well. 

Hopefully you can imagine this a little in your head?

I’ve some great animations on it within Master Your Tyres but if you search for “quarter car over bumps” you might be able to find some animations on google. 

So, if there was no vibrations, bumps or other disturbances, having more unsprung mass might make some sense, because you can’t get away from vibrations it doesn’t. 

Perhaps a slightly different question might be:

“What do I do to get the most grip in the wet?” 

And if that is your thinking, then this article gives you my perspective:

It is a complete free lesson from my Master Your Tyres course, with an image of the contact patch in wet conditions. 

Once you see this, I’m sure a light bulb moment will be had - as it was for me. 

Look at the centre of the contact patch and see how much the pressure is changing this. 

I’ve had drivers do this for the first time and they were so blown away by the extra grip compared to everyone else they were wondering what everyone else was up to! 

In answer to your question then, no more unsprung mass will not give you more grip in the wet (or dry).

Hope this fuller explanation is more helpful in understanding why :)