Tire Shake, the Bottom Line

by Henry Walther

From my experience, is that it is a combination of things that ends up resulting in tire shake. Just like a good tune up, you can arrive at it from a number of different routes. Getting out of it isn't as easy as you would hope. As has also been pointed out here, tire shake is a changing phenomenon. Fix it here, and you might still have it there.

In my simplified view, I see it as the tire trying to 'stand up' after it has 'squatted', and when there isn't enough power or clutch to pull the car up on the tire, it sits back down again. Then it rapidly repeats this process over and over again until there is finally enough tire speed that the centrifugal force helps the tire to stand up. Sometimes getting more aggressive with the power or clutch will help. Sometimes taking clutch away helps the shake, but that also kills the ET. More or less tire pressure is another tool that can be used. You might want the tire to bite harder, or you might want it to slip a bit more. The bottom line is that you are trying to get after the tire at the point where it needs it the most.

I have been privileged to see some ultra high speed video of tire shake (shot from underneath the car), and what goes on with the tire is beyond your imagination. Not long ago the Goodyear engineers would tell you that mathematically today's fuel cars cannot accelerate as quickly as they do. They were baffled how they could achieve the rate of acceleration they had in the early part of the run. After they saw the same video that I saw, they understood. They were working with a presumed contact patch area to formulate their calculations, when in fact the contact patch of rubber to the ground was much larger than they figured. The footprint is about one and a half times as long as everyone thinks it is. After I saw the video I went to that starting line at the Las Vegas race and watched a lot of cars run to try to see this with my own eyes. I couldn't. It took the high speed photography to see it. But, what it also explained to a small degree was why tires shake. The cord angle is laid over more severe then you would imagine, and trying to get that straightened back up, so the tire can stand up, takes some beans.

There was an engineer named Chuck Hallum who wrote an SAE paper on drag tires and tire shake about a half dozen years ago. He was a fluid dynamics engineer by training, but had a lot of knowledge in other disciplines. A condensed version of his paper was published in National Dragster. At the time he wrote it, it seemed a bit far fetched, so it was largely dismissed. When the video I mentioned was shot, it validated his theory. He used to hang out at Racepak on occasion, so I had the chance to talk to him a few times about it, but unfortunately he was killed in a single car traffic accident (put his Viper on its lid) a couple of years ago, so we never finished our conversations. Interesting guy. I wish I still had access to his mind.

Do any of you remember the door on Garlits trailer on which he had written a long list of the things that can cause tire shake? He ran out of door before he ran out of reasons. Truth is, it is the sum total of your combination that gets you into tire shake. That is why there are no easy answers to resolving it, or why one person has it, and a seemingly similar combination doesn't. Obviously, as the sidewalls of the tires became more flexible, the easier it was to find shake. Today, when a car starts shaking the tires, it usually means they aren't making the power they had planned to make.

Here is another shake phenomenon that you can have some fun watching. Watch the Pro Stock cars when they shake the tires. Every wonder why they all tend to turn right or left when they shake the tire? If you were privy to seeing the data from their shock travel sensors you'd know why. When the tire goes into shake it sets up a harmonics (as has been described as the paint can shaker experience by some of the drivers here). When that happens it also drives the valving in the shocks bezerko, and the shocks collapse. Trouble is, they don't collapse at an equal rate, so one side of the car dives before the other, and this causes the car to turn one direction or the other (like wedge in a circle track car). Another thing that happens at this same time is that the floats in the carburetors take a dive in the float bowl and the engine stops gaining RPM even though the driver still has the throttle wide open.

Ah yes, shake is a wonderful thing, and we haven't even talked about all of the strange shit it can break.

Now, knowing that talking about tire shake is akin to talking politics, and that for every opinion there are two more that disagree, I'll get out of here and let someone else take the podium and explain how my theory is all hogwash.

I know a subject like tire shake isn't high on everyone's list of stimulating conversations, but I'd say there are quite a few of you left here that enjoy a good tech talk.


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