# Speedway Motorcycle Racing Physics

1. Jan 19, 2014

### bdub_24

At least in the US, it's rare to find anyone that is familiar with speedway racing, so here is a very basic description of the sport:

Four riders race around a dirt oval track on single-speed motorcycles with no brakes. They slide sideways around the corners.

Here is a sample video:

I used to race in the US, and one of the techniques you learn to gain drive is: at the apex of the corner, tuck your body down to the left side of the bike, push the bike into an upright position (while still leaned left), and drag your left foot back toward the rear wheel (they don't do much of this in the video above as the track wasn't demanding). The attached a photo demonstrates what I'm talking about.

I understand that dragging your foot back will shift some weight toward the rear wheel to increase friction, but I've never understood why standing the bike upright is supposed to help considering you're still tucked to the left, thus your center of gravity is shifted left. They say it gives you a bigger "footprint" on the ground, but I know from physics that shouldn't be the case. I don't feel like this situation can be explained with managing tire temperature as one would explain the size of drag racing tires.

Any ideas?

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2. Jan 20, 2014

### Lok

You raise a good question. And all I have are a few thoughts:
An upright tire has a different behavior than a tilted one as I would imagine it is more rigid when pressed on the side, which would influence the suspension characteristics of it.
The tire treads would "cut" the dirt completely differently in tilted vs upright. Not that you cannot cut different treads on a tire for better side performance but in tilted it would hit the dirt from sideways and impart sideways kinetic motion instead of a sideways Force. So the overall behavior is different.
The specific sideways slide of this sport does not leave a lot of choice for the rider to balance the bike in a tilted position (how wrong am I here?).

3. Jan 20, 2014

### mic*

To add to those thoughts, if the bike were leaned back further, perhaps you could not get as low off the side of the bike, because if you did, too much mass would be behind the contact part of the rubber with the ground. You could lose control and end up sliding in an undesired way - falling.
To get low, lowers the centre of gravity of the entire mass of rider and bike which would have to be beneficial in this sport. Because this is effectively being done by "flattening out" the mass of rider and bike (instead of having them stacked one on top of the other) you could simplistically interpret this as a "bigger footprint" for the mass (even though the more obvious "footprint" of the rubber remains relatively unchanged).

4. Jan 20, 2014

### bdub_24

You can definitely make it around a corner without using the technique I mentioned, but I'm wondering if it has more benefit for a rider that turns a tighter radius in a corner. It is well-known that the fastest line around the track is to come in slightly wide, turn the tightest radius that the rider can handle, then come out tight and straight to maximize the amount of time accelerating.

To better explain the cornering, you want to lay down the bike to scrub off speed when approaching the corner (done as late as possible), the discussed technique is applied right at the apex of the corner, then the bike is straightened out to head down the straight.

I think this technique would be more likely to be used when making a tight turn, so I think that the balance issue you guys have discussed is probably correct. Although it doesn't necessarily provide more drive directly, it contributes to faster times since it is used to allow for a tighter radius.

To help analyze the tire explanation, here is an image of the rear tire tread pattern.

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