Why do these bikes have a high speed wobble?

[R1chie]

I posted this here because I wanted a racers perspective.
In the later half of this video (sorry if it is a repost) You have to watch a little way into the video before you can see the modern bikes.
http://media.putfile.com/TT2005retro
The bikes have what I consider a tremendous slow oscillating high speed wobble. I have never felt this on my 04 r1. I have felt going about a 130 while slightly wheelied in a turn, when the wheel met back up with the pavement a very fast oscillation wobble. My handlebars almost shuttered and danced back and forth maybe a half an inch in each direction about 5 times and then it stopped. It was not unnerving or scary. It twitched about 5 times in the amount of time it took one of the bikes in the video to wobble once.

So my question is, why are these bikes wobbling like this? Is it just they are old technology or are the bikes not set up correctly or are they set up this way on purpose so they turn much better sacrificing straight line ability? Or is there some other reason. I want to know because, I don't want my bike to do what theirs is doing when I am going fast. It even appears they wobble while on the rear wheel while in a wheelie!

Also I am building a super moto kx500 and because of the different sizes of the wheels 17-17 compared to 21-18 and the fact it has no steering damper like the R1 does, I am a little worried about the change and will it change geometry due to the front wheel being smaller and placing the rear of the bike higher. Normally in motocross if you raise your back end steering gets sharper and may even knife but you can also pickup head shake. I want understand why this is happening so I can prevent it on both bikes. I sure anyone that races knows of this effect, why it happens and how to prevent it. If you have some time type the answer or a web pointer would be nice too.

[The GECCO]

You are on the right track with the idea that it is something done to improve cornering at the expense of straight line stability. The two main culprits are the tires and the geometry.

Tires - race tires have a much different profile than street tires. the cross section of a street tire is very rounded so that there is always a fairly constant contact patch size regardless of the position of the bike (ie, straight up and down, leaned over slightly, or leaned over a lot). However, the cross section of a race tire is much more triangular shaped for two reasons.

1) less contact patch (meaning less rolling resistance) when straight up and down, and
2) a larger, flatter contact patch when the bike is leaned over (for obvious reasons)

it is this very small contact patch that makes the tire "nervous" at high speeds. if you ever watch serious wheelie/stunter guys you'll see that they greatly lower the rear tire pressure to give a bigger contact patch and more stability while on the rear wheel.

Geometry - very basic, a shorter wheelbase and steeper fork rake makes a bike turn better at the expense of straight line stability. The trick is to get the best of both worlds. to illustrate the point, think of an extreme example - a chopper bike with 5' long forks stretched WAAAYYYY out. It'll go straight forever with absolutely no input from the rider and be very stable.....but don't try to turn it fast, it'll just plow the front tire for a few miles and continue on it's way. Somewhere towards the other end of the spectrum are our sportbikes with steeper raked forks and shorter wheelbases. The fastest way to adjust these things is to slide your fork tubes up and down in the triple clamps. lowering the front (more fork showing above the clamp) makes the bike turn quicker because the rake is now steeper and the trail is shorter (and as a result the wheelbase is shorter). But, the closer the front wheel gets to being directly underneath the steering head, the more unstable it becomes. an extreme example of this is the front wheels on a shopping cart. their contact patch is BEHIND the steering head and you can see how unstable they are.

so, as long as you keep your street tires and no radical geometry changes your street bike should be fine. the rake and trail on the KX (or any dirt bike) make for a very stable roadrace machine.

[R1chie]

You are on the right track with the idea that it is something done to improve cornering at the expense of straight line stability. The two main culprits are the tires and the geometry.

Tires - race tires have a much different profile than street tires. the cross section of a street tire is very rounded so that there is always a fairly constant contact patch size regardless of the position of the bike (ie, straight up and down, leaned over slightly, or leaned over a lot). However, the cross section of a race tire is much more triangular shaped for two reasons.

1) less contact patch (meaning less rolling resistance) when straight up and down, and
2) a larger, flatter contact patch when the bike is leaned over (for obvious reasons)

it is this very small contact patch that makes the tire "nervous" at high speeds. if you ever watch serious wheelie/stunter guys you'll see that they greatly lower the rear tire pressure to give a bigger contact patch and more stability while on the rear wheel.

Geometry - very basic, a shorter wheelbase and steeper fork rake makes a bike turn better at the expense of straight line stability. The trick is to get the best of both worlds. to illustrate the point, think of an extreme example - a chopper bike with 5' long forks stretched WAAAYYYY out. It'll go straight forever with absolutely no input from the rider and be very stable.....but don't try to turn it fast, it'll just plow the front tire for a few miles and continue on it's way. Somewhere towards the other end of the spectrum are our sportbikes with steeper raked forks and shorter wheelbases. The fastest way to adjust these things is to slide your fork tubes up and down in the triple clamps. lowering the front (more fork showing above the clamp) makes the bike turn quicker because the rake is now steeper and the trail is shorter (and as a result the wheelbase is shorter). But, the closer the front wheel gets to being directly underneath the steering head, the more unstable it becomes. an extreme example of this is the front wheels on a shopping cart. their contact patch is BEHIND the steering head and you can see how unstable they are.

so, as long as you keep your street tires and no radical geometry changes your street bike should be fine. the rake and trail on the KX (or any dirt bike) make for a very stable roadrace machine.

This was a great answer, thanks.

I suspected this and it is similar on dirtbikes but like you said sportbikes have a geometry more suitable for cornering. I did not notice MRA bikes doing this, was I not paying attention or they are more stable than the ones in the video.

I bought slicks for my KX500. They look more triangular than my street tires. I am curious how stable they are. I guess I will have to wait till next year.

[The GECCO]

I did not notice MRA bikes doing this, was I not paying attention or they are more stable than the ones in the video.

Our bikes do it, too, but mostly at Pueblo where the speed on the straight gets pretty high...you could probably see it if you were standing on the side of the track with the same perspective the camera gives you in the video....you can ask next years track marshal, but I doubt he'll let you stand out there

[Mista Black]

from the perspective of a corner worker standing just off the track you notice a lot of wobbling going on at the races.

[Lel]

what a lot of people tend to forget is the amount of input that is put into the bars by your hands. I was told by a very fast rider to try riding w/ my finger tips, and it made me realize how much i really am putting input into the bars, or ie the 'death grip', That is where you will see a lot of your acceleration, partially leaned over wobbles (like you had mentioned)

From watching the video, some of the wobbelling looks as though when they are coming to a turn... thats just the bike sliding around and minor corrections. ie they are transfereing so much weight to the front tire from the rear that its just bound to get a little squirly, keep in mind too those arent tracks, they are street surfaces.

Other than that, your bike probably wobbles a lot more than you realize, the chasis of the new bikes are so good that the rider just doesnt feel it. Watch bikes coming out of turn 7 at pueblo, or 8 (the crossover) and i guarentee you will see some wobble in almost every class.

Jason

[Mista Black]

Next time your at Pueblo for the MRA races, Rich, come stand by the crossover and watch them heading toward turn 9 at the end of the back straight (from the crossover til they're out of sight). They get some movement like Lel said...