Tank Slappers

[DavieSV]

I have increased the length of the rear shock (the rsv shock being adjustable)

Initially I thought a bit too much, as it was trying to turn too tight.

I thought I may have it about right, because it feels very neutral mid corner, take the weight off the hands and the lean angle takes the bike on the perfect line.

I was thinking this may have contributed to the slapper because the rake angle of the forks is quite low now. I have the forks legs 5mm up through the top yoke aswell.

hmm i dont suppose it easy to measure is it

[yorkie_chris]

What shock length you at?
What spring rate?

Way I understand it (not 100% on this, going on f-in about)... swingarm angles most important thing is to control squat. Longer shock increases swingarm angle > thus increase effect of chain tension in reducing squat.

If you go too far with this then back end may go way 'stiffer' than you want and transfer fast bump loads to front leading to TL1000S-esque problems. Or simply transfering weight to front steepening rake.


On curvy I think sweet spot is around 355mm. I tried closer to 360 and found some strangeness in how the bike accelerated. No problems with stability though.

[yorkie_chris]

Originally Posted by barwel1992

hmm i dont suppose it easy to measure is it

Tape measure, pen and paper and MSexcel

[Red Herring]

Originally Posted by Richie

re-rail...
Tankslapper... from another forum....


A tank-slapper (wild and rapid swings ...........

Richie, with harmonics you are going into wobble and weave territory. They are an entirely different kettle of fish to a tank slapper and your advice ought to reflect that.

Well, from a lot of youtube videos and reading on this stuff it seems weight forward is the way to go. As is anything else that dampens the fork side to side occilation.

I'm not sure about gripping the bars tighter though as this may lead to broken/dislocated thumbs if it turns into a big one. Gripping the tank with the knees is a good as it changes the harmonics of the system (to a lower frequency). As does leaning forward. A lower front tyre pressure means a tankslapper is less likely to happen in the first place.

This is about thge clearest video of an extended slapper and in the slo-mo you can see that as soon as he shifts his weight forward the slapper reduces. As soon as the rear tyre breaks traction the occilation stops. This can only happen if you unweight the rear. leaning back seems the wrong way to go.

http://www.youtube.com/watch?v=hNlW7...eature=related

This is an unusual one thought because the cause looks like a spinning up rear on the white lines. The resulting occilation is the same though.

C

[Richie]

Originally Posted by Red Herring

Richie, with harmonics you are going into wobble and weave territory. They are an entirely different kettle of fish to a tank slapper and your advice ought to reflect that.

It wasn't my advice to give, so I've edited previous post to reflect this.
:0) Richie

[yorkie_chris]

Originally Posted by Berlin

Well, from a lot of youtube videos and reading on this stuff it seems weight forward is the way to go. As is anything else that dampens the fork side to side occilation.

Why would that dampen it?

If you move enough weight to compress fork then it will change natural frequency because you will change how the restoring force acts. But I don't think moving yourself forward is enough to compress the tyre enough to create any meaningful damping.

I think only thing that makes sense so far is to grip tank and relax on bars (if they're still in your hands!), you don't want to fight it and just transmit the shaking to the rest of the bike.

[Mej]

i had a near miss on the 750 not too long back, and that was a combination of a fair amount of acceleration, and going over a hill. As i went over the top the front end came up and when it landed it had the shakes, i kept the power on but the steering damper did its job.

Wheelie
HTH