Rear wheel skip - why?

[The Pastor]

like the wheel is stopped but the front sprocket keeps turning?

[AlBundy]

Could be the chain slapping against the chain slider.

[vifferman]

like the wheel is stopped but the front sprocket keeps turning?

I dunno Eggs Zachary, but I think the back tyre is skidding then gripping and hopping, and meanwhile it's still connected to the engine which is trying to turn it.
Plus the chain has some slack in it, which is usually in the bottom run while the engine's driving the rear wheel. Add some silly business as described above, and possibly a little movement of the swingarm, and the chain could well be having the slack swapping from below the swingarm to above it and flapping around.
Try braking hard with the rear and see if it still does it.

[The Pastor]

i think the bike might "shorten" when you brake hard and causes more slack that normal?

[AlBundy]

The chain is going to be at it's tightest when the front sprocket, swingarm pivot and rear sprocket centres are all in line. Any time the swingarm is in a different position, you'll have a slack chain to varying degrees on either side...

That is why it is advised to adjust the chain with these three lined up. Manufacturers sometimes give a measurement, eg + or - 25mm from static when on the side stand...

On the KTM I have to press the chain towards the swingarm and have a 5mm gap at a certain point, which I assume would correspond to a well set chain when the above three centres are alligned.

Interestingly enough, the new BMW off-road bike, has the front sprocket and swingarm pivot in the same location, so that would take the above scenario out of the equation...

[dipshit]

No, don't blip the throttle ordinarily so I'll try that.

Then that is 80% of your problem. Watch this vid as an example of how *not* to down change. http://www.youtube.com/watch?v=UO8TuakquPQ Note how his bike gets very unsettled going into corners sometimes... and even right at the end at 5:59 the rider loses control completely and goes off the track.

Blipping for down shifts gives you much more control and stability, especially when riding on wet roads or needing to change down when cranked over even just a little bit.

After looking at the Ducati MS forum I'll check the tie-rod between the frame and the swinging arm too. That will lower the back end which is negative for handling but positive for stability. The Ducati ST4 has plenty of weight already on the front - heavy steering compared with other bikes, so lengthening the wheelbase slightly and lowering the back end won't do any harm. I'm going to be a nana for a while....

Giving a bit more rear sag to steady the rear end under brakes isn't so much for weight distribution - it is for stopping the rear shock from topping-out.

When on the brakes (or even just heavy engine braking) the front of the bike dives and the rear end tries to lift. If you haven't got much rear sag to begin with, then the rear shock will top-out and the back end will lose grip and feel skitterish and light. So any rear pre-load you take off will reduce that feeling. Of course you need to fine tune this with not making the rear feel too low and slowing down the steering. I am only talking about small adjustments here. Fine tuning your bike.

[SHELRACING]

I dont know if this will help, but....

when we build our race bikes, sometimes they get a lot of chatter in the
rear end under heavy front braking, just before you enter the corner , as you described.
To eliminate this we adjust the pre-load on the rear shock.

Press down hard on the center of the bike. Usually this will be just before the seat on the petrol tank.

It should have the similar travel in the front suspension as in the rear.

Maybe the back end is solid and the front end moves up and down a bit.
You can adjust the rear shock to make it softer. Then try again, until they both travel a similar distance.

If it seems ok, then check your tyre pressures. On a hot day you need only
one psi less, to make a huge difference to the handling.

Hope your little off, hasn't put you off riding!

[Motu]

This is what those magical slipper clutches or MV's clever partially open inlet valves are about. .

They are a complicated way to go about getting back what they always had with the 2 strokes.Two strokes are the very best bike to ride in any discipline - road racing,MX or trials....engine braking is evil incarnate.

Of course I have now sold my smoker and am back to dealing with stepping out on corner entry.

[puddytat]

hmmm I do most of my slowing with engine braking - is this not a good thing? does everyone else just use the brakes only?

Naaah, generally its quite o.k,but if your thrashin' it all the time & using your engine to do the braking at high revs it'll not help your crankshaft much...
mate o mine with a monster pretty much only uses his engine braking when we're out fanging (he's quick) & his motor is pretty tired already at 30,000 ks....

[Winston001]

Sorry to hear...

Question... Did you panic brake and look for a place to crash, or did you look through the corner and try and make it through.

Often people are actually going slow enough to make the corner but focus on the outside of the corner and crash, instead of through the corner, where they can easily make it.

I think you are spot-on.

Have been chewing this over and thanks for the advice given. I don't know if anyone is interested but here is my memory of events - which probably took 5 seconds at the most.

Approaching corner which leads to a T-intersection so need to scrub off the speed - front brake and change down possibly twice, start to move my body over into the anticipated bend -> WTF? not slowing and almost into corner -> whoa now the bike is skipping from side to side -> panic -> sit up to get control -> yep, good now -> WTF??!!! marker post coming at me very very fast -> I'm in the off-camber gravel on the outside of the corner -> mustn't brake in this stuff, I'll wipe out -> I'm looking at grass, flaxes, and concrete fence posts -> so naturally that's where the bike goes too -> try to ride it through -> weaving on the grass -> over the bars and land on my chest and hands.

Stunned, I took my helmet off, something hurts but what about the bike? Its just lying comfortably against the flax having a snooze facing 180 degrees to where we were moments ago. No breaks, scratches, nothing but a bit of grass on the pegs.

I hauled it up while the adrenaline was still flowing and pushed it across to the road. Then I paused to take a deep breath while the bike looked rearing to go, sniffing for more corners, no worries.....

So I now think the bike would have gone around that corner if I hadn't straightened up, and had been disciplined enough to keep looking to where I needed to go.

Nevertheless I'm going to check the rear suspension, chain and anything else tomorrow just to ease my mind that it wasn't a mechanical problem.

Bass - Can I be so rude as to suggest that if you have any doubts at all, go and get the medical attention. It gets the ACC process activated before it's too late - just in case - y'know what I mean?

Excellent point. Need to do that.

[AlBundy]

Sounds like a VERY lucky escape.

It's still interesting that you didn't slow down a lot though.

I'm kind of thinking maybe you rely on your rear/engine/compression braking more than you realise, so when it 'locked up', you didn't have the same amount as you'd normally have. Add to that, the bike behaving horribly, just overwhelms the mind.

As you say, it happened really quick. I'm sure a lot of people would probably find themselves in the same situation... Accidents happen all the time, we can only try and learn from them and try to be better...

[Winston001]

Cheers Al and I suppose my wish is to figure out what went wrong as well as share an experience which might help others.

[AlBundy]

How's the hand? It's worth checking, since the scaphoid is usually the bone that breaks in these types of crashes.
You may not notice much pain but it'll get you in the end if you don't catch it early...

Have you read any books on riding? Some of the things they say are sometimes really obvious but they get one thinking 'Ah yeah, true bro...". Lol...

[Winston001]

How's the hand? It's worth checking, since the scaphoid is usually the bone that breaks in these types of crashes.
You may not notice much pain but it'll get you in the end if you don't catch it early...

Ah thankyou, I was googling late last night trying to find that specific bone. I knew there was one which broke but couldn't determine it from the anatomy sites. So yes, that is my only worry and it is worth checking. Right now my hand is a lot better and just feels like torn ligaments. Having said that, I still cannot turn the key to start my cage.

[scracha]

Blippy throttle prior to downshift is the way to go...but it doesn't work on some bikes (makes the slipper clutchy thingie on the 748 get all upset). Clutch out quick on upshifts, clutch out slooooow on downshifts.

Oh..if it's still hopping about as you're entering the corner then whip in the clutch before tipping it in to settle it all down a little. Coasting around the corner aint a great feeling but it's better than a locked up rear wheel suddenly gripping and spitting you off. Don't release the clutch whilst cranked over either