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Discussion Starter #1
I was fortunate to ride a track day on the weekend where I could compare my 999R racebike back to back with my D16RR (16.5 inch Dunlop Ntec slicks, full race exhaust, stock gearing).

While there are lots of interesting comparisons (the 999R is more friendly and foregiving, the D16RR is almost too precise) one problem I encountered is that while the 999R will predictably lift the rear wheel slightly at the limit under heavy straight line braking (ie 4th gear to 2nd gear 90 degree corner), the Desmosedici front wheel will grip, release and grip again resulting in a high frequency pogo effect. The front wheel is locking up.

I can brake later and harder with the 999R. I have to wait for the front end of the D16RR to calm down before I can drop it onto the desired line while the 999 on race compound DOT Dunlops allows me to trail brake pretty hard right to the apex.

I could not compare lap times, but they would be pretty close. The 999 allows me to stick my neck out pretty far on corner entry, while the D16RR is virtually supersonic in the upper gears and walks away from anything at the track day down the main straight. The 999 wheelies very hard in low gears (2nd or even 3rd), the D16RR will paint long black lines with the rear slick and will not wheelie.

I have read that locking the front wheel under braking means the C of G is too low. Any way to raise the center of gravity on a D16RR? I do suspect that I have slightly too much sag in the rear and maybe can drop the forks in the triple clamps a few mm. It took me forever to get the 999 perfectly balanced (I cheated and asked someone else for the numbers), I fear that it might take even longer to sort out the Desmosedici.
 

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What about raising the rear ride height a bit?
Other potentials I can think of would be 1) is the tire up to the task? I'll assume so but had to mention it. and 2) is the front suspension completely bottomed and road irregularities causing it to hop? Just random thoughts....good luck with what you find on such a desirable pair of bikes. :)
 

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Discussion Starter #3
I had not contemplated the front bottoming out. The front tire is a fresh KR106 slick, tons of side grip with normal wear pattern. I may have to check bottoming. There may be some some very small ripples in the pavement there where the cars brake and turn in, but I have never noticed it before. Rear tire life is very short, maybe 30 laps of a 4km circuit (shorter than expected) before the side grip goes away. The rear tire gets lots of waves in it, I suspect from spinning.
 

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Hi John,

I first wanted to reply an hour ago but thought I´d let someone else take a guess.

It sounds like you don´t really know where you are with your suspension.

Wavy rear tyre might be undersprung rear shock and overdamping. Hence the tyre has to compensate for what the shock is not able to suck up.

The KR108 tyres can be a pain in the arse as there are loads of compounds out there. Most of them are really sensitive to temperature, which is why you might not be getting a lot of life out of your rear tyre. Once you get the correct tyre, you can use it for ages.
As for the front, I had the same problem with my SPS a while ago using Brickstone tyres. I softened the rebound by 2 clicks and the problem went away. I would have described it as front chatter instead of tyre hopping, but maybe it´s the same with your problem.

My suspension guy mentioned that problems like these are likely to come from the rear of the bike. Incorrect spring rate or too much damping. Maybe check that first. And run a zip tie on both ends...
 

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Consider that you may have too much rebound damping up front.

This is of course assuming you have the correct spring, and sag has been set. If the rebound is so stiff that it doesn't allow the front to to extend fast enough to follow even small undulations in the pavement, it will tend to skip across.. Under braking the forks can "pump up" - because they can't extend as much as they compress with each small bump.
Same problem can be the source of high speed head shake...
Easy enough to check on your next excursion - turn out the rebound a click or three, then repeat the lap...
 

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Discussion Starter #7
That might make some sense as the bike had significant headshake issues at speed when I first tried it. I added about 5 clicks to the steering damper to sort it out, which seemed unusual at the time. The frork travel band is completely to the bottom of the fork when I pull in so it is bottoming. Maybe packing down too.

It will be 2-3 weeks before I can take it back to the track, but I need to establish some baseline sag numbers and check the compression and rebound settings on both ends. I will likely need a new rear spring, I doubt anyone in the UAE can respring a pair of gas charged ohlins front forks. A trip to JHP may be in order as they may be closest if I need to go that route. In comparison we got an ideal setting on the 999R by changing the rear but using the stock front fork springs.

I also need a new rear tire.
 

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Cool bikes John. Nice clutch, brake pump and, ti exhaust (ohh the hassle with the crackage... :)) and SBK front rotors on the 999.

The Desmo looks cool. I´d like to own one.
 

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Discussion Starter #10
No cracking, the system is an Akro prototype I got from JHP in the UK in 2007. Amazing fit. I get the RS frame, EVO airbox and RS swingarm on Friday... The bodywork is old and crappy, but good enough to race with.
 

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When a tire is acting up it's because the it is trying to do the job the suspension is suppose to do (overloaded). The Desmo's are more weight biased to the front than a 999 or 1098 so Ducati puts 10.5 fork springs in them, (999R come with .95) too much unless you are 190# or more from what we found. Is the fork far from the bottom? (should get to within a few mm's) If so take some pre-load out and try that. What is your sag? (38 to 40mm is good). If it's too stiff the tire takes all the load = chatter. The Gas forks are no different when it comes to a spring change than an R&T, is separated from the gas section.
 

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Discussion Starter #12
Front tire wear looks fine, the forks must be bottoming completely at some point as the travel ring is fully bottomed after a session. It is just that the wheel locks up under straight line braking and the front end "hops".

The rear feels planted, but the tire wear is terrible.

My thinking is that the rear has too much sag, which is dropping the C of G and taking too much weight off the front wheel, causing it to lock up and skid under heavy braking. I also suspect that the rear has too much compression damping causing excessive tire wear although I have had that problem before and the wear looked very different to this.
 

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Well if the fork is bottomed then that is the most likely problem, add some pre-load or add some oil to the forks. Again...what is your front sag?
Front tire wear looks fine, the forks must be bottoming completely at some point as the travel ring is fully bottomed after a session. It is just that the wheel locks up under straight line braking and the front end "hops".

The rear feels planted, but the tire wear is terrible.

My thinking is that the rear has too much sag, which is dropping the C of G and taking too much weight off the front wheel, causing it to lock up and skid under heavy braking. I also suspect that the rear has too much compression damping causing excessive tire wear although I have had that problem before and the wear looked very different to this.
 

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If the forks are bottoming under braking - it's not the rear.

Problems in the rear generally manifest themselves when coming out of turns and/or under acceleration (don't flame me - I said generally..)...

It's either you don't have enough compression control - incorrect sag, not enough spring rate, or too little damping - that allows the fork to completely compress prematurely, or -
The rebound settings are not allowing the fork to extend enough on the return stroke to equalize the amount of travel during the compression - so the forks keep compressing to zero avail travel. The forks become basically solid bars, resulting in skipping or chatter....
I know it's contrary to normal thinking that rebound settings can affect bottoming, but it does...
 

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All the symptoms fit the bottoming out hypothesis. Taking out some rebound will eliminate packing down, so this is also consistent. Put a plastic tie wrap on the fork and see how much travel you are using. Once you bottom out, smooth road or not, the tire will skip. Very rarely will it skid, the front end will flex enough to result in a judder or chatter.

Before you just start turning knobs, measure the sag, should be 35mm to 40mm. Turn the compression and rebound full soft and adjust the preload until you get the numbers. Turning out the compression and rebound isn't necessary, but if you do there is just one less thing to wonder about.

Once the sag and preload are set, write everything down. Preload should be counted as number of turns in (clockwise) from full out (counter-clockwise). Write down your current compression and rebound settings as well. You might also compare them to your other bike just for reference.

Then set your compression and rebound in the middle of the range. I measure these as clicks out from full "On". On most forks you would turn the screw clockwise all the way until it stops and then back it out. On the Ohlins for the 1098, there are about 25 clicks out from full on, so half way on that fork is about 12 clicks.

Then, with your tie wrap on the fork, see how much travel you are using. You can adjust compression a bit to see if this helps. You can add a turn or two of preload. Or, you can add 10cc of fork oil to each fork to help eliminate bottoming. (You can add the fork oil twice if needed.) If none of this prevents bottoming, the next step is stiffer springs and maybe valving, depending upon your goals.

Hope that helps!

For reference, on my 1098 I have 37mm sag, 10 turns of preload, 10 clicks out on compression and 13 clicks out on rebound. I have stock springs and internals, and I weigh 182 lbs in the buff. Bike works very well with these settings.
 
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