In part 4 of this project, I attempted to fix a high speed brake shudder issue by making my own fully floating discs out of the factory semi floaters. I’ll admit I was a little nervous about grabbing the lever on my first fast lap for obvious reasons. Modifying the brakes on a vehicle should never be taken lightly and should always be tested incrementally. As I starting going faster and gaining confidence I was braking later than ever with this new setup. These are certainly the best brakes I’ve used on a race bike regarding initial feel, braking power and fade resistance.
As speed increased, new issues arise; testing never ends. By the second session of the day I was really feeling comfortable and getting on the throttle quite early coming out of corners leading to higher speeds on the straights. Nola Motorsports Park has a long start/finish straight. On my Triumph 675 I would see an indicated 155 mph or so before braking for turn 1. When building this Ducati 998, I chose gearing that would allow just over 160 mph in 6th assuming the more powerful but heavier Ducati would see similar speeds. I was wrong. The Ducati is apparently a rocket ship, albeit a deceiving one. The twin cylinder engine in the Ducati only revs to 10,500 rpm and creates a very deep baritone sound, whereas the Triumph will go 3,000 rpm further and screams by comparison. This makes the Triumph louder and ‘feel’ faster. I click into 6th gear on the Ducati as I pass the start/finish line and run out of revs approximately 2-3 seconds before my braking marker. The Ducati has no speedometer, but gearing calculators say this should be about 162 mph. Next time out I’ll lengthen the gearing a little bit to resolve this. The current gearing setup is a 15 tooth front and 40 tooth (+4 from stock) rear sprocket; it looks like a 38 tooth rear might be the way to go here.
I also noticed as I braked later with each lap that the rear wheel was starting to come off the ground and create some instability. I tried to most easily curtail this by reducing some rear rebound damping in the shock which would allow the rear spring to push the tire into the ground more quickly and maintain grip. This helped some but wasn’t a complete fix. I decided not to pursue this any farther because when I change the rear sprocket to a smaller size (longer gearing) the rear wheel will have to move farther back on its eccentric adjustment to maintain proper chain tension. Ducatis with single sided swing arms use an eccentric hub for rear wheel mounting. The single sided arm looks great and makes for fast wheel/tire changes but creates some geometrical issues if not adjusted properly. Because the hub adjustment is eccentric, the circular movement means as the rear wheel moves rearward or forward the ride height also changes. When ride height changes, chain pull angle center of gravity and other geometries are also affected. It’s important to check ride height before rear hub adjustment then use this turnbuckle style link to raise or lower it back to the correct position. The rear wheel moving farther back also creates a longer wheelbase which should also help keep the rear wheel on the ground during hard braking. I’ll try this solution out next time before any more suspension adjustments are made.
Man Rob, I love your series, even though I am not a hardcore bike guy, this is sweet stuff!
Thanks Mike. Hopefully the warmer weather comes around early next year. Part 6 will see some upgrades to the bike as mentioned here as well as a new Solo2 to get some quantitative data to share. Looking forward to next time out!
“making my own fully floating discs out of the factory semi floaters. […] These are certainly the best brakes I’ve used on a race bike regarding initial feel, braking power and fade resistance”
You certainly should give the details of this conversion. It sounds fascinating.
Tan, take a look at part 4 of this project.
For more details on that. It’s linked in the bottom of the article here. Thanks for reading!
Love this serious. Clearly challenging bikes to run one’s self. Very curious fuel dilemma. Hopefully not causing a leaning out issue. Vapor lock?
Thanks Miguel! Ducatis in general are not difficult to run, but it does help to understand how they work. They definitely don’t have quite the “set it and forget it” nature of modern japanese machines, or even of my Triumph for that matter.
Interesting you mention the vapor lock, because I kind of though the same thing initially. However with vapor lock the bike is essentially starved of fuel unless the cap is opened. I did replace the fuel cap as a precaution, and because fiddling with a key in a 25yr old lock cylinder at the track is a complete pain in the ass. I used to clear out vent lines by blowing compressed air through the vent line, but I no longer have access to compressed air. I’ll try with a little air pump at home before the next outing to be sure. Thanks for your input!
Ha most definitely they require specific knowledge and tools. So I appreciate you revealing the details of their nuances. My old SR500 is a much better candidate for hammering away and learning on, for now!
On the vapor lock thing, I had it happen to my ’13 F800GS BMW a couple times despite being relatively new. Hot day, full tank, but issue was low RPM and it would die completely. Very annoying. Popping the cap and not over filling the tank seemed to remedy it. (I suspect overfilling interfered with ventilation in my case)
I’ve been reading a lot about fuel system and supply issues lately. As I am pretty sure low fuel pressure at max revs cost me a car engine last summer. Unfortunately – no data.
The simplicity of an SR500 is certainly to be appreciated. I’ve had vapor lock issues in the past, but not on this machine specifically, and it was a very hot day, so you may be correct. I’ve blown up a car engine or two from low fuel pressure under high turbo boost before also. I never had any quantitative data either, but the engine felt very fast for a split second then made bad noises lol. Pretty clear.
Just came across this page as I have a 998 myself and am looking to upgrade the brakes. I always thought the braking was great until I tried a friend’s newer Ducati (and another friend’s Kawi for that matter).
As to your fuel issue, I don’t know if you’ve sourced the problem, but I regularly have vapor lock issues at the track. I need to pop open the tank before I can start the bike, but once I’ve done that the bike starts up fine and has never given me issues on the track, even during long track sessions. Love to hear what your “fix” was.
Mike I’d love to say I have a fix to
share. I don’t believe vapor lock is the issue, as it doesn’t remedy even after I open the fuel cap.
I’ve got a sneaking suspicion it’s electrical. The worst part is I can’t duplicate it as the bike isn’t registered or road legal. So I get to the track and after 30min or so I get the problem and the bike won’t start for my next session. Hugely annoying.
I’ve since removed the fuel pump and filter again to double check everything and verify fuel pressure. I have another day planned in a couple of weeks, so we’ll see if the issue persists. Next step I think is making a new harness and finding a new ECU.