R7 front end chatter when I switched from Pirelli to Dunlop

Hey all. I figured I would post here with all of the knowledge on this board in hopes of getting some more information.

A little about the bike, I am running stock forks with Ohlin's carts, 9.5 springs (in the bike currently), Robem triple, Robem rear linkage, Ohlins TTX GP rear shock.

I rode Dunlop's for the first time at Roebling last week and it was very unsettling. I had a lot of front end chatter any time I was coming off of trail braking in turns 4, 5, 6. I ended up going down in turn 4 in a small low side due to the chatter.

I was running an R5 front / R7 Rear. Played around with the tire pressure and went up to 40PSI in the front. It seemed to get rid of a lot it. But I only had one session at the end of the day after getting the bike and myself ready to go back out and push again. I know there is a thread on this already. The bike felt amazing on Pirellis, I made the switch due to wanting to try to do the Moto A round at Road Atlanta.

I figured if anyone has any pointers to get me in the right direction, I would greatly appreciate it!

 

I'm sure you'll get useful suggestions to address your issue, but be aware that there are now noticeable ripples in those turns caused by the outside (loaded) tires of cars. I raced there three weeks ago and found the ripples to be noticeably worse than in February '23. They're not all that hard to miss once you know they are there. Apologies if you're already aware.

 

The experts will be along shortly, but I’ve talked with a number of racers that have chased that chatter on the R7’s for sure a while. Try 42psi hot in the front and then play around with spring rates (I’ve heard a bit stiffer that you would normally run). I know the N2 Bobblehead Championship bike (under Blake) was running R1M forks as part of the solution to get the bike handling right.

The frame of the R7 is very simple a bit of a noodle and not a proper twin spar aluminum racing chassis. So the chatter seems to be baked in to some degree, at least with the Dunlops. Can you go back to Pirellis if they worked better for you? Edit: I missed the MA race part. Ignore the return to Pirelli question then….

 

Have you tried going up to the R3 and running at 38?

 

I rode there in November at the James Bock test, and I didn't really notice it on the race line. Thank you for the heads up!!

Yea I have heard a lot about that. It's just so weird how I had zero chatter issues on the Pirellis but out of nowhere I have it on the Dunlop's. I will definitely try upping the pressure! Thanks!

 

No, I only tried the R5 front. I plan on trying an R3 this weekend.

 

It won't help your setup, but when we switched from the R5 to the R3, 2 of the 3 riders said the front end felt slower, despite running the same times. A softer tire may have that feel because the compound is more plush. I would also try the R0 just for comparison.

 

I have no claims of being an expert, but I have a good amount of time with the Dunlops from an analysis perspective, and the Pirelli's from a my own racing perspective.

Everything you're describing sounds like a second order effect coming from a natural frequency oscillation originating from the rear wheel.
Taking compound (SCx-3 / R0-7) out of the conversation for a moment, Pirelli's generally have a softer sidewall than Dunlop.
With all things being equal (combined weight, damper/spring settings, and suspension geometry), it's unsurprising you're having what feels like from end chatter.

Before you add multiple variables into your troubleshooting, take the simple approach and add a couple clicks of rebound to the rear shock, and try again, continue adding rebound until your front end chatter is resolved, but before you introduce a new issue like packing the shock.

You can spend ages making change after change to the forks to get them working properly, for what could simply be a damper setting change on the rear shock.

Cossalter writes on this in 8.9.3 of Motorcycle Dynamics.

 

How much do you weigh and what is your pace? These bikes are a bit of an anomaly as they require very light front springs. Also, Roebling being a flowing track with no hard braking requires a pretty soft setup. Unless you're over 225 lbs, I would say .95s are too heavy.

 

Thank you! I'll try that out this weekend as well.

I'm 178lbs, I'm newer to the R7. Second time ever riding and first time on it at Roebling on Pirellis I got down into the 17s. So, I figure maybe drop another second?

 

Id get a .90 and a .85 That would give you .95, .925, .90, .875 options. Honestly I would start at a .875 at Roebling. Rd Atl and Barber you may need to go up.