My Handling issues , figured it out after Summit Point

[DaveW]

Rd Am was another mixed bag of results, but for the 1st time in several years I was feeling comfortable and confident in the car. The car was so good we twice lowered the rear wing setting down to 5.6-deg (the lowest I've ever run) with respect to the chassis. We also lowered the front wing. As a result, the car was a rocket on the straights, but still had excellent grip in the turns.

Lap times were getting better each session, and I finished 6th in the 1st race. In another lap, I would have passed another car to finish 5th. I was a bit annoyed because the race was shortened by 2 laps even though we finished well within our time limit.

We were looking good in Q2, but after a stop to measure tire pressures and temperatures, the car would not restart because the starter pinion was slipping out of engagement with the ring gear (see next post for the root cause). We could have bump-started for the race, but during Q2, the car also developed a vibration, so we packed up and went home to avert any further damage.

I'll correct this issue, and our next (and last) race of the season will be at Summit Point since we're skipping NJMP. Summit will be a good test of the setup because it's a low-grip track where I was really frustrated last year, leading to my discovery of the front swaybar MR issue.

[DaveW]

...I'll correct this issue, and our next (and last) race of the season will be at Summit Point since we're skipping NJMP. Summit will be a good test of the setup because it's a low-grip track where I was really frustrated last year, leading to my discovery of the front swaybar MR issue.

I found the cause of the starter issue - one of the starter-bracket mounting bolts fell out (I'll Loctite it this time since it's not accessible when everything is together), allowing the starter to move away from the flywheel. The skipping teeth also caused the starter bolts to its bracket to loosen, exacerbating the issue. Next step will be to pull the trans off the engine and assess how much damage this did. I have a spare flywheel, which I'll install if necessary.

UPDATE 7/6/24:

I got lucky. No apparent damage to the ring gear, and the starter pinion is apparently in good shape (teeth look good and gear OD is the same as a new one). I got the trans on and the starter bolted into place.

I red-Loctited a stud into the hole where the bolt came out and used jet-nuts on both fasteners to hold the starter on. This should prevent a repeat of this issue. Before I totally reassembled the car and while I could get at everything I checked the clearances and cranked the engine to make sure everything relative to the starter worked as designed. The engine cranked perfectly.

There was no obvious cause for the vibration I felt, but it may have been the loose starter vibrating in harmony with the engine.

Now I just have to finish putting the car back together.

[DaveW]

We went to Summit Point with the same setup we had successfully used at Rd America, i.e., low wing settings, etc. I wanted to see how the excellent grip and feel from RA would carry over to SP.

We started off well, but I was still looking for more grip on the low-grip SP track, so over the weekend we at various points tried softening the damping, lowering the front and rear ride height, lower tire pressures, softer springs, less stiffness in the swaybars, etc.

Some of these changes initially seemed to help, but in race 2, handling went off midway through the race and we wound up with rubber pickup on the tires, plus lap times suffered. So apparently we went too soft and there was not enough stiffness to scrub off the pickup. In addition, the car seemed a bit too sluggish in its response to inputs.

So, in retrospect, lowering rates, damping, ride height, etc. from the Rd America setup was the wrong way to go. Apparently, I need to stiffen the car a bit to enable predictable use of higher slip angles.

Next time out (next year at Rd Atlanta), we will experiment with slightly stiffer springs and damper settings based on the Rd America setup.

So the saga continues...stay tuned for next season.

Edit: The one thing I didn't do is make any significant change to downforce - see posts # 44 & 46.

[DaveW]

...

Next time out (next year at Rd Atlanta), we will experiment with slightly stiffer springs and damper settings based on the Rd America setup.

So the saga continues...stay tuned for next season.

From some information I recently became aware of, apparently I'm still using too much downforce and overloading the tires such that they don't want to slide, making breakaway unpredictable. That also accounts for not being able to scrub off rubber pickup, as I noted in the last post.

[2BWise]

From some data I recently became aware of, apparently I'm still using too much downforce and overloading the tires such that they don't want to slide, making breakaway unpredictable. That also accounts for not being able to scrub off rubber pickup, as I noted in the last post.

Would love to understand what data you're referring to. Off the top of my head there's only 2 scenarios I can think of that would cause this kind of behavior. 1) Tire load sensitivity absolutely falls off the cliff above a certain load or 2) Frequency response of the tire doesn't support super the high ride frequencies required to support the added DF putting the tire out of its window. The 2nd one seems the more likely of the two scenarios but would be interested to hear your findings.

[DaveW]

From some information I recently became aware of, apparently I'm still using too much downforce and overloading the tires such that they don't want to slide, making breakaway unpredictable. That also accounts for not being able to scrub off rubber pickup, as I noted in the last post.

Would love to understand what data you're referring to. Off the top of my head there's only 2 scenarios I can think of that would cause this kind of behavior. 1) Tire load sensitivity absolutely falls off the cliff above a certain load or 2) Frequency response of the tire doesn't support super the high ride frequencies required to support the added DF putting the tire out of its window. The 2nd one seems the more likely of the two scenarios but would be interested to hear your findings.

A bit of a preface: Last spring I finally resolved the front swaybar MR issue, so the incurable understeer that I previously discussed is gone. When it was present, more downforce seemed to help. So I'm still learning what the car wants with that issue resolved.

This latest conclusion is based on empirical knowledge that at least 2 very fast Citation FC drivers have reduced their front wing size (and overall downforce) and gone faster, even on slow tracks. I had been using a front wing that I've used forever that has a 14" chord. Over the past season I've been reducing both front and rear downforce almost every weekend I've raced, and each time the car has gotten easier to drive and faster, but, IMO, I haven't gone far enough. So the next time I drive the car it'll be with a 10.5" chord front wing and less rear downforce to match.

My thinking on this is closer to your #1 than #2. Bias tires have always wanted more slip angle than radials, and if one can't get to the higher slip angle w/o overloading/over-deflecting the tire it becomes unpredictable. In addition, at Summit Point I had pickup on the tires and very low grip at the end of the last race, so that confirms that I had not gotten to the slip angles required.

I know I am running tire pressures that fast drivers/cars have been running successfully, so too low tire pressure is not my issue.

[DaveW]

As I've said before, we have been suffering from a lack of grip in all corners, especially the slow ones. So based on some advice, I set the front swaybar full soft at Mid Ohio, and that seemed to help a bit so the car was slightly faster both in Q2 and R2.

Because I was out of adjustment, and I don't want to eliminate the swaybar entirely, I modified the front bellcranks to reduce the swaybar link attachment point radius to the BC pivot to 7/10 of what it was initially before the FRP PIRC weekend.

Since our PIRC weekend was so short (our RF suspension was damaged by an errant FA and I didn't have all the parts to complete the repair), we didn't really get to evaluate the front swaybar link radius change I made, but in the 1 practice I ran there it seemed good, so I made another set of bellcrank holes to reduce the front swaybar link hole location radius further to ~0.58 of what it was initially. That lowers the swaybar stiffness to ~1/3 of its initial value. This is as low as I can get with this approach, since I had to clearance the bellcrank so the link rodend wouldn't bind.

We missed the Road America FRP weekend due to no replacement steering arms.

At the Watkins Glen FRP weekend, we made significant progress in obtaining better low speed grip. The lower swaybar stiffness was a definite move in the correct direction.

Fast forward to race 2 (~90F with a lot of TransAm rubber on the track) we lowered bump & rebound damping and also tire pressures a bit and the car was better, and I finally started to feel comfortable in the car, even though it was hot.

Bottom line is I think we're finally on the right track with decreasing the overall roll stiffness way more than I ever thought would be necessary. I have known that too much roll stiffness from any cause (friction, too high roll centers, too much damping, etc.) will degrade grip, but I had not realized how much reduction in roll stiffness was necessary for the Hoosier spec FC bias tires.

I'm looking forward to Summit Point which is a low grip track, and if we go well there, it will be confirmation of this approach (vastly lower roll stiffness).

If the latest front swaybar setup in its softest setting is not soft enough, the next step will be to disconnect the front bar completely.

[Lee Johnson]

As I've said before, we have been suffering from a lack of grip in all corners, especially the slow ones. So based on some advice, I set the front swaybar full soft at Mid Ohio, and that seemed to help a bit so the car was slightly faster both in Q2 and R2.
*
*
*
Bottom line is I think we're finally on the right track with decreasing the overall roll stiffness[?] way more than I ever thought would be necessary. I have known that too much roll stiffness from any cause (friction, too high roll centers, too much damping, etc.) will degrade grip, but I had not realized how much reduction in roll stiffness was necessary for the Hoosier spec FC bias tires.

I'm looking forward to Summit Point which is a low grip track, and if we go well there, it will be confirmation of this approach (vastly lower roll stiffness) [front only or rear also?].

If the latest front swaybar setup in its softest setting is not soft enough, the next step will be to disconnect the front bar completely.

Dave-- much of your earlier discussion on these handling issues was focused on front end modifications including "MR" (motion ratio, I presume) and specifically regarding the front sway bar. In this last post you use terms (I italicized above) including "overall roll stiffness" without being explicit about whether "overall" included front & rear; or perhaps front-only but including both chassis spring stiffness & roll bar stiffness (maybe also to include tire stiffness reduction thru lower pressure?)

One inquiring mind would like more clarity .

PS -- I always like your posts for their analytical nature, but, like a squirrel, I maybe too self-distracted to follow it all without a clear trail of crumbs.

Lee

[S Lathrop]

Dave-- much of your earlier discussion on these handling issues was focused on front end modifications including "MR" (motion ratio, I presume) and specifically regarding the front sway bar. In this last post you use terms (I italicized above) including "overall roll stiffness" without being explicit about whether "overall" included front & rear; or perhaps front-only but including both chassis spring stiffness & roll bar stiffness (maybe also to include tire stiffness reduction thru lower pressure?)

One inquiring mind would like more clarity .

PS -- I always like your posts for their analytical nature, but, like a squirrel, I maybe too self-distracted to follow it all without a clear trail of crumbs.

Lee

A simple answer to your question is that in calculating roll resistance, there are 3 different spring rates that come into play. First. is the rate to the springs themselves. And that is the force to displace a wheel vertically. Second is the spring from the anti roll spring system. Again this is expressed by the what it takes to displace the corner vertically. The third spring rate is the force it takes to displace a wheel relative to the the wheels at the opposite end of the chassis. The third is referred to as the chassis rigidity.

A Zink Z!0/Z11 was about 1200 foot pounds per degree. The the second chassis I tested was the Z14 which was the Z11 but with an aluminum monocoque chassis. It tested over 4000 ft./lbs. The current Citation tube frames are better yet. As the chassis became stiffer there was a big learning curve that came with the increasing stiffness. Details that were not significant with a soft chassis become very important as the car gets a lot stiffer. A sot chassis will make good grip but it may not be very responsive to driver inputs. A poorly done suspension system on a stiff chassis will leave the be super frustrating to get the handling as you want it to be. I speak from experience with these problems. Dave was instrumental in getting things right as we developed our designs.

[DaveW]

Dave-- much of your earlier discussion on these handling issues was focused on front end modifications including "MR" (motion ratio, I presume) and specifically regarding the front sway bar. In this last post you use terms (I italicized above) including "overall roll stiffness" without being explicit about whether "overall" included front & rear; or perhaps front-only but including both chassis spring stiffness & roll bar stiffness (maybe also to include tire stiffness reduction thru lower pressure?)

One inquiring mind would like more clarity .

PS -- I always like your posts for their analytical nature, but, like a squirrel, I maybe too self-distracted to follow it all without a clear trail of crumbs.

Lee

By overall roll stiffness, I am referring to the stiffness related to the chassis' input response time. With a stiff chassis, if it's too soft in roll, the car is sluggish and hard to drive. If it's too high, the car is usually easy to drive, but will lack grip.

And in my and others' experience, the front suspension's roll stiffness has most of the influence on overall roll stiffness and response time. The rear roll stiffness is more effective in tuning understeer vs oversteer.

Steve mentioned that above, but I wanted to state it in a different way.