My Handling issues , figured it out after Summit Point

[DaveW]

As some of you have seen, I've been struggling with my Citation's handling since we modified the suspension after my Watkins Glen wreck in 2018, especially since we switched to the SCCA FC-spec Hoosier bias tires. We've been continually trying different spring rates, ride heights, damper settings, swaybar motion ratios in the rear, etc., etc., and some things got better, but others got worse.

At Summit Point, which I usually like, I threw a lot of changes at the car, none of which helped much, and some (before race 2) that made it worse. My times and race results throughout the weekend sucked.

I pride myself on getting my car working properly, but have been getting VERY frustrated with the lack of improvement. The situation was very confusing, and it was getting so bad that I, for the 1st time on the drive home, thought maybe it was my driving that was the issue and considered no longer driving in competition.

Monday, the day after Summit Point, I began seriously looking for design or mechanical causes and I finally identified the issue. There is an error in the front suspension geometry that hit me like a ton of bricks. It was so obvious that we (Steve Lathrop and I) should have never let it happen.

Luckily, it should be pretty easy to correct. So, again, we are looking forward to next season with FRP !!

[2BWise]

What'd you find?

[DaveW]

What'd you find?

The swaybar link attachment points to the bellcranks made the swaybar increasing MR (rate) in bump. The result is similar to the effect (understeer) of front increasing-MR-in-bump geometry. New bellcranks will correct it.

[DaveW]

The old (RH bellcrank photo) and new (opposite side drawing) are attached below.

[jtaphorn]

Dave says, "There is an error in the front suspension geometry that hit me like a ton of bricks. It was so obvious that we (Steve Lathrop and I) should have never let it happen."

OMG Dave, the rest of on the forum recognized the solution as so obvious that we didn't make the suggestion knowing that you would figure it out before to long.

Of course, that is my attempt at humor. You are illustrating the difference between the pros and the rest of us wankers. I am looking forward to your next driving impression after the change. Thanks for sharing.

[BeerBudgetRacing]

Sorry to try to educate myself at your expense....

The swaybar link attachment points to the bellcranks made the swaybar increasing MR (rate) in bump. The result is similar to the effect (understeer) of front increasing-MR-in-bump geometry. New bellcranks will correct it.

The old and new are attached below - the new one will be similar to the RH photo

It looks to me that this is the right bellcrank, dampers are behind (left in picture) and ARB is forward (right in picture).
So the swaybar attachment on the new one is the circled point.

On the old one, the attachment point starts moving both laterally and rearward and the further in moves the less lateral and more rearward movement - thus increasing the MR.

On the new one exactly the opposite happens because is starts with maximum rearward motion and decreases rearward movement and increases lateral movement thereby decreasing MR.

Am I understanding this correctly?

[DaveW]

Dave says, "There is an error in the front suspension geometry that hit me like a ton of bricks. It was so obvious that we (Steve Lathrop and I) should have never let it happen."

OMG Dave, the rest of on the forum recognized the solution as so obvious that we didn't make the suggestion knowing that you would figure it out before to long.

Of course, that is my attempt at humor. You are illustrating the difference between the pros and the rest of us wankers. I am looking forward to your next driving impression after the change. Thanks for sharing.

Having the front swaybar working properly will require some more setup changes, but at least, I hope, I won't be chasing my tail.

[DaveW]

Sorry to try to educate myself at your expense....

It looks to me that this is the right bellcrank, dampers are behind (left in picture) and ARB is forward (right in picture).
So the swaybar attachment on the new one is the circled point.

On the old one, the attachment point starts moving both laterally and rearward and the further in moves the less lateral and more rearward movement - thus increasing the MR.

On the new one exactly the opposite happens because is starts with maximum rearward motion and decreases rearward movement and increases lateral movement thereby decreasing MR.

Am I understanding this correctly?

I'm having trouble understanding what you are saying. Let me say it differently.

In both bellcranks, shown in full rebound, the radius to the pushrod is at a greater angle to the pushrod than the radius to the shock. So as the BC rotates under load, the pushrod angle to its BC radius has more rotation to bring it to 90-deg to its radius than does the shock. This means that the lever length of the shock force increases slower than the lever length of the PR, by definition this is decreasing MR.

The shock and the swaybar link are essentially parallel to each other.

So, in the revised BC, the shock pivot is on the same radius line as the swaybar, so they behave similarly.

In the original BC, the swaybar is on a radius line ~30-deg clockwise, so BC rotation CCW under load increases its lever length faster than that of the PR, making it increasing MR compared to the shock or PR.

So as the car rolls or goes into bump, the swaybar geometry acts like an increasing MR suspension geometry, and understeer results, especially under bump travel and gets worse with increasing roll angle.

This results in swaybar stiffness increase that under certain conditions, decreases understeer, because it decreases roll. It also means that softer front springs might be counterproductive, because they allow more roll and the swaybar MR on the more-loaded side increases. That's what makes it a confusing setup.

Clear as mud, right?

[BeerBudgetRacing]

Clear as mud, right?

I believe I'm understanding.

Looking at the VD Bellcrank the connections of the shock and sway links are at the same radius but at slightly different angles compared to yours at the same angle but different radii .

So the VD bellcrank will have the (although very slight) same condition you experienced
but you setup will require a stiffer ARB.

I think that is the definition of "compromise" in building/packaging these car.

[B17overhead]

One has to look at all link angles vs the pivot on both the pushrod side as well as the ARB side. The pushrod side linear bump motion has decreasing angular rotation of the bellcrank - The 90-degree radius (not the simple straight line point a to point b radius) to the pivot from the pushrod becomes longer which slows the belcrank rotation. So, to determine net change in ARB MR from the pushrod, one needs to look at both input and input rod angles and how they change vs the pivot in bump.

[DaveW]

Another aspect of this geometry error:
Front roll stiffness generally seems more effective in tuning the car's overall roll stiffness, and the rear is more effective in tuning understeer-oversteer balance.

So:

In most of my race weekends since the 2018-19 repair/upgrade incorporating the new swaybar setup, I've suffered from a lack of overall grip. Correcting this error means that the front swaybar will now be much more effective in "hitting the sweet spot" in roll stiffness since the rising rate (MR) swaybar issue will be gone, and the car's overall roll stiffness will no longer be rising rate.

[S Lathrop]

I have some experience with the VD front bell crank setup.

The big difference and why this showed up to be a problem is the motion ratio that I came up with worked fine, until I made the chassis way stiffer in torsion. If the motion ratio had been below .5 for the shocks, this would not have been an issue that bothered anyone. But as you approach 1 to 1 shock motion to wheel motion, things become exaugurated. This applies not only to the sway bars and springs but the shocks as well. A stiff chassis, say over 2000 ft pounds per degree will show issues

Shocks have what is sometimes called nose pressure. That is the force it takes to move the shock. If one shows the dyno curve of a shock without zeroing out the nose pressure, you will see that the force to move the shock might be as high as 30 pounds or so. That is the point on the graph where the shock goes from negative to positive pressure. Guaranteed it is never zero but it can be very close to zero. But most of shocks we run. it takes some amount of force to move the shock shaft. 30 pounds is not uncommon for a lot of the shocks we use today. There are "thru shaft" shocks that have very low numbers but, again, 30 pounds is not uncommon for most shocks.

The reason all this is important is that if you have a high front motion ratio, the load on the suspension has to exceed 30 pounds before the shock moves. But if you car is only 1000 foot pound per degree in torsional stiffness, you don't have much of a worry because the chassis twists to accommodate the loading. But if your car is 4000 or above ft. lbs. per degree, and the motion ratio is above .5, this can be an issue. The car will slide a bit before the suspension moves and the car grips up. Shocks that work well with a stiff chassis and high motion ratio are expensive.

Sorry if this explanation is as clear as mud. Poor Dave has been the person who has had to work through all this.

[DaveW]

I differ a bit from Steve in my understanding of what "nose pressure force" actually does.

While high internal pressure in a (non-thru-shaft) damper creates seal friction (not good) and a relatively small, but measurable damper spring rate, the pressure-related force itself (Steve mentioned 30-lb) acts exactly like slightly preloading the spring perches. I.e., it raises the ride height a small amount that is compensated for when one sets weight balance and ride height during an alignment.

However, having said all that, too much roll stiffness from ANY source (springs, swaybars, dampers, tire pressure, suspension friction, roll-center height, etc.) can, especially when the track is not abrasive, cause a serious reduction of grip, sometimes referred to as "skating" where the tires slip across the surface and don't properly engage with the track surface.

Several (~5) decades ago, before the effects of stiff dampers and excess friction were totally understood, the chassis was often referred to as the "5th spring". This chassis flex allowed too stiff, high-friction suspension and dampers to be used w/o killing grip.

I had an instance (1999 Runoffs) where stiffening the chassis by tying the Pinto cylinder head to the roll hoop with an aluminum plate caused a (at that time) mysterious loss of grip. Removing the plate restored grip to its previous level.

Edit - Effects of excess stiffness, friction, etc. from my 2015 handling presentation, linked here:
https://www.apexspeed.com/forums/att...3&d=1539115694
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...reductions in grip are mainly due to the interaction of load transfer rate with tire and tread-rubber characteristics, some of which are noted below:
•Rubber coefficient of friction depends in part on the time rate of load variation –i.e., too fast, and the tread rubber cannot fully conform to and interact with the road surface, and grip will be reduced. Higher-hysteresis compounds are inherently, due to their greater damping coefficient, slower to conform to the road surface, and thus are more affected by friction and RCH.
•Both suspension friction (load increases before motion can occur) and too-high roll center (always-present geometric roll stiffness) will result in almost immediate transfer of load from inside to outside tire as soon as cornering force begins.
•Conversely, load transfer due to spring rates plus swaybar stiffness develops only as the chassis roll angle begins to increase, i.e., more slowly.
•Therefore, limiting chassis roll-angle by increasing friction or roll center height may result in lower grip compared to similar roll-angle and response-time control achieved with swaybars, dampers, and springs.

[DaveW]

It was a resounding success. The pervasive understeer, heavy steering, and overall low grip issues are totally gone.

We now have to bring the rear handling up to match the front. It's nice to be working on a different issue.

More to come as the season progresses.

[E1pix]

Maybe I should say nothing… but as an outsider if it’s not comfortable, you can’t possibly go like what we all know you’re capable of.

I’ve wondered since the crash if eventually you’d ask if “that time” had come. Any of us would.

But in your case, with your history, I honestly believe it has not. And I never BS about this.

Here’s to carrying on, you and Sherrie both have inspired me since 1972. You still got it.

[DaveW]

When I can no longer drive, analyze and/or work on the car to my standards, I will hang it up. I did that with racquetball when my body would no longer take the beating from playing 3 or more times a week (necessary to keep me sharp) any more.

But I still enjoy the challenge in all three aspects of racing, and as I get older, there's even more satisfaction in doing it.

BTW, Eric, I like your signature: "Once we think we’ve mastered something, it’s over"

It seems like I learn (or re-learn) something new almost every day.

[bgaff6]

When I can no longer drive, analyze and/or work on the car to my standards, I will hang it up. I did that with racquetball when my body would no longer take the beating that playing 3 or more times a week (necessary to keep me sharp) was possible.

But I still enjoy the challenge in all three aspects of racing, and as I get older, there's even more satisfaction in doing it.

Not sure I follow much of what you speak but am impressed and thankful you share with the rest of us mortals! Can’t imagine what you would think of driving my FF but I am sure it would be helpful.
Keep up the passion and sharing wisdom with your racing pals.
brian

[E1pix]

When I can no longer drive, analyze and/or work on the car to my standards, I will hang it up. I did that with racquetball when my body would no longer take the beating that playing 3 or more times a week (necessary to keep me sharp) was possible.

But I still enjoy the challenge in all three aspects of racing, and as I get older, there's even more satisfaction in doing it.

Thanks for taking my comments as intended.

Your last sentence is just the same for our mountain trekking. We absolutely dread our last hike — real, live *dread* — and hope we get bored with it and find something else before that decision is a forced one.

Doubt it’ll work out that way, but seeing drivers like you and Harvey Templeton and Dan Carmichael throughout my life may give a clue as to a proper rebound after we can’t backpack to 13,000 feet any more. Figuring on 2040 for that. :-)

All our best to you both.

[DaveW]

FYI:
https://www.apexspeed.com/forums/att...3&d=1539115694