Finding the roll center and engineering its dynamics

[Sid_RallyX_82]

One of the Engineering Explained videos was on roll center and how to find it by using imaginary lines through the control arm pivots on Double Wishbone and another line between each side's point of intersection and for McPhearson Strut, following the line of the LCA and a line perpendicular to the steering access at its uppermost point and again a line between each side's point of intersection, but, being that I can't take a cross section of my car out of it and make adjustments, does anyone have any ideas on what I CAN do? I would just ask for the setting You guys use, but almost everyone here is lowering their Festi for track and I'm going to raise mine for rally. Thanks guys!

[Greywolf]

I don't feel exactly happy with that guys explanation of what happens with a McPherson suspension.
(The guy on Engineering Explained)

I'm going to look around a bit and see if I can find a better analysis.

We also have a strange rear axle situation, and I'm not sure if we have a flat tracking pair of rear wheels/tires, or if there is a certain amount of independent movement. The "K-frame" rear axle mount looks similar to later VW Beetle setups that were designed to keep the wheels straight up and down instead of "Pigeon Toeing"

Comments anyone?

[Advancedynamix]

If I were setting a festiva up for rally, I would leave the ride height stock and use different shock valving and spring rates/bump stops to control body attitude. These cars are actually already set up pretty well, except the shock valving is way off the mark. These cars are exceptional off road, and the short wheelbase means you don't need tons of ground clearance. In fact, I'm willing to bet you could even lower the car a bit if you could fit decent rally tires in the fenders.
The key to success is to keep it simple. If you spend too much time and energy trying to change everything then your just spinning your wheels. I bet Chris Duplessis could take a stock chassis festiva with a dohc b6 or bp and be competitive in it. Spend less time working on the car and more time driving it and making it reliable. Just my .02

[firebush357]

So where do yall do RallyCross Racing? It looks like a lot of fun and I would like to setup a car for it but the only track around me that I know of wants $200 a day to play on the track... That seems quite high but I think it is setup more towards professionals.

[Advancedynamix]

I don't feel exactly happy with that guys explanation of what happens with a McPherson suspension.
(The guy on Engineering Explained)

I'm going to look around a bit and see if I can find a better analysis.

We also have a strange rear axle situation, and I'm not sure if we have a flat tracking pair of rear wheels/tires, or if there is a certain amount of independent movement. The "K-frame" rear axle mount looks similar to later VW Beetle setups that were designed to keep the wheels straight up and down instead of "Pigeon Toeing"

Comments anyone?

The trailing beam on a Festiva is very independent. The geometry change of the rear wheels is dependant on toe and camber settings. With 0 toe and 0 camber the wheel will remain straight through its travel (not considering lateral beam flex, which is a good thing). With negative camber and 0 toe, the wheel will toe in with compression and out with extension of the suspension. This helps turn the car. I recommend negative camber on these cars.

[Sid_RallyX_82]

K, I have a lot of questions. First, what spring rates should I use for what terrain? Is soft good for rough? Stiff for large jumps? Or does that affect valving more than spring rate?

[Advancedynamix]

I would run KYB gr2 rear shocks for the Aspire with small body bilstein sleeves and 120lb rear springs for rally. Adjust for large jumps wI think your bump stops. You may want to shorten the shafts an inch or 2. The stock kyb rears have very soft valving, which will be good for washboard dirt roads. I'd run MK2 VW GOLF front coilovers shocks. The spring rate largely depends on what engine/trans combo you use.

[Sid_RallyX_82]

Shortening the shafts is to prevent bottoming the shocks on hard landing, right? I read about that on your Advanced Susp. Mods thread. Also, the bump stops you said can be used to control body roll, balancing weight transfer when accelerating out of the curve, and such like that. How do I determine how tall and stiff they should be to bring everything together?

[Advancedynamix]

No, shortening the shafts adjusts the amount of droop you have. Droop is the major cause of body roll. You'll want more droop on a rally car than I would suggest for a road coarse car, but it still can be reduced to a minimum to help control body roll.
Your bump stop stack is really up to you. There is no right answer here, it's all dependant on your driving style. This is one of the reasons I recommend getting out and driving the car rather than spending all your time building a parts list on paper. You won't know what you need unless you know what the car does and what you want it to do better.

[moz]

Finding the roll center and engineering its dynamics

Awesome info Charlie.
My friends kid wants to get into autocross and motorkhana events so I recommended a festiva.

We have a massive abundance of 4 doors here. Would the rear doors add to body flex? Would welding the rear doors shut achieve anything positive?

Should we start a thread called "ask Charlie"? [emoji12][emoji12][emoji12]

[Sid_RallyX_82]

I've mostly only driven in mud and hard-pack dirt and had one day of fun in the snow, but from all those, my car oversteers a lot if I'm not at least 25% on throttle. When I bring my gear with me (extra jugs of water, 30lb bag of tools, spare tire, etc...) I put it all in the front passenger seat to keep with your weight forward thing and it helps a lot, so I think I should start with better tires, instead of my 155/80 winter tires and do some more weight reduction. What exactly is "droop"? I've heard and read this term, but I can't find a definition... And how will the bump stop stack be affected by my driving style? Give me a few examples like if I drive -like this- then I'll want my bump stops -like that-
Thank you guys so much!

[Sid_RallyX_82]

Looking at Mk2 Golf struts on Autozone. What do you guys think of these?
Gabriel Ultraimage.jpg
Gabriel Guardian image.jpg
KYB Excel-Gimage.jpg
Just wondering what I can expect from each of these

[Sid_RallyX_82]

Aaaaaaaand the very expensive Bilstein B6
image.jpg

[Advancedynamix]

Awesome info Charlie.
My friends kid wants to get into autocross and motorkhana events so I recommended a festiva.

We have a massive abundance of 4 doors here. Would the rear doors add to body flex? Would welding the rear doors shut achieve anything positive?

Should we start a thread called "ask Charlie"? [emoji12][emoji12][emoji12]

I would be more concerned with the added weight and length of the 4 door chassis.
Sometimes I can't help but be frustrated at how much focus poeple place on eliminating chassis flex. Flex being a bad thing is one of the most common fallacies in the automotive performance world. Flex is NOT a bad thing. It can be excessive and destructive in a poorly designed car, but the Festiva is a far cry from poorly designed or built. Eliminating chassis flex from a well designed unibody is much like replacing the tires with hard plastic rollers to reduce sidewall deflection. Chassis flex is an important part of the cars suspension system. Auto manufactures spend millions of dollars designing and testing the car's chassis to get it to flex just right, then "tuners" spend 50 bucks on steel and welding rod to eliminate the flex without thinking twice about what purpose this flex may have.
That being said, a longer and heavier festiva won't be able to carry as much speed around corners without becoming unstable.

I've mostly only driven in mud and hard-pack dirt and had one day of fun in the snow, but from all those, my car oversteers a lot if I'm not at least 25% on throttle. When I bring my gear with me (extra jugs of water, 30lb bag of tools, spare tire, etc...) I put it all in the front passenger seat to keep with your weight forward thing and it helps a lot, so I think I should start with better tires, instead of my 155/80 winter tires and do some more weight reduction. What exactly is "droop"? I've heard and read this term, but I can't find a definition... And how will the bump stop stack be affected by my driving style? Give me a few examples like if I drive -like this- then I'll want my bump stops -like that-
Thank you guys so much!

Droop is basically the amount that your wheels drop when you raise the car. You need it in the front because you don't want your driving wheels to leave the ground, but the rear is not that necessary on a FWD car. If you jack up a stock festiva, you'll see that they have a lot of suspension travel that is taken up by the cars weight (a lot of droop). This suspension travel is trying to lift the car, but it doesn't have enough spring pressure to lift the car. However, when you make one side if the car lighter with lateral g force (in a corner) the spring will be able to lift the inside of the car and cause body roll. If you eliminate this extra shock travel, you will reduce the body roll significantly without the need of stiff springs or antiroll bars. This allows the chassis to absorb bumps easily, and maintain traction around corners.
Bump stops really aren't there to stop your travel, they are there to add spring rate at a given place in the travel. They can be abrupt or gradual. They can be tuned to induce oversteer at a given point in a corner, or they can be used to reduce "packing" over high speed washboard sections. In order for me to explain the proper use of bumps stops to tune your chassis, I would have to write several chapters on driving technique and car control. It's just easier to give you a suggestion for a problem that you are having with the handling of your car.

Looking at Mk2 Golf struts on Autozone. What do you guys think of these?
Gabriel Ultra[ATTACH=CONFIG]19697[/ATTACH]
Gabriel Guardian [ATTACH=CONFIG]19698[/ATTACH]
KYB Excel-G[ATTACH=CONFIG]19699[/ATTACH]
Just wondering what I can expect from each of these

I would go with the KYB shocks. They are the best bang for your buck. The Bilsteins will be too stiff for your light little car on dirt.

[Sid_RallyX_82]

Ok, so to reduce droop, you said to cut some length from "the shafts". Is that the piston rod which bolts to the top mount? Or the rod separating the assembly from the bottom mount? "Packing" I assume is the suspension staying mostly compressed through washboard? Sorry I'm not so clear about the terminology. Trade schools teach us about being repair techs, not performance tuners