Finding the roll center and engineering its dynamics

[moz]

Whoa! I missed this reply altogether.

Thanks for the info Charlie!
Much appreciate you taking the time to respond and explain.

[Advancedynamix]

Moz, I did understand your concern. My response, however, was not meant to be directed at you. My apologies for that. It was simpliy a rant to all those who put chassis flex issues as a priority over weight savings.
Weight in the rear of these cars is very bad for performance, even just door handles, latches and added crash bars (inside doors) further back will make a difference. However, a 4 door has much more to support the doors than just that. The body of the car is reinforced to accommodate the extra doors. I have not tested a 4 door car to say for sure weather flex is an issue, but I highly doubt it's a problem. The rear wheels on these cars are more along for the ride than a controlling member of the chassis. Wheel alignment is dictated by the trailing beam which is not as susceptible to chassis flex as a multi link suspension (the beams strength is built in).
Having driven both caged festivas and stock chassis festivas on the race track with sticky race tires, I can honestly say I prefer the flex of a good straight chassis without a roll cage.
Cars that need extra reinforcement in the rear are typically either very heavy FWD or RWD/AWD cars. Lightweight FWD cars only need it in cases of very extreme speeds that require more downforce than the car was ever designed to take. Some cars are so horribly designed from the factory that they fall apart when driven aggressively with race tires, but I have yet to see this problem with a post 1975 Mazda designed chassis.

[moz]

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 don't think you understood my question Charlie.
I'm aware of flex being a good thing, but too much flex can be a bad thing also right?

So my concern with the 4 door is that it has two extra holes behind the pillar and perhaps different chassis dynamics than the 2 door.
Do you thing the two doors at the back would promote extra flex compared to a 2 door?

Re weight and length:
Yeah I see where store coming from. But the difference is marginal in weight and I'm not even sure if the wheelbase is any different between the 2 and 4 doors.
The wagons and sedan models are longer, I know that much

[Sid_RallyX_82]

Cut it below the register? Any chance you have a picture on hand? It sounds like I need a lathe...

[Advancedynamix]

You can cut the piston shafts just below the register for the washer that holds the top mount. I drill and tap both sections and use a stud to hold them together and then I weld the two parts (after the desired section has been cut out to shorten them). The first set I shortened has been thrashed for over 100k miles now with no problems.
Yes, that is shock packing. Shock packing is a common problem when street cars are driven fast off road. If your rear shocks pack and go stiff then your little car will just want to spin in circles at the hint of a corner. A long, progressive bump stop will help prevent this. This is also why I recommend the stock fitment kyb shocks for the rear of a rally car. They have very soft rebound dampening. This will be less prone to packing than the VW MK1 shocks that I use on street and road coarse cars.

[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

[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]

Aaaaaaaand the very expensive Bilstein B6
image.jpg

[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]

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!

[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]

[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.

[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]

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]

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?