Finding the roll center and engineering its dynamics

[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]

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

[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

[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]

Whoa! I missed this reply altogether.

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

[Advancedynamix]

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

Yes, a lathe will be necessary to do it the way I do. You could risk it by just cutting and welding the shafts. I like the added security of a threaded stud in between the welded pieces.



Here I have cut the shaft to my desired length, then I threaded and chamfered the shaft.


The top part of the shaft is then threaded and chamfered to match.


The top is then attached to the shortened shaft with a threaded stud.


And tightened.


The shaft is then welded back together.

[Sid_RallyX_82]

Ok I don't have a lathe, so if I could avoid paying a machine shop to do it, what do you think might be the next best way to do it? Make a jig?

[Advancedynamix]

Look for shocks that are already shorter. That's one of the big reasons I use the VW mk1 shocks. Maybe a cheaper shock (Monroe or no name ebay shock) would have softer rebound dampening than the KYB shocks.

[Sid_RallyX_82]

So that I have a reference for shorter, what are the dimensions of the stock equipment?
Length collapsed:
Length extended:
Total travel:
Thank you for all the help

[Advancedynamix]

From the lower bolt center to the register where the washer sits that holds the top mounting bushing is;

23.5" extended
16" compressed
7.5" travel.

[moz]

You Americans are slowly "inching" towards the metric system.

Why can't y'all be more universal!?!? [emoji12]

[Eaglefreek]

You Americans are slowly "inching" towards the metric system.

Why can't y'all be more universal!?!? [emoji12]

Inching. When I was in elementary school in the 70's, we were told the US would be converted to the metric system by the time we got out of high school. Obviously, it never happened.

[moz]

The NRA used to be the NIA (national imperial association) back in them days.
Charlton Heston cut his teeth on making sure all American measurements are done in fractions or by irregular body parts.
Inches based on thumbs, feet based on.... Well, FEET!

I wonder which body part they would've used if they wanted an increment between inches and feet.... HMMMM.... [emoji848]
[emoji57]

[moz]

Aaaaand the fun is spoilt.
Between inches and feet lie a hand (4") and a span (9")

I've gotta get my mind out of the gutter and stop looking at how many "hands" are in my pants. [emoji13]

[Sid_RallyX_82]

From the lower bolt center to the register where the washer sits that holds the top mounting bushing is;

23.5" extended
16" compressed
7.5" travel.

That's for the front, right? Do you know the rear?