Howard Coleman

iTrader: (6)

lots of good comments here but

the first question to answer is:

are you going to be driving on the street?

second question is:

what tires?

if the car is primarily street driven and if it has non race tires...

the first item on the agenda is getting the longitudinal balance correct. which means softer rate in the rear. front engine rear drive cars need to hook the rear. softer rear checks the box.

mazda knew that. the FD came w:

293 pounds per inch/5.2 KG front
195 pounds per inch/3.5 KG rear

i owned both a shock dyno and a digital spring test instrument. i dyno'd 31 OE shocks as well as different coil overs and zillions of springs including FD OE.

in order to properly balance the FD it needs about 30% less spring in the rear.

OTOH, if you want to do the drift thingy you add rear spring so the rear doesn't optimally hook up... so you do equal rate or perhaps higher. or pump up your tires to 50 psi (not recommended)

"but i have Ohlins and they are 11/11."

according to someone i talked to at SBG, after pressing him on WHY 11s in the rear, the answer was there wasn't enough travel to accomodate a softer spring. SBG has since re-engineered the rear, good for them.

conclusion one... 20-30% softer in rear for longitudinal balance

as to overall rate:

executive summary:

8/6 for most including me, 10/8 for more track oriented.

the stiffer the spring the less time the tires is actually contacting the road rather than being in the air. the stiffer the spring the less input to the driver before breakaway. softer is way easier to drive at the limit. the FD has a racecar suspension. one of the keys is that the camber goes negative as the tire rises into the wheel well. ("negative camber gain on bump"). we would be talking about the outside wheels in a turn not losing their important negative camber. most double A arm cars don't have camber gain and wash out/understeer/push in a corner. in order to make them work it is necessary to run a really stiff spring to limit suspension travel.

not so w the FD.

i could go into corner weights and motion ratios etc... but the summary is you want around 1.5 to 2 inches of travel to keep the rubber on the pavement and for feedback near the adhesion limit. put a tie wrap on your shock, go for a serious drive and check the travel.

you might start the process by taking a look at the spring rate of your tires. set them 30 f and 27 r cold.

i currently run a set of Pettit Trak Pro single piston coil overs. i was happily running a set of Tein HAs 8/6 when Cam called and offered a test set. why not? i dyno'd them and they looked good. eventually i got around to focusing on them and realised i was chattering over the bumps. i set them full soft and they still chattered. please do understand that just like you can't run lower boost than the wastegate spring, you cannot get a lower compliance from a damper setting than the spring rate. a 10 is a 10 no matter what damper is attached. i simply visited my favorite spring manufacturer site, Eibach, and bought a set of 8 inch by 2.5 I D springs. Eibach makes plastic insert to enable the typical 60 mm coil over spring.

here's a link to my front springs:...$75 each

https://eibach.com/us/c-9-ers.html

Eibach is the real deal as to springs... pick your rate in 25 pound increments, add an Eibach adapter for your coil overs and you are good to go.

ArmenMAxx

iTrader: (21)

Ill leave my 0.02 here

I recently installed my JRZ's with 13k springs. 285 square 200tread tires, tanabe front swaybar stock rear, GT wing. On soft, feels very compliant on the street similar to your typical off the shelf ohlins. I need to more spring for the track, this is with compression full stiff.

Billj747

Pettit was 10k? What do you mean by chatter? What was the car doing?

j9fd3s

iTrader: (3)

the driver we tailor our stuff for (several national trophies, couple of in class track records, lots of gray hair lol!) likes the car to understeer a little with no throttle, basically so if he's on the gas a little early its ok.
we collected all the data, he's faster, and part of it is that there is no easing on the gas, its like a square wave.

if this graph is throttle/brake the blue box is faster, for us




he also likes the car to respond quickly, and not move around a lot (like a gokart) so its pretty stiff.
we were running like 950/500 in a 2360lb miata on 205's. one time we had it at a car show, and on the way back to the trailer i got waved into traffic, and went around the block in SF!
you'd need to calculate the wheel rate FD vs Miata, but it rode really well, and was quite soft on the track. Ohlins FTW

out process for stuff is pretty dumb, and racing is simple, its either faster or not. we just add spring until it got slower.
on the street though its totally different, speed isn't a measurable metric anymore.

MilesBFree

iTrader: (2)

Thanks @Howard Coleman . It's been a very long time - hope you all are doing well.

I saw you recommended: "8/6 for most including me, 10/8 for more track oriented"

Thanks for clarification on the softer rears - i appreciate it

Edit: the link to the Eibach site gave me a random spring rate of 700/~12.5, but it appears that is due to my browser having cookies turned off. That clarifies the question i have now deleted.

MilesBFree

iTrader: (2)

Thanks j9fd3s. That kinda describes my driving style too - very responsive and not a lot of movement unless i input it (or screw up :-) ). Generally, understeer is annoying and my first instinct is to try to drive out of it but that isn't really do-able - need to back off it instead. That really scrubs off speed and makes both me and the car miserable. Although now being older i am not sure i CAN still drive it like that - requires a lot of car control to balance oversteer and personally i think some of that has gone away for me, being brutally honest with myself

gracer7-rx7

iTrader: (16)

Race and track driving is a perishable skill set. Takes seat time to get back up to speed. Sometimes lots of seat time... Sigh....

MilesBFree

iTrader: (2)

Thinking more about my old setup (600lb / 10.7kg front, 300lb / 5.3kg rear)... The alignment Tri-Point recommended for autox had a bit of toe OUT in the rear. I didn't dig that - it was quite squirrely and too much oversteer even for me - so I had another alignment to set it back to standard. But in any event, I am wondering if their drivers liked a looser back and so the spring rates were very soft compared to the front.

Thinking 650lb / 11.6kg front and 500lb / 8.9kg rear, to be in the 25% less range that falls within @Howard Coleman 's 20 - 30%.

Valkyrie

An FD will still understeer somewhat even with even spring rates, in my experience. Given the weight balance, motion ratios, and stock sway bars. Some people here actually use higher rates in the rear. The latest HKS track-oriented coilovers are 16/18! The last gen were 16/16.

BLUE TII

iTrader: (9)

I agree with that.
I put softer than stock front sway bar with the R1 rear bar, bushings to eliminate stock toe change, zero F&R toe alignment and about 100lbs off the front end to eliminate understeer on my FD on 11k/11k Ohlins and square wheel/tire.

Id like to get a fuel cell in the rear bin area to get the gas tank ahead of the rear axle to help fix the changing chassis balance with fuel consumption.


From what I have seen from watching Japanese programs it seems to me it is the FDs with GT wings that run higher rear spring rate to help decrease the downforce induced understeer.

Valkyrie

This is probably true. It’s rare to see an FD run without an aftermarket wing of some kind.

MilesBFree

iTrader: (2)

OK, I am having doubts...

I was set yesterday to order the shocks from Guy and the Eibach springs directly, but when I sent the info to Guy to confirm springs he replied with a couple of changes that introduced doubt:

1. Go with 800/600 as a good all-around spring rate. The Penske 8300s will handle those rates better than other brands, per him. OK, I can go with that.

2. Use 7" height front and 6" rear. OK.

3. Add 150lb "helper" springs to the rear. I put "helper" in quotes because his definition is different from Eibach's. Eibach's is: "Keeps Main Spring Seated When Suspension is Unloaded". They list theirs as 11lb and the Tri-Point shocks that he built back in the day had those. Photo below of my compressing a helper spring between two fingers. I replied to confirm the definition and he said yes 150 lbs. The Eibach web site shows their tender springs as 150lbs, and definition as "Provides a Softer Initial Rate When Paired with a Main Spring", which seems to be more what he is describing. I did more searching to make sure my understanding and Eibach's is the same across the spring industry and it appears it is. I occasionally mis-speak so not too too worried about the definitions, but adding 150lb tender springs is a new variable at the last minute and worries me...

4. I asked for more info on the total height of the springs with the 150lb tender added to the 6" main spring - does adding the tender spring to the main spring change the recommended height of the main spring? Also, the Eibach tender spring in 2.25" ID is available in 2.72" and 3.39" heights - which one? Awaiting his recommendation. Stymied on ordering springs...

One of the reasons I went with him is that this was to be turn-key but it is getting to be a bit less so

Valkyrie

Any spring that’s is fully compressed at normal ride height isn’t going to change the handling one lick.

Assuming a 1:1 motion ratio, it only takes 450 pounds to bottom out a 3” 150 pound spring. An FD corner weight about 700 pounds.

The spring length will depend on how much droop you have.

BLUE TII

iTrader: (9)

The 150 Lb spring will be collapsed in a static state as noted.

The 150 Lb spring will only be in effect when a tire is off the ground. Then the 150 Lb spring will come into play as the tire settles back down to the ground and you get a more gradual (dual) spring rate.

The FD is narrow and we fit wide wheels/tires, so you will see autox pictures of FDs cornering with one or both inside wheels off the ground fairly regularly.

It feels kind of like a damaged sidewall when you 2 wheel it (gets squirmy) but it increases your cornering grip as it puts more vertical force on the outside tire's contact patch.

When the inside tires settle back down you (counterintuitively) suddenly lose grip as the outside tires now lose the increased vertical loading, so it is nice to have this unloading happen more gradually (the 150 Lb spring).

MilesBFree

iTrader: (2)

Thank you for that clear explanation. Much appreciated!