On my 914 project I'm putting together some DIY rear coilovers. Most folks seem to get 8" springs whether by Ground Control suggestion or recipe I'm not quite sure. I have read one or two have received 10" springs.
Without calling vendors and asking a bunch of questions while never spending a dime with them, I'd like to know if there's a reason to choose one length over the other. I would imagine the more spring the better and yet I also understand there would be limitations. In the end I'll probably get 8" but by golly I kinda want to know why they are the one to get.
In reply to captainawesome :
Assuming that there's enough range of adjustment to fit either length, the shorter spring may help keep the springs and perches clear of the tires. Longer springs allow more travel and resist sagging better, but are heavier and if they're long enough you have to start worrying about lateral buckling (only heard of this being an issue on high-end offroad rigs).
No issue with tire clearance in this application. I often see tender springs employed with the 8" but don't recall if I have with the 10". I wonder how much 2" of spring can weigh?
You need enough spring length to not coil bind at full bump, but short enough to achieve the ride height you want with the perch adjustability available to you. The front springs on my AW11 are a 6" barrel type spring, the front springs on the rally BRZ are 14"- without knowing the numbers involved in the 914 I have no idea what length you'd want. I tend towards the longest springs that can fit, assuming the perches can be adjusted low enough to keep you out of coil bind and get the height where you want it.
In reply to captainawesome :
Taking up 2" of length with a tender spring should be lighter and could offer even more travel, although that last bit of droop travel will be extra soft which would be fine for a track or street car but wouldn't be ideal for landing from jumps.
I'll assume the best way is the mount the shock and cycle it to see how much range there is, but how to measure coil bind without the physical coil in hand?
In reply to captainawesome :
Some coil spring manufacturers will list the spring travel, or you can kinda ballpark it by looking at the wire diameter and gap sizes in the spring.
In reply to captainawesome :
If you can find the specs for the spring it will usually list block height, which is the length at which the spring hits coil bind.
dps214
SuperDork
3/13/25 11:37 a.m.
Any worthwhile spring manufacturer should list solid height; I know both eibach and hyperco do. I had to play this game last year trying to get as short of a spring as possible while maintaining stroke.
¯\_(ツ)_/¯ has it - it's all about the geometry. If you're starting from scratch, try to figure out how much the spring will compress at your static ride height given your particular spring rate. This is related to corner weights and motion ratios. Then figure out what that means for perch location - too long and you won't be able to lower the perch low enough to reach your ride height. Too short and you won't be able to raise the perch enough.
Also figure out how much travel you need - that's directly related to your shaft travel. If the shaft travel is greater than the spring travel, you're going to have coil bind and you will break things.
The Eibach website lets you plug in ID, length, rate and type (leave blank for ALL). That'll give you your block height and your travel (which add up to the free length, so if you have two of those measurements you also have the third).
https://eibach.com/race-spring-search
Helpers will basically let you cheat with a shorter spring. Keep in mind that once the main spring has unloaded, the only thing extending the suspension is the unsprung weight working against the damping. They're really only there to keep things from getting floppy when you've got the car on jackstands or a lift. With high spring rates, you may need helpers.
Tenders are helper springs with a non-trivial spring rate. They'll help extend the suspension when unloaded, so they'd be a better choice if you think you're going to be full unloaded frequently, like a rally car.
A higher rate spring will not compress as much as a low rate spring, so you'll probably end up having to go shorter as you go stiffer. For example, we use 8" springs on our Fox suspension for road cars but my track car runs (I think) 6" at one end due to the high rate and low ride height.
This will be a road car with maybe once a year autocross. No rally or situations of extreme full droop that I can imagine either.
The rates that folks seem happy with most for a street car is 140lbs which is fairly low compared to what I've run before. I may end up with 150lb due to selection available. There's a better selection of springs in the 10" range than 8". Helper or tender springs aren't cheap and if a 10" does the same as a 8" with tender/helper then I like money in my pocket better. Heck maybe the tender springs would still be needed with 10".
I guess I was just curious how someone landed on the magic 8" spring number. I'll see if I can get a bit of napkin math going this weekend and find out hopefully come to a similar conclusion.
