Using Honda springs for coil overs

[roman]

NoHachi Wrote:Coilovers don't keep springs captive...at decent ride heights. You need a helper for that.

Keeper springs only add extra downwards suspension travel, below the static ride height. The ride height 'up' from there is determined by the spring rate of the main spring, the weight of the car, the length of your shock absorber, and how high or low your bottom spring perch is with a particular spring.

Quote:Requirements for spring travel are calculated from two influence factors. First you want to be able to absorb spring compression from maximum cornering force. Anywhere between 0.8 and 1.6G's depending on setup. Then you want some extra margin on top of that because you will always overshoot steady state by a margin and also want to be able to absorb bumps during hard cornering.

Yes, but then how do you calculate the affect that roll has on your car at 1G? you'd need to know swaybar spring rate, static and dynamic roll centre position, centre of gravity, leverage on spring by suspension geometry, and weight of the car at each corner presumably. And then work out how to calculate how much this is going to tip your car at a certain G force.

And then its still a guesstimate at best, until you take your car out to test.

Dont get me wrong I'm all for having as much suspension travel as possible! But when ride height becomes lower and the spring rates get harder, there's less scope for potential suspension travel anyway. It's the limits of hitting wheels against bodywork etc that is more a determining factor in how much suspension travel under compression that you can have.

Quote:4.8kg is normal for a honda civic but pretty high for an AE86. YOU CANNOT COMPARE SPRINGRATES BETWEEN CARS WITHOUT KNOWING MOTION RATIO's AND CORNER WEIGHTS! Hondas have high motion ratio's so 4.8 is equivalent to something MUCH lower on a McPherson car. I'm willing to bet 4.8 is pretty sweet on an AE86 street car, so worth a try. Not good for slammed status though.

I agree that 4.8kg spring will be equivilent of a softer spring in the case of the double wishbone, because the spring is further inboard and the leverage on it requires a harder spring to accomodate. (is that what you mean by motion ratio?)
This isnt bad news though, it just means that in case of using this spring for Mcpherson strut / AE86 etc theres good spring rates to be had even with the full length spring, if 4.8kg or there abouts is a suitable rate for the application. 4.8kg or there abouts just seems low compared to what everyone seems to reccomend as a good setup though.

Quote:Oh and most AE86's weigh around 1000kg without the driver, nearer to 1100kg with driver and full tank of gas. Just look around on the internet for pics of cornerweight scales and don't trust people who pull numbers out of their asses like I am doing at the moment.

Haha yep.

I'm hoping my car (carina) will be under 1000kgs, but I think the motor/box/diff will make it too heavy for that in a slightly bigger shell than AE86.

[NoHachi]

Quote:Keeper springs only add extra downwards suspension travel, below the static ride height.

Isn't downward suspension travel exactly what you need to keep springs captive?

Quote:Yes, but then how do you calculate the affect that roll has on your car at 1G? you'd need to know swaybar spring rate, static and dynamic roll centre position, centre of gravity, leverage on spring by suspension geometry, and weight of the car at each corner presumably. And then work out how to calculate how much this is going to tip your car at a certain G force.

Jep, thats what you do. And for the static situation its pretty damn accurate You seem to have a decent grasp on the subject, so why not give it a try. Most of the stuff you need can be found on the forums. Its an estimate, but it really helps you understand whats happening with the car.

Try 4.8kg first and hack a stock rear spring to get you lower and closer to 3.5kg/mm. People consider 8/6 a good combination because they are told that it is a good combination. The number of people with experience with multiple spring rates and actual hard driving are minimal. Just everyone copying each other and the need to validate their own purchases.
I dropped from 9/7 to 5.5/4.5 and the car is still very stiff, rolls a very small amount (most roll still comes from tire compression) and handles like a dream. Its more compliant on the street in that it now absorbs the biggest hits instead of being launched. In no way would I consider it comfortable and I think If I were to make it a dedicated street car, I would drop spring rates to 3.5/2.5 or something similar. Thats what most rally winning Escort mk2 cars run.

[roman]

Ive had some ideas about how to raise the roll centre in my car, in order to use as softer springs as possible while maintaining equivilent roll resistance to harder springs. But then, the car id still susceptible to dive under braking? and there isnt a way to raise the roll centre at the rear in the same way. Im not obsessed with having 8kg springs haha... The point of this thread was more about a way to experiment with various spring rates in a predictable way that doesnt cost heaps of
money. Im definitely going to try out a lot of configurations, thats half the fun of tinkering with a car.

[NoHachi]

You can adjust the rear roll center with a Watts or Wobb link. There isn't much point in doing so, roll centres don't even really exist in the real world, so don't fret to much about them.
If you want less roll, try and find a way to stiffen up the sway bars. I'm sure you could figure out an easy way of making them adjustable.

[roman]

I disagree about roll centres not really existing, roll centres distance from COG ar why boats tip into corners, and cars tip out of them.

Adjusting swaybar rates is kinda counteracting the extra force encountered by having your COG far away from roll centre, as opposed to minimising the force in the first place by raising roll centre.

Raising the roll centre makes swaybars less necessary, or a softer swaybar rate acheive the same thing. If you've got soft springs and hard swaybars, you've still got a hard total spring rate affecting single wheel bumps, and a soft spring rate for preventing dive under braking.

Which is the opposite of what I'd prefer.

I've got a few ideas to try experiment with from here anyway.

[NoHachi]

Google around for force based or kinematic roll centres if you want to learn more. There is a lot happening when changing roll centres: load paths through the suspension, speed of load transfer etc.
Minimise roll centre and you introduce large forces in your suspension arms and shock load your tires.. usually not fast. Toyota got it pretty close on the ae86.

[roman]

NoHachi Wrote:Minimise roll centre and you introduce large forces in your suspension arms and shock load your tires..

Yes, but when you lower your car the car there's a massive migration of the static roll centre away from where it usually is, which brings a lot of extra roll.
And even worse, as the car tips, the roll centre moves further from the COG, making it even easier for the existing amount of lateral force to tip the car even further, and so on.

When you raise the roll centre in a lowered car, at best you're just bringing back to where it is at the factory height.

Given the geometry I'm not even sure it's possible to raise the roll centre up far enough to be very close to the COG.

Out of curiosity Nohachi, what sort of height do you have your car at?

[NoHachi]

I run at oxcart heights. Dunno how to give you an exact number (all reference points are tweaked), but basically it sits on shortened housings, with ride heights that put the dampers near 50% bump, 50% rebound travel at static ride height. There is about 5cm of bump travel available now, without bump stops.

[NoHachi]

http://forums.pelicanparts.com/porsche-a...ect-1.html

Roll centre discussion..interesting at times.
Still not sure if its really that important in en AE86, but RCA's have the benefit of restoring some of the cambercurve as well, so it won't hurt. Its the rear I'm interested in. With the stock setup, roll centre is WAY up on the car. Above the front to be sure. I would like and try to lower it using a watts link.

Roll centres are complicated. They shouldn't move around as much as they do.

[NoHachi]

Staniforth states:

Quote:"All racing cars [roll centers] are probably now located within one inch below to two inches above ground. Low roll centres give less weight transfer to the outer wheel, smaller or nil jacking effect but high potential roll angles."

Jacking forces shock load tire (they are instanteneous) and raise the COG at that end of the car. This is supposed to be bad generally, but can help warm up tires on very light vehicles (something I will be trying on a MX5 that has trouble getting 225/45 R888's up to temperature.)

Someone should try and revisit that CAD model we made together and plot a roll centre migration curve. Is Beanbag back on the internets yet?