Valkyrie

I have coilovers with a fairly similar spring rate (10 kg front and back) and I think I don't have enough stroke in the rear. In one rear corner it seems to be slamming into the upper mount. Not good.

Can anyone tell me how much stroke Ohlins DFV shocks have in the back and front?

jacobcartmill

iTrader: (6)

sometimes you just gotta stroke it, man, even in the rear.

lOOkatme

iTrader: (19)

Add preload to the spring to gain some bump.

Valkyrie

I was talking to guys from Pan Speed yesterday and they said preload on the rear on FDs is a no-no. It's also a terrible idea to have a left/right difference.

I actually had a bunch of preload on the side that was bottoming out (but none on the other side). Taking it off make an improvement, if anything.

I just want to know how much stroke Ohlins DFV have because I want to know how much stroke a well-designed coilover with relatively soft springs has.

I probably only have two inches (4-5 cm?) of stroke with the car on the ground, which is not nearly enough for 10 kg springs.

lOOkatme

iTrader: (19)

here is the measurements from my ohlins dfv.

front 4.1" and rear 3.6"

I am running 8KG/6KG set up with mine. I have .25" preload front from what I remember and .75" rear.

I do not bottom the car out and the car handles and rides great.

Pre-load just means you have a little higher starting point. It just centers bump/droop.

I think the bumpstop takes up 1" of space or so, and is used in use.


just for viewing sake on a miata.

Valkyrie

Are those measurements from full droop, or 1G?

I measured my front stroke as 12 cm at full droop, or 4.7", which is pretty reasonable.

I could have sworn my rears only had about half that, though. I suspect if I had 3.6" (9 cm) in the rear, I wouldn't be having this problem.

My car doesn't have bump stops for some reason. Probably a bad idea, seeing as how easily it bottoms out... at this point, a little preload probably isn't helping.

Just FYI, the guy from Pan Speed said that rears should have no preload, and the fronts should have none or maybe up to 4 mm. He said 10 mm was *way* too much.

OTOH, they tend to run much higher spring rates on their cars.

lOOkatme

iTrader: (19)

Preload is dependent on spring rate. softer springs offer much more mechanical grip and droop. if you have a corner weight of 700lbs a 350lbs spring compresses 2 inches without preload.

the panspeed might run 1400lbs spring, which compresses only .5". They need aero on that car to keep the car on the ground. If they hit a 1/4" hole or drop off in the road they only have 350lbs of spring pushing the wheel back down, on a 350lbsin spring the spring is pushing down 612lbs.

running preload on high spring rates is not going to work well. softer rate springs you pre-load to make it work and center the bump/droop travel of the suspension.

My measurements are from the shocks off the car being measured for full distance. subtract 1" for a 1" bump stop.

Bump stops actually are a great way to tune a vehicle with softer rate springs as well.

Valkyrie

Yeah, you're effectively giving it a progressively stiffer spring rate.

ZDan

Only if motion ratios are 1.0. FD has motion ratios of 0.61 front, 0.69 rear. 700 lb corner weight (actually corner weight minus unsprung corner weight) at the rear will put 700/0.69 or 1014 lb on the spring, will compress a 350 lb/in spring by 2.9"

0.725" at the spring under 700 lb. at the wheel.

1/4" hole at the ground = .1725" stroke at the coilover (at rear of car with 0.69 motion ratio). With 1400 lb/in spring, force in the coilover drops from 1014 lb to 773 lb. Force at the wheel goes from 700 lb to 533 lb (vs. 658 lb. with 350 lb/in springs).

Yes, with softer springs you need more preload.
With the "stock" 11kg/mm springs and no preload, the rear springs compress 1.65" at the coilover, 2.4" travel at the wheel.
With 6.25kg/mm (350 lb/in) springs the rear, with no preload the rear springs would compress 3" with 4.4" of wheel travel.
With those springs you need 1.25" of preload to have the same 1.65" stroke under static load as the "stock" 11kg/mm springs.

lOOkatme

iTrader: (19)

you are right, I spaced the motion ratio's.

I used a calculator like the one below to figure out my spring rates.

Automotive Spring Rate Calculator

I used for inputs.

front

corner weight of 700lbs
unsprung weight 75lbs
Dim A .61
Dim b 1.0
spring angle 85
shock ride height (4.1 * .5) + .25" (preload) = 2.3"

spring rate needed = 448lbs/in

Rear
corner weight 650lbs
unsprung 75lbs
dim A .69
Dim b 1
spring angle 85
shock ride height (3.6 * .5) + .75 (pre-load) = 2.55"

Spring rate needed = 328 lbs/in


If you don't preload at all you want
front = 501lbs/in
rear = 464 lbs/in

my buddy runs 9/8 on his rx7 and preload the rear a little bit. runs great. I could see why ohlins chose 11/11 for their coilover set up.

Valkyrie

I just measured my rear stroke, and it's only 6 centimeters with the car off the ground.

That's 2.36 inches!

That's more than an inch less than the DFV.

I estimate I only have about an inch and a half with the car on the ground.

lOOkatme

iTrader: (19)

Sounds like a perfect candidate for adding in a little preload to the spring to better manage that bump travel.

mrselfdestruct1994

iTrader: (1)

My rear DFVs have 51mm shaft travel before engaging the bumpstop which is 30mm tall with a solid height of 7.5mm. There is 73.5mm total shaft travel until the bumpstop is solid.

The rear motion ratio is .75, so there is 98mm total wheel travel available. At 25.5" rear ride height, there is barely enough space in the wheel arch to allow that.