Ohlins Road & Track for FRS/BRZ suspension install/review thread

[remhex]

Yup increasing bump travel reduces droop travel.

[JIM THEO]

I still don't understand how preload increases bump travel, in reality it only decreases compression as if the spring was harder, thus the same bump travel is enough for more spring load but the actuall bump travel remains the same!
I could say the perfect desription is preload increases resistance of the spring to compression so the same force results in smaller compression!

[DocWalt]

Increasing preload raises the car

[remhex]

If your car gets too low, you hit the bump stops. If you raise it with preload, the car is lifted away from the bump stops. More preload= more cowbell.

[JIM THEO]

Originally Posted by DocWalt

Increasing preload raises the car

Raising the car couldn't by itself increase bump (height adjuster raises the car also but does nothing for bump), preload adjuster raises the starting point of compression thus giving more space before piston reaches it's lower limit at bump stop or spring bind, whichever comes first depending of adjustments, spring length, actual piston travel.
But if it is that way the more it increases bump reduces rebound so the perfect adjustment will be when bump and rebound have the same travel and ride height is determined only by the height adjuster.

[remhex]

Not too far off but I wouldn't say equal bump and droop is always the best. Depends of what your doing and how much travel there is to work with.

[2Wavy]

Preload doesn't change the force behavior of the spring. No matter what you do to preload, the spring will always compress to the same resting length based on the weight of the car. The only thing you're doing by adjusting preload is raising or lowering the spring perch which will adjust your height and droop/bump travel.

[jamal]

If I adjust the height of the where the spring sits, how would that NOT change the ride height? If the shock is sitting higher in it's travel, how would it not have more room to compress?

[JIM THEO]

Originally Posted by jamal

If I adjust the height of the where the spring sits, how would that NOT change the ride height? If the shock is sitting higher in it's travel, how would it not have more room to compress?

I didn't say that, I said that preload isn't there for height adjustment, this can be adjusted from the lower height adjusting bracket.
Preload serves a different purpose at least as I understand it, as already compressed the spring "resists" more when a load acts on it which doesn't change it's original rate but raises the point in it's spring force curve for the same compression travel (as per the diagram I posted before) and bear in mind even linear coilover springs aren't linear <20% and >70% of their travel.
Plus the ability to change bump/rebound travel as recently realised.

Otherwise can you explain me why Ohlins set 20mm preload for BRZ and 2mm only (just to keep spring in it's place) for MX5?

[jamal]

Originally Posted by JIM THEO

and bear in mind even linear coilover springs aren't linear <20% and >70% of their travel.
Plus the ability to change bump/rebound travel as recently realised.

Otherwise can you explain me why Ohlins set 20mm preload for BRZ and 2mm only (just to keep spring in it's place) for MX5?

Yeah, but how "not linear?" We're talking a pretty small variation with any decent spring. Saying a spring is only actually linear for like 50% of the travel is way blowing things out of proportion.


Ohlins calls for all that preload because their 4/3k springs are too soft for this car and it needs that to get the correct proportion of bump to droop travel. Do the math for a 3k spring on the rear of this car. Actual wheel rate is 1.68k using the previously mentioned 0.75 motion ratio. We'll call the corner weight 250kg, so the suspension will compress 148mm with zero preload. That's a lot, almost 6 inches. How much rear travel does the suspension even have?

E: these numbers are a little fudged. MR of 0.77 and unsprung corner weight of 200-225kg is more accurate, and gives suspension compression at the wheel of more like 120mm. Adding 20mm of preload works out to about 35mm of height change. Which are now more reasonable numbers for actual travel of this car.


Anyway, what I'm getting at is preload is only used to make sure the suspension has the right amount of bump/droop travel for a given spring rate. In cases with a stiffer rate you'll often have "less than zero" preload and need a helper spring to take up the difference at full droop.

And not every set of suspension has a separate threaded lower mount, RCE/KW, JRZ, Bilstein for example. The extra adjustment is not always as useful as you might think, as the suspension can only move so far before things start hitting other things.

[JIM THEO]

So is similar with SAG from motorbikes I know of...could someone with last gen Ohlins MP20/21 do the math please?
http://www.reliablespring.co.uk/calculator.htm

[remhex]

Lol at goin down the rabbit hole on suspension stuff. I'll get a good review in after lower control arm install. My preload would be way different if I was on normal roads. My driving conditions are more like road rally with occassional gravel and dirt and if the forecast sticks, snow and ice hopefully. As of now, I'm at 30mm rear preload, 24mm front. 12 clicks front and rear. Shudder bumps and potholes feel much better, but the rear bottoms out harder and bucks about the same as stock. It feels like the rear has much less travel than stock but I don't have any numbers for ya. Great quality product, easy to setup.

[remhex]

So...I'm giving up on a complete review for now since I'd have to swap back to stock and drive a while to give an honest review. I ended up going a completely different route with preload. The bucking was annoying and I had a spurt of wet non-icey weather to go play in with enough time to play with settings. I started at the recommended 20mm preload front and rear for my springs. Still bottomed but the buck was less severe. I figured since that actually helped, I may as well back it off even more. Now I ended up with 12mm front preload and 15 rear...lol. It rides...nicer. I'll probly end up tweaking it many more times depending on tires, springs, etc..after I read up some more and edumacate myself.

[JIM THEO]

Nice, if the instruction manual is for the 230mm 4/3 Kgf/mm springs yours need less preload, I asked local dealer if I change rear springs with 4K and his suggestion was about 12-13mm preload so yours could run even less with the 5K springs.