I'm running Ohlins R&Ts on my e46 M3. The Ohlins front shocks don't have a ton of metal to metal travel-- 92.5mm total.
My front springs are 325 lb/in driver, and 300 lb/in passenger. This puts me at 1.81 hz on both sides.
So, I need to allocate my 92.5mm optimally... but what it's the optimal distribution? The car is weekend car that sees 5-15 track events per year.
Depends on how hard the springs are. The optimal setting would be to find out how much compression travel the car uses on track plus how much is taken up by the weight of the car, allocate that much to compression and the rest to extension.
GameboyRMH said:Depends on how hard the springs are. The optimal setting would be to find out how much compression travel the car uses on track plus how much is taken up by the weight of the car, allocate that much to compression and the rest to extension.
That's why I included their frequencies...
Frequencies aren't really relevant, the spring rates are though. On a heavy car like that with relatively soft springs you'll need lots of compression travel. You might want to try the "zip tie data logger" method - put zip ties on the shock shafts tight enough that you can just barely move them with your hands and then do some test runs. These will be pushed upward and will show the maximum compression travel used. This is then how much compression travel you need to allocate, and whatever's left is available for extension travel. It's much worse to run out of compression travel than extension travel. If you want to add more extension travel, you can add tender/helper springs.
With pre-selected spring rates and fixed motion ratio how exactly do you expect to influence bump vs droop travel?
It's going to be whatever it is unless you run series springs that enable you to have a softer spring rate in droop.
I would just set the ride height to whatever you want for optimum static geometry, validate that you have sufficient springrate to keep you off the bumpstops and you get what you get for droop. If the spring falls off the perch get some super soft springs just to hold the springs in place.
Unless I am missing something. Which has happened before and will again.
GameboyRMH said:Frequencies aren't really relevant, the spring rates are though. On a heavy car like that with relatively soft springs you'll need lots of compression travel. You might want to try the "zip tie data logger" method - put zip ties on the shock shafts tight enough that you can just barely move them with your hands and then do some test runs. These will be pushed upward and will show the maximum compression travel used. This is then how much compression travel you need to allocate, and whatever's left is available for extension travel. It's much worse to run out of compression travel than extension travel. If you want to add more extension travel, you can add tender/helper springs.
Frequencies are super relevant-- they factor in the weight of the car and suspension geometry..
On the scale of e46 M3s, it's not that heavy-- 3080 lbs. :P
Sadly zip tie data logging isn't possible, as it's an inverse mounted strut with the piston fully enclosed.
nocones said:With pre-selected spring rates and fixed motion ratio how exactly do you expect to influence bump vs droop travel?
It's going to be whatever it is unless you run series springs that enable you to have a softer spring rate in droop.
I would just set the ride height to whatever you want for optimum static geometry, validate that you have sufficient springrate to keep you off the bumpstops and you get what you get for droop. If the spring falls off the perch get some super soft springs just to hold the springs in place.
Unless I am missing something. Which has happened before and will again.
I plan to set it with the ride height adjuster. Higher ride height = more bump travel, less droop travel. Lower ride height = more droop travel, less bump travel.
If need be, I could also machine new spacers to change the position of the shock relative to the spring perch.
Because of the weight/stiffness ratio you'll naturally have a healthy amount of extension travel - the springs will compress a good few inches just under the weight of the vehicle. So your best bet at finding the optimal height might be to just lower it until you start hitting full compression, and then go back to the last setting.
If you're not changing your struts and you're not willing to change rate because you're at your target frequencies, then the only other variable is ride height. In that case I'd consider optimal arm geometry (roll centers, etc.) more important than travel unless you're bottoming out compression as Gameboy mentioned.
In reply to Matt B :
I'm not overly concerned about suspension arm geometry because the entire range of ride heights I'm considering is within the range of acceptable suspension geometry.
I can also machine new spacers, if need be, to change the position of the shock vs top spring perch-- setting bump/droop independently to ride height.
So... can we focus on the question I asked? Is there an optimal ratio of bump to droop travel?
There isn't really an ideal ratio across cars. You want to set things so that you're not on the bumps under normal conditions and dedicate the rest to droop. This can vary on an individual car due to tire choice and how rough/bumpy a particular track is.
^This, there's no optimal ratio.
Yes, there is an ideal ratio. It's something along the lines of "enough bump travel that you're not constantly bottoming out the suspension or riding the bump stops at static ride height":"whatever droop travel you're left with".
dps214 said:Yes, there is an ideal ratio. It's something along the lines of "enough bump travel that you're not constantly bottoming out the suspension or riding the bump stops at static ride height":"whatever droop travel you're left with".
Haha I guess I can agree with that 
60/40 is what I set my car at.
And the damping rates will also influence when the car starts hitting the bump stops on what track, as will the antiroll bar.
As others have said you'll need to find a way to to measure when you start hitting the snubbers...then raise the ride hight to stop it.....or add more compression damping....or more antiroll bar....or more spring rate....or......
If the question is more about perfect worlds, with perfect everything include travel matched to the track, then I like 1/3 to 1/2 droop...depending.....
mke said:And the damping rates will also influence when the car starts hitting the bump stops on what track, as will the antiroll bar.
As others have said you'll need to find a way to to measure when you start hitting the snubbers...then raise the ride hight to stop it.....or add more compression damping....or more antiroll bar....or more spring rate....or......
If the question is more about perfect worlds, with perfect everything include travel matched to the track, then I like 1/3 to 1/2 droop...depending.....
That really was my question. As in, if I'm setting everything independently, what is the optimal ratio.
There's probably not so much an ideal ratio as an ideal amount of each to have. The goal would be to have enough bump travel that you're rarely getting into the bumpstops and enough droop travel that you never max it out and start to pull a tire off the ground.
rslifkin said:There's probably not so much an ideal ratio as an ideal amount of each to have. The goal would be to have enough bump travel that you're rarely getting into the bumpstops and enough droop travel that you never max it out and start to pull a tire off the ground.
Sadly the only thing I can't change is total travel-- that's limited to 92.5mm/3.64". So, I need to make the most of it.
With limited travel ignore the droop. Just accept what ever you get when the other parameters are met. Unloaded wheels contribute little to performance, just let them dangle free of the ground.
Which means testing. I'd prioritize staying off the bumps over droop.
Roughly 2:1 in my book, but short travel shocks are always problematic. As noted, give up droop for enough bump to keep you off the stops most of the time, but don’t let it all go because then you get bad behavior over crests and when you unload a wheel. It’s a weak point for Ohlins.
Yea, and it's a BMW. One of the front wheels will always be off the ground, just a rite of passage
You'll need to log in to post.
