Bump vs Droop Balancing Act

Hello Folks.

New here and wanted to get your expert input to something I’m still unsure about. I am attempting to build a coilover setup that offer weekend warrior handling yet offer good daily driving manners. I’d like to have the discussion on technical/measurable details and not focus on “brand” item. For starters, I’m using an affordable set of coil overs. Yes, it is not the best example but it is affordable and I am using this as a learning experience.

[B][The Setup][/B] My vehicle has a total weight of ~3500 lbs with a f/r 56/44 weight distribution, which would put it 1960 lbs front, divide by 2 = 980 lbs sprung weight per corner at rest. I don’t know the exact numbers but I would guestimate about 980 – 120 = 860 lbs un-sprung weight per corner at rest.

The coilover shocks are 33 way adjustable monotube cartages and has an internal bump stop build into the base. Set to the softest setting, I pressed the shaft all way the down and get a net shock travel value of ~3.5”. This is likely when it hits the bump stop so I would max it at 3.7”” before bad things happen…

The kit springs are 2.5” diameter, rated 448 lbs/in, and measure 7.125” length at rest with a coil bind value of ~3.15”. 7.125 - 3.15 = ~3.975” total max spring travel (likely ~3.7”?).

[B][Questions][/B] 1) Is Bump or Droop more important for comfort? I’ve read that a good DD rate is 60% Bump / 40% Droop. Does that sound right? 2) Since the springs are 448 lbs/in and assuming the unspring weight is 860 lbs, does this mean at rest the spring has already lost (860/448 = 1.95”), 1.95” for droop travel?? I would only have 3.975-1.95= 2.025” of spring travel?? 3) Since that shock only has 3.5” of travel, does this mean I would only have (3.5 – 1.95=1.45) 1.55” bump travel!?!? Am I looking at this right? 4) I image when at hwy speed and air pressure and down force in play, this would further compress both spring and shock travel down further by a little bit. Any I understating this correctly?
5) The standard kit spring seem to give me 40% Droop / 40% Bump.

The kit spring’s numbers seem off for my needs. It seems to strive for a 50/50 bump/droop. This is nice but the front end is just too heavy, it seems. Also, the 445 lbs/in spring rate x 1.58” remaining shock travel for a max bump of 708 lbs. Is there any correlation between this number vs the car’s unsprung weight of 860lbs? I an thinking of going with 560 lbs/in spring, which would net me 1.85" remaining shock travel for a max bump carring weight of 1036 lbs. 1036 seems much better than 708 when hitting a pot hole at HWY speeds. It would seem the spring is under rated for this car? In this instance, it seems it would benefit me to go up in spring rate (see green ones I’ve highlighted, yes one of the formulas is off in the pic). Based on my butt dyno, I get the feeling these coilover shocks can handle an extra few hundred lbs/in no problem, just need to adjust it for higher rate.

It's another discussion on whether or not the coilover shocks, set to the stiffer setting, has the proper damping profile to handle a stiffer spring. It did not come with a shock dyno. In the end, I would end up with a more 'taught' travel but it is less likley to crash at HWY speeds, sound right? If so, that is exactly what I need.

Eibachs are more affordable but the Swifts have actualy number for Max vs Usable spring rate, whereas Eibach only has one number without reference what it measures. Any experience on this?

Appreciate any comment you can offer. Thanks!

Coilovers with adjustable spring perches allow you to set the ride height independently of the spring rate.
So you don't need to juggle spring rates and free lengths to maintain the desired travel in bump and droop.
That makes it easier to keep the car off of the bump stops.
It's worthwhile to either get shocks with this feature or fit a kit to do it.

I set spring rates based on suspension frequency.

I set ride heights such that the suspension will preferably bottom before the chassis carves a divot into the ground, but that the suspension doesn't fully bottom any more than a few times in use. If it doesn't scrape, it's not low enough - but you shouldn't be pummeling the bumpstops either.

I shoot for zero spring pre-load.

I prefer not to have the springs fall out on full droop - but that isn't a necessity. Zap Straps or whatever to keep the parts in location.

Keeping in mind - I ain't no engineer.

Preload isn't really a thing, though. Not until you start to reach boundary conditions. Once your springs go loose, you are now at the end of your droop travel.

I set up my suspension to get maximum bump travel, then use the shock with the longest possible shaft travel. Droop does make a surprising difference in high speed control on questionable terrain, less so on a track.

These coilover shocks come with independent height adjustable perches. Fronts are spring (2.5” ID) over shock (adj. base) and rears are spring (3” ID, adj perch) and independent strut (adj. base).

I shoot for zero perload as well.

Keith Tanner wrote: Preload isn't really a thing, though. Not until you start to reach boundary conditions. Once your springs go loose, you are now at the end of your droop travel. 1 I set up my suspension to get maximum bump travel, then use the shock with the longest possible shaft travel. 2 Droop does make a surprising difference in high speed control on questionable terrain, less so on a track.

Hi Keith.

Good to see your response! Former NA owner. 1)How are you using the configuration of the car you are referencing? Why do you lean towards bump? More shock travel would be the answer but that’s not available within the limits of these shocks (3.5” stroke MAX). :(

2) So are you saying Droop is beneficiary for rough roads? I read that 60% bump and 40% is a good balance. Any experience/feedback on this? Given that I only have 3.5” to work with and considering the weight of the front end, it’s not there’s a whole lot of range hence I want to get the most that I can.

Also, the 445 lbs/in spring rate x 1.58” remaining shock travel for a max bump of 708 lbs. Is there any correlation between this number vs the car’s unsprung weight of 860lbs? I an thinking of going with 560 lbs/in spring, which would net me 1.85" remaining shock travel for a max bump carrying weight of 1036 lbs

More thoughts/questions!

I realize I am comparing wrong numbers or using the terms incorrectly. If I’m not mistaken, the 708lbs is the total max bump weight to counter the “unsprung mass!” – is this correct? Effectively, this is how hard the spring can compress via bump (up) motion that is driven by the unsprung mass (x velocity of impact). With all things equal, going to a higher spring rate (569 lbs/in = 1036 lbs max in this case) allows the front end to manager more weight from a harsher impact…! Is this correct?

Based on an example that I came across, an easy way to translate this is stick both arms out and flap them. How add just a few lbs in your hands and the flapping becomes much harder. Say if the weight in the hand was suddenly/abruptly pushed up by an external force (bump in the road at hwy speed), the upward traveling mass would be much greater. The unprung mass, however small, becomes a multiplier. I have a better appreciation of how much influence unsprung weight plays into the ride quality equation and why lighter rims/tires are a good investment (not to mention the lower inertia for acceleration).

Sooo… a stiffer spring would be more desirable, would that be correct? I should now secretly sneak in that I upgrade the calipers and rotors to larger (cough heavier) setup. :p The wild card is how well these shocks can absorb/transfer the energy from the stiffer springs. Any advice is GREATLY appreciated. My coilocvers are disassemble and I would like to buy springs and put them back together.

More, more questions: What do you guys use to avoid coil bind? These coilovers came with needle roller bearing up top and delrin (+ metal washer/ring) thrust sheets for the bottom. Works great when new buuuuuttttt the needle roller bearings froze up after the first winter. It is not a suiteable long term solution. I am thing of using delrin thrust sheets for the top as well since it endures winter just fine but does not move nearly as freely as roller bearings. Any practical feedback on this?

How am I using the car I'm describing? Well, I started off doing a rally suspension for rough tarmac. Then we discovered how well it worked on the track. So it became how I approached suspension overall, for street or track.

I focus on bump first because you almost always want the car (and CoG) as low as possible, and that's limited by ground clearance and suspension travel. If you ground out before you bottom out the suspension, then you do need to adjust your maximum bump

But, I won't sacrifice droop too much. You need some, it comes into play on crests and the ability to lay down power mid-corner - especially if you've got that inside wheel dancing across some rough curbing. On a street car, you'll feel it with the car's stability over things like railway tracks.

Remember that shock travel isn't your goal, wheel travel is. Shock travel is just a factor. If you can move the shocks so that you increase your wheel travel, that's excellent. Not easy to do on a street car. You may be surprised at how much wheel travel comes from a 3.5" shock stroke. You may also be disappointed.

More importantly, don't look at spring rates. Look at wheel rates. To get these, you'll need to know the motion ratio of your suspension, and it will likely change through the suspension's range of motion so you'll have to average it or concentrate on a particular range.

Avoiding coil bind is done with intelligent suspension design. You want the shock to bottom out on the bumpstop before the spring binds. This may mean using a longer spring, as they have a greater difference between free length and bind length. Going to a 1" longer spring will require your perch to drop by 1", so make sure your packaging allows this.

Unsprung weight is best viewed as a percentage of overall vehicle weight. Light cars are more affected by it. That said, lighter is always better as long as you have the strength and cost under control.

Keith Tanner wrote: How am I using the car I'm describing? Well, I started off doing a rally suspension for rough tarmac. Then we discovered how well it worked on the track. So it became how I approached suspension overall, for street or track. I focus on bump first because you almost always want the car (and CoG) as low as possible, and that's limited by ground clearance and suspension travel. If you ground out before you bottom out the suspension, then you do need to adjust your maximum bump But, I won't sacrifice droop too much. You need some, it comes into play on crests and the ability to lay down power mid-corner - especially if you've got that inside wheel dancing across some rough curbing. On a street car, you'll feel it with the car's stability over things like railway tracks. Remember that shock travel isn't your goal, wheel travel is. Shock travel is just a factor. If you can move the shocks so that you increase your wheel travel, that's excellent. Not easy to do on a street car. You may be surprised at how much wheel travel comes from a 3.5" shock stroke. You may also be disappointed. More importantly, don't look at spring rates. Look at wheel rates. To get these, you'll need to know the motion ratio of your suspension, and it will likely change through the suspension's range of motion so you'll have to average it or concentrate on a particular range. Avoiding coil bind is done with intelligent suspension design. You want the shock to bottom out on the bumpstop before the spring binds. This may mean using a longer spring, as they have a greater difference between free length and bind length. Going to a 1" longer spring will require your perch to drop by 1", so make sure your packaging allows this. Unsprung weight is best viewed as a percentage of overall vehicle weight. Light cars are more affected by it. That said, lighter is always better as long as you have the strength and cost under control.

That makes a lot of sense on the need for droop, which is why I’m asking if 60/40 is indeed a good “rule of thumb” (not sure of origin).

This is street car so I won’t get to move shock’s position to impact wheel travel. I suppose I should take some measurements for MR and calc my WR. I was hoping I can get away not having to do this :p. I have a high level understanding of what WR and MR strives to archive but I’m not 100% sure on how to consume/interpret the result –as in what should I strive for? I’ll try to get some hard numbers this weekend. http://eibach.com/m-america/en/motorsport/products/suspension-worksheet

RE coil bind, thanks for the suggestion on “longer” spring. That was also my thought and motivation to consider 8” spring. My setup currently has my ideal height (allows me to clear deep alley ways and steep parking ramps) and still have 2.25” of perch gap to spare (or I see these as targeted towards the ‘slam-me-now’ crowd)! I would love to have that length back as stroke travel..  Should I go as high as 9”? I would have excess usable spring travel but more length for the spring to twist (aided by delrin thrust sheets too). Would this solve my spring bind issue and give me an all season maintenance free solution!?!?

Pic of my original perch gap. Branding covered to avoid bias.

For giggles, I decided to try wrapping the exposed threaded sections with Teflon tape to see if it can protect it from salty winters. Turn out it does and admirable job and the threading underneath is clean as a whistle! :D I realize the purpose of the shock absorber is to dissipate heat. Do you think the added layer of Teflon tape might hinder this?

I know some of these question are off topic to title of the thread but figure I ask away while I have your attention. I hope you don’t mind and I really do appreciate your feedback. It is hard to find someone of your knowledge level to interact with. Cheers!

I've never bothered trying to find an ideal ratio for bump and droop. For me, it's "as much as I can package" - with packaging being my limitation.

I can't comment on the ideal spring length for a [REDACTED] suspension bolted to a [REDACTED] for hopefully obvious reasons. The spring needs to be long enough. Too long doesn't really bring any real problems until you get really long, too short is a definite no-no.

I'm not sure what "perch gap" is. Do you mean the amount of available adjustment to lower the perch further? If so, then if you have 2.5" of adjustment left, then you can run a spring that's 2.5" longer than your current one without affecting your ride height, all else being equal.

One way of tuning suspension is to do it by frequency. You can actually measure your frequency fairly easily.
http://www.autospeed.com/cms/article.html?&title=Measuring-wheel-suspension-rates&A=113057

I find that all the spreadsheets and calculations only get you so far. At some point, you've got to get out there and start swapping parts. You can get into boundary conditions (coil bind, weird suspension geometry at the ends of the range of travel, lifted wheels) and some of it just comes down to what feels good.