Droop Question

I know a little bit about the suspension of open wheel circuit cars, but very little about rally cars, so I have a few questions.

It seems to me like you can only increase the droop travel on a rally car, you are basically stuck with the bump travel the car comes with, outside of lifting the car, which it doesn’t seem like anyone does, at least not drastically.

Would it be beneficial to have as much droop travel as possible? I ask because I would think beyond a certain point, say six or eight inches of droop travel, your remaining travel would for the most part only be used when the car leaves the ground. This isn’t necessarily a bad thing, but it brings me to my next point.

For the sake of argument, say you have a car with 6 inches bump travel from the factory ride height, and 10 inches of droop, with your awesome rally prep. But say this car only had 4 inches of droop from the factory. You could achieve this by running whatever spring rate got you the ride height you wanted, of course, but since you didn’t increase the bump travel at all, this spring rate would have to be way softer than factory to get through all that droop travel with the car sitting in a parking lot. And even softer springs is not what you want.

So people run short, stiff springs. A spring that was the correct rate to give you 6 inches of bump at ride height, only sagging 4 or less inches, (not 10). Well, naturally, this spring rate is going to be way too stiff for those 10 long inches of droop, so you run a second spring in parallel, a helper spring, with a really low spring rate (the helper spring). Because you want your helper to be coil bound say 2 inches of droop before you get to ride height (so you have a margin of error in staying consistent with your springs rates when the car rolls) This creates the condition of the helper needing a super low spring rate.

I have already assumed that you are only going to need all 10 inches if the wheel (or car) leaves the ground. So say the wheel comes slamming back down at the ground after you just jumped your car, the first 10 inches of your travel will barely resist the car’s impact as all you have is this helper spring to compress completely before you get to the “real” spring.

So here is the question: Why have all that travel if it gets your wheel on the ground 6 inches sooner, but doesn’t much help with decelerating that corner of the car in the downward direction?

I think the answer might lie in the fact you could run a ton of high speed damping on your dampers, but that is just a guess. Maybe rally cars don’t need much more travel than they have stock?

Sorry for the really long question…..

Well, for 6 inches of comp and 10 of droop, you would have to have a shock with 16 inches of travel total... then it comes down to your ride height placement.

How I did my rear end (with Johns JVAB coil overs [over due plug haha]) I set my 4 links how I wanted them, got them to be level, and then decieded the amount of droop/compression i wanted.

I measured from the end of the shock body to about 1/4-1/3 of the bump stop (i dont remember where exactly) and came out with 9 or so inches of travel. Maybe high 8.**

I then said I want slightly more compression then droop (as I'd rather absorb HARD hits then STICK to the ground all the time ) and compressed the shock down to my desired "ride height." I just compressed it 4 inches or whatever my droop was going to be.

Then I stuck it in the turret and lowered it down to the axle's shock mount, and measured where the top eyelet would be on the turret, so I can cut it down. (which is done) Now when I bolt the shocks onto the car, it SHOULD sit nicely where i want it, having the droop and comp as i would like.

Anyway, This way (to answer your original question. lol) I'll still have the droop for when the car gets light over crests and such to keep the tires on the ground [hence letting you brake, steer, gas it...in theory], but by biasing the travel to the compression, I'll have the ability to absorb more faster.

I think your right in the high speed compression in the shock, as with my mountain bike, I run a REALLY soft spring (for droop, go figure) to hook up in the corners (as im light...) and need to change my oil to a heavier weight so it dampens a bit more as to help smooth out the jumps/drops/impacts/crashes

Im not sure that helps, or even answers your question. But thats my story.

Sorry for the long post

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Welcome to the cult of JVL drink the koolaid or be banned.

First a long discussion is indeed what would be required if you were to try and finger out what the top WRC cars were doing because they have a LOT of movement, certainly over 10"-250mm, likely using careful enlargements and calipers at least 11-12".

But they have bodyshells modded to accomodate that (extended towers, extended wheel housing and arches cut away and obviously CV joints that MOVE a LOT!!!)

So aside from idle speculation, since we don't have any real numbers or dimensions, or even very good photos very often, not a lot to gain wondering about WRC cars.

Now the stuff we do have are very often limited in travel in stock form and the more styling is a factor, the more the limitations.

Sub-a-rats and Evobitchis have about 160mm travel tops in front, a bit more in the rear.
Old school Volvo 250s have 210 in front and I don't know in back but those cars I'm doing for guys will have 9.3"

E30 Bowel Movement Wonder have barely 160mm front, maybe 175 rear.

VWs about 160mm front, 190 rear.

Maybe 40-45% is droop. So you see your numbers are miles off

And the limitations are everywhere, foremost among them being budget.

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John Vanlandingham
Sleezattle, WA, USA

Vive le Prole-le-ralliat

www.rallyrace.net/jvab
CALL +1 206 431-9696
Remember! Pacific Standard Time
is 3 hours behind Eastern Standard Time.

I was hoping you would weigh in on this John.

Yeah, it seems like WRC cars are running quite a bit of droop, maybe 10" from ride height would be my really rough guess. Sure the more droop the merrier, but what do you do about spring rates? And if you do what I think you do (run long helpers) how does all that droop help? With a lot of high speed damping? Maybe they run relatively soft springs and have really stiff ARBs? If that was the case they would still squat and dive a whole bunch, so I dont think that is it...



Edited 2 time(s). Last edit at 10/29/2008 01:00PM by DirkaDirkaJack.

Yesterday I measured 3.5" of droop on a gc8 subaru with rs&sp suspension, front and rear. Which is about 89mm for you mm guys.



Edited 1 time(s). Last edit at 10/30/2008 06:17AM by Carl S.

Droop is cool but not something to try to add.
Your right that it is pointless if there is no spring to back it up.
using the corolla's rear as an example -
A relatively soft rate with a really long spring.
I've got 16" of 150lb/in in the rear with a 4in tender spring (tends to keep things concentric).

with the shock setup, the tender is down to about 1" and has basically 20lb/in rate.

since the back of my car weighs about 1100lbs (wrc car~1400) and 1" is a give-away, 4.5in of static compression

if I got a spring with 0 for stack height and never hit bumps I would find a 3.5in long spring. (air spring?)

If I went to eshocks.com and ordered the 12" travel bilstien shocks i'd have to make a decision,
2 more inches of compression means checking to make sure the coils don't bind when I put another 600lbs of force into the rear, and moving the top of the shock up 2in or the bottom down, so it sits at the same level.
2 more inches of droop means my helper spring is now 3inches long, and my spring stack just wiggles in the wind when I air it out.

I'll tell you a story of a short travel new sentra spec-v who went over a bump on taholah and instead of the spring landing back on its seat, landed off center and broke the lower spring perch right off. it bent the shock shaft, and finished the stage with the gas tank dragging.

how do the WRC cars get such incredible droop? I suspect progressive springs, with soft initial rates.

There is a good part to droop - articulation.
like the rock crawler trick. I can put my front wheel ~14in off the ground with the other 3 touching.
drop a wheel in the ditch? ok
when you're in a glorious drift, using all the tires grip, it transfers right onto my spinning rears.
http://picasaweb.google.com/acrane/PacificForestRally#5261875885178093202

http://picasaweb.google.com/acrane/OregonTrailRally#5056496380133338402
I'll try to share a picture

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"I put the hurt on dirt" - adam crane
http://CraneRallyCrew.com
corolla gt-s "Patches"
Op: S.S.



Edited 2 time(s). Last edit at 11/02/2008 12:29PM by acrane.

Interesting question, but like has been said, the idea of 16" total travel on any stock based car is not realistic. The numbers of 6.5" to 8" total travel is the norm for most cars being rallied; getting a total of 10" is a big chllenge. One thing not mentioned yet is the suspension geometry, 'specially of a McPherson strut car. You will be challenged to get 10" total travel; it will require some suspension control arm mods and relocation of the pickup points to keep the geometry useful through 9"-10" of total travel.

Maybe that answers the question: significantly longer travel within the confines of a 'normal' cars control arms, trailing arms, etc. to keep the suspension geometry in whack is the real limiting factor. Look at the long arms adopted by off-road/CORR/SCORE vehicles. They do that for a good reason.

Mark B.

Carl S Wrote:
-------------------------------------------------------
> Yesterday I measured 3.5" of droop on a gc8 subaru
> with rs&sp suspension, front and rear. Which
> is about 89mm for you mm guys.

And 89mm is about what percent of 210mm? 42%.
See above post suggesting 40-45%
>
>
>
> Edited 1 times. Last edit at Oct 30, 2008 by Carl
> S.

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John Vanlandingham
Sleezattle, WA, USA

Vive le Prole-le-ralliat

www.rallyrace.net/jvab
CALL +1 206 431-9696
Remember! Pacific Standard Time
is 3 hours behind Eastern Standard Time.

good point about ditch hooking Adam, I think that is probably key to all the droop travel.

Basically i had this whole droop idea because i was thinking about prepping a GTS. I mean, there is nothing really stopping you from setting up the back of a GTS (or any solid-axle car) like a SCORE Baja truck. And with a little creativity in the front, you could probably get to a seriously long travel car. But would you need it? Would it be worth it? Is that going to make you faster in stage rally?

I guess if you are building a WRC car it makes some sense. But, for a regional rallier, there are TON of better things to invest in, IMO, wiht both your time and $$. Lots of fast, good rally cars out there with < 8" total travel. (But none with <6" !) Just building/buying a car and learning how to drive in rallies would do far more than 99% of us could ever do with long travel. Anyhow, I would expect the long travel suspension stuff to weigh more, with more unsprung weight, etc.

Please say you are not going to worry about this further, execpt as an academic exercise.

Regards.
Mark B.

ask tom b. about not worrying anymore, and he'll tell you about shitty compromises.
If you're going to build a car, make a reasonable attempt to get good stuff on it.
somehow, for many of us, fabrication and internet wheeze time is free, a great investment.
the VW R32 went the extra for suspension, (how's it doing)
the sentra in the story above, has won gp2 now that they fixed/ sorted the rear suspension.
John lane had choice things to say after getting his stuff up to snuff.
I'm a believer too. #1 skid plate #2 suspension #3? gearing?comfortable seats?mudflaps?

i'm a super moderator, which outranks your junior status -
update your expletive deleted signature Dude.

list some better investments - john v can help you.
3 DNFs? while you sort out off the shelf stuff?
or $1000 extra dollars upfront to not worry about little damage.
the 16" example was the conversation starter- we know we aren't building monster trucks.
discs, calipers, wheels and tires are unsprung,
springs and dampers are counted as ?half? or something involving the motion ratio.
Timm- i need my back pack back.

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"I put the hurt on dirt" - adam crane
http://CraneRallyCrew.com
corolla gt-s "Patches"
Op: S.S.

^^^^ Adam speaks the truth.

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-Tom
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