Putting together a set of custom D-Specs

[lorenkb]

It's getting to be time for a new set of struts, and I thought I'd get some feedback on my current plans.

A few months ago I picked up a set of Tokico D-Specs from our very own rally Talon driving badass, Odi, aka FEAL Suspension. At that point I wanted to do a bit of machine work on them along the lines of a Ground Control kit, only a bit more suited to the rigors of offroad. Sniper1rfa got to it before I did, with his wonderfully built set found here (http://dirtyimpreza.com/forums/showthread.php?t=8674).

http://i39.tinypic.com/6pqfyx.jpg

First up, a few numbers that I'm basing everything on:

Curb weight in use: 3400 (lbs)
Motion Ratio: 0.96
Weight Distribution: 58/42 front/rear (%)
Unsprung Weight: 101/92 front/rear (lbs)
Sprung Weight: 885/622 front/rear (lbs)
Spring Rates: 225/200 front/rear (lbs/in)

These crunch out to Suspension Frequencies of 1.52 Hz in the front, and 1.70 Hz in the rear. I think these will work well, going by the references made here (http://books.google.com/books?id=BJUVdn5SMBgC&printsec=frontcover&dq#PPA41,M1).

My quick and dirty measurements show the D-Specs as having 6.5" of travel in the front, and 9" in the rear. All accounts point towards the D-Specs being identical dimensionally to the stock struts. I measured my wagon as it sits, and in the front just over 4.25" are used in static compression to take up the front end's weight. This leaves a whopping 2.25 inches of bump travel... not exactly what I'd call ideal for offroad use. I haven't gotten around to measuring the static compression of the rear struts just yet...

Right now I'm working towards having 3" of droop travel front and rear (the rear may change once I measure what stock actually is), leaving 3.5" of bump travel up front, and 6" in the rear. That will put the front end a little over an inch above stock ride height.

Keeping all that in mind, I started looking at springs, Eibach and Swift in particular. Long story short, the Eibachs just don't cut it. Using their 12" springs in the front and 14" in the rear, both are 0.1 inches or less away from coil bind at full strut travel. They would work as long as I never hit any jumps with a passenger, and what's the fun in that?

Here's how I went about my spring length validation:
- I compiled a list of springs and their maximum stroke length from Swift's and Eibach's website.
- Using the above mentioned sprung weights, I calculated the static compression of the strut/spring front and rear.
- I calculated the preload in the spring required to attain the droop/bump travel I wanted.
- Add the total strut travel to the preload required, and subtract from max stroke of the spring. A negative number = coil bind before max strut travel. A positve number is what I call a bump margin.

Here's a summary of the results so far:
Front, 3" droop, 3.5" bump:
- Swift 12" spring, 225 lb/in
- Max stroke of 8.5"
- 3.93" static compression
- 0.93" preload required to attain 3" droop
- Bump margin of 1.07"
Rear, 3" droop, 6" bump:
- Swift 14" spring, 200 lb/in
- Max stroke of 9.8"
- 3.11" static compression
- 0.11" preload required to attain 3" droop
- Bump margin of 0.69"

Any and all comments or questions are welcome. I'm looking for some validation on my methods to make sure I'm on the right track and nothing is grossly wrong. At this point the only thing I see as a wild card is making sure 3" of droop in the rear is in line with 3" of droop in the front (keeping the car level). I'll be checking that out tomorrow.

[JacksonRally]

I still think you have something measured wrong. I'm measuring (by actually crawling under the car and measuring how much shaft is showing at ride height) and getting almost 4"!!! This is a rough measurement cause of the difficulty getting your hand/tape measure up there. There is no way there is only 2.25'' of shaft available at ride height.

[lorenkb]

Can you do me a favor and try to duplicate my measurement? I measured from the ground to the fender for the first measurement. Second measurement was jacking the front end up to where the tires have just barely left the ground, and measuring from the ground to the fender again. Subtract the two and you get droop travel (unless I've gone completely retarded here, which is entirely possible...)

[JacksonRally]

Yeah I will when I get back to the shop on Wednesday.

(give me a call!!)

[sniper1rfa]

I agree with lorenkb - there is very little travel before the bumpstops as stock. I never measured, but i would be surprised if you have more than 3" total bump in the front. The rears have a ton, though.


EDIT: by the way, i strongly recommend using a set of stock bumpstops. I'm on daystar softs and they just aren't long enough in the rear. daystar hard in the front would be OK, but you need a stock set in the rear.

[JacksonRally]

Doesn't matter if the bump stops are on the strut or not, you have to measure the total shock/strut shaft that is showing at ride height. Then that is the available up travel on the strut. I actually got different bump stops that cover the entire shaft at ride height....mostly because my struts are blown but it keeps them from bottoming on the smallest of bumps.

[sniper1rfa]

Right, by "not much before the bumpstops" i was thinking something along the lines of an inch. :)

I have ~3" of bump (not including the stops) on 225lb/in springs, about 1/4" above stock ride height. So I would imagine stock has ~2.75" bump travel. Less for a WRX.

[JacksonRally]

I get what your saying, but you have to include the amount of room that the stops take up. So if you say stock is around 2.75" and the stops range from (just guessing from the last time I saw a couple of stock ones) 1.5'' to 2.0'' tall then you get 4.25'' to 4.75'' of up travel on the strut, which is plenty.

[sniper1rfa]

Nope, an inch to the stops, 2.75 total (without the stop on the car). Remember that there is only 6.5" travel total available in the strut itself.

I keep thinking about trying to shoe-horn a rear strut into the front to get some more travel...


EDIT: it's hard to be clear...:crazy:

[JacksonRally]

Yeah you don't get any more travel until you get a real coilover. But hey with what you are doing and what Loren wants me to do I'm sure we will come up with something. I do love the fact that the Tokicos are a good strut and cheap enough to play with.

[Rally OBXT]

I keep thinking about trying to shoe-horn a rear strut into the front to get some more travel...

Oh, I like this idea. Could this really work?? Only one way to find out right?? :mrgreen:

[lorenkb]

Oh, I like this idea. Could this really work?? Only one way to find out right?? :mrgreen:

Sure you could, it would just involve a lot of cutting and welding on the strut tower, and possibly the hood. I'm not to that point yet. My wild ass guess is that a better idea would be to do custom hubs and LCAs, and adjust the motion ratio (having the wheel travel farther than the strut).

[JacksonRally]

My wild ass guess is that a better idea would be to do custom hubs and LCAs, and adjust the motion ratio (having the wheel travel farther than the strut).

Ssssshhhhh. This I already have drawn up. But the SAI will no longer be run through the strut, which makes for my super top secret design which I won't tell you all about yet :muhaha: I have some rough drawings that I will shortly be bringing to an engineering firm to make the 75mm struts I am designing. This will NOT be for a rally car.

[Tommy Gun Rally]

boner

[c3hammer]

This is a very intersting discussion. Actually quite on target for what I was going to start a project on :)

I play on quite a few roads that have rather large rocks embedded in them. Some in the neighborhood of 4" - 8" high. Traditional road suspension that sags a third to half of total travel is completely untenable in this type of situation.

I'm very familiar with desert racing motorcycle suspension and there we tend to run sag at 1/4 of total suspension travel. You set up a bike to specifically have the right spring rate and preload such that you sit down exactly a 1/4 of the travel statically. This is not setup like that for jumping per se, but is to keep the wheels in contact as much as possilble when blasting through baby head sized rocks.

As a newb to automobile suspension, why are you (we) so close to the middle of the shock travel in sag? It seems that road suspension to suck up pot holes is diabolically opposite of what you'd want in an off road vehicle :)

Cheers,
Pete

[sniper1rfa]

Well, for one thing these suspensions are not actually for offroad use. At least, mine isn't. It's for unmaintained roads. I work under the assumption that i'm not going to run into a whole lot of really severe bumps at speed. :)

Not enough travel in the struts, really, for good offroad use anyway, and it'd be hard to lift the car that much and still have a useful alignment.

[JacksonRally]

I am going to be putting together a video to show what my car does with my lift kit and stock struts. I want to show how capable these actually are off road. This is also going to reference the "before" before I make the new suspension.

[lorenkb]

I finally got around to measuring the rear. As suspected, it has the same ~4" of droop at ride height. So, it looks like my plan for 3" droop front and rear will keep a level ride and give me ~1" lift.

Kyle, did you ever get a chance to measure the front/rear the way I did?

boner

I'd ask if that was a shifter in your pocket, but I guess you're just happy to see me after all!

Well, for one thing these suspensions are not actually for offroad use. At least, mine isn't. It's for unmaintained roads. I work under the assumption that i'm not going to run into a whole lot of really severe bumps at speed. :)

Not enough travel in the struts, really, for good offroad use anyway, and it'd be hard to lift the car that much and still have a useful alignment.

I agree. I'm trying to massage a relatively stock setup into giving me a bit more offroad ability. In the end I think I'll end up with something that will be OK for messing around with, but it certainly won't be true offroad suspension.

As a newb to automobile suspension, why are you (we) so close to the middle of the shock travel in sag? It seems that road suspension to suck up pot holes is diabolically opposite of what you'd want in an off road vehicle :)

Well Pete, I'm right there with you. The WRX may have been rally inspired, but in my newb opinion I think that stopped shy of its suspension setup.

[sniper1rfa]

The WRX may have been rally inspired, but in my newb opinion I think that stopped shy of its suspension setup.


Disagree there. While it's limited slightly by the stock front shocks, the STi at least is quite well set up for rally.


Keep in mind that rally is not offroad. With just a longer strut you can get plenty of throw out of the stock equipment for anything you could throw at it on a rally stage.

[carl]

I would have been fine on 100% stock suspension at our stage rally this past weekend - the road was very smooth and flowing though, not a hard workout on the suspension pieces. Gravity Park will be a better test or so I've heard

[lorenkb]

With just a longer strut you can get plenty of throw out of the stock equipment for anything you could throw at it on a rally stage.

Too bad it didn't come with a longer strut then eh? Hence my comment on the inspiration stopping just shy of the stock suspension. ;-)