Debate: RR set ups

[DirtRacer9x]

We have yet to find a very knowledgable/factual answer on the subject of Coil over in front vs shock behind? I personally like coil over and can't see the high/low speed working the same when shock behind because of indexing. Thoughts, theory's, or anything you can input on the subject would be nice to hear.

[MasterSbilt_Racer]

We have yet to find a very knowledgable/factual answer on the subject of Coil over in front vs shock behind? I personally like coil over and can't see the high/low speed working the same when shock behind because of indexing. Thoughts, theory's, or anything you can input on the subject would be nice to hear.

Shaft speed is reduced if you mount it on the back of the cage. Not sure what else there is to say?

[DirtRacer9x]

So Master what type of changes would you make to a shock you ran in front to one behind or vice versa?

[Matt49]

Moving the same shock you had on the front of the RR to the back of the RR is going to result in decreased dampening in both directions regardless of speed. So you would need a more aggressive shock to control the un-sprung weight. And it would be LESS granular in tuning due to the loss of motion ratio. AND you'll be adding additional un-sprung mass with whatever slider or dummy shock you're going to put on the front of the birdcage to hold the spring.
None of these are good things.

Maybe I should have just asked this first: Why would you want to do this?
For answers on why you wouldn't want to do this, see above...

[MasterSbilt_Racer]

So Master what type of changes would you make to a shock you ran in front to one behind or vice versa?

Short answer, it must be stiffer. It will operate at lower shaft speed, so you must have similar damping at reduced shaft speed when compared to the front location. The shape of the curve would require much more analysis to answer.

[DirtRacer9x]

Reason I ask is I've seen a continuous problem with many cars running shock behind on a dry track. I'd say 90+% I've raced with can't get through the corner and I go right under them. Then the shock companies we've worked with just say to have more compression when shock behind. I don't believe that's the "simple" fix and there would be a lot more fine tuning involved due to the ratio/shaft speed channging and yes some unsprung weight changing.

[Matt49]

Reason I ask is I've seen a continuous problem with many cars running shock behind on a dry track. I'd say 90+% I've raced with can't get through the corner and I go right under them. Then the shock companies we've worked with just say to have more compression when shock behind. I don't believe that's the "simple" fix and there would be a lot more fine tuning involved due to the ratio/shaft speed channging and yes some unsprung weight changing.

Maybe they should consider putting the shock back where it belongs.

Are you seriously running against a bunch of people that are trying to make this work? "90%" having trouble making it work implies that you're running against at least 10 guys with this setup and 9 of them are struggling.
Why are you trying to fine tune a setup that you're already outrunning with something else. Keep it simple. You're way over-thinking this.

[DirtRacer9x]

Maybe they should consider putting the shock back where it belongs.Are you seriously running against a bunch of people that are trying to make this work? "90%" having trouble making it work implies that you're running against at least 10 guys with this setup and 9 of them are struggling. Why are you trying to fine tune a setup that you're already outrunning with something else. Keep it simple. You're way over-thinking this.I'm actually just seeing if a shock company can get on here and answer this question. Because cant find one that can and not only that but they call someone else everytime to try and find the answer it seems because they don't know. So I'm just looking for a really good explanation is all from someone who builds shocks and dynos them for rr behind. To me it would be easier to change for everyone on the crew. I've always seemed to smash my hand or finger on rr coil over set up when putting it up in there haha. But Matt49 I agree I like rr in front coil over. Just seeing what everyone else thought on the subject and has any input of feel and results.

[Matt49]

Dyno information is useless if you don't understand what motion ratios are involved, what spring rates are involved, etc. I'm sorry but I've explained it about as well as anybody can and MB did the same thing but with much less wording.
But to recap: When the RR goes up and down on a 4-bar car, the front of the birdcage has a motion ratio greater than 1 versus the axle and the back of the birdcage motion ratio less than 1 versus the axle. How much the difference is depends on a number of factors including bar angles, bar lengths, shock drop distance, etc. But the bottom line is that front of the birdcage moves a lot more than the back relative to the chassis so if you put a shock behind with the spring up front, you will need more dampening force (stiffer rebound and stiffer compression) to make it work. Which is why it won't work just by moving the shock without compensating for this. You'll have trouble getting the RR to conform with the racing surface. If you find a shock company that can provide a better or different answer than this, I'd love to speak with them also.
...maybe I'm just doing a really bad job of explaining this...

[DirtRacer9x]

No I fully underhand Matt49. I'm just continuing to see this and wondering why because it never made sense to me and couldn't find an answer from shock companies as to what valving to start with. I found it interesting the lack of knowledge on the subject is all when calling around. They want to send you the same rr shock and don't even ask where it's mounted.

[let-r-eat]

I see where you're coming from DirtRacer9x. But Matt and Masters answered your question. Much more dampening involved and with additional dampening comes all the trouble associated with that dampening. *fade, control, sensitivity,*

Like grabbing a hold of the ratchet at the head instead of the end of the handle and trying to control the load on the socket.

[hucktyson]

You would have to use data acquisition to find out the typical shaft speeds seen in front then calculate the difference in indexing between front mount and rear mount and figure a ratio of difference and you would see exactly what the rear shock would have to be I order to act like it would in front . Shock manufactures mostly never took the time to calculate this since it's rarely run that way. And then remember your ratio would change depending on what holes you have your 4 links in..

[DirtRacer9x]

Yes I understand what everyone's saying. Just didn't make sense to me why they're sold by Sho l companies and the don't tell you front or rear. Thank you everyone. Hope I keep seeing them behind more on rr because those seem to be the easy ones to pass in the slick.

[hucktyson]

That makes sense because of the cage indexing away from
The spring it doesn't load the right rear on entry making those cars loose. They probably turn in really well on hammer down stuff.

[DirtRacer9x]

That makes sense because of the cage indexing away fromThe spring it doesn't load the right rear on entry making those cars loose. They probably turn in really well on hammer down stuff.Nailed it on the head. Tacky they're fine but dry they chase that a$$ up the track it seems every time. Swoop right under them.

[save the racers]

Well before SCD completely derails this thread lets TRY to somewhat get back on track.

For a common denominator. Lets say you have a RR with 17in upper and 15in lower. The upper bar has 18degrees and the lower has 5 degree. The shock mount is 6inches below the axle centerline(i believe that is close to standard on most cages). The wheel compresses 3inches. how far does the front mount travel and how much does the rear travel?

[Matt49]

Well before SCD completely derails this thread lets TRY to somewhat get back on track.

For a common denominator. Lets say you have a RR with 17in upper and 15in lower. The upper bar has 18degrees and the lower has 5 degree. The shock mount is 6inches below the axle centerline(i believe that is close to standard on most cages). The wheel compresses 3inches. how far does the front mount travel and how much does the rear travel?

This could be very easily modeled. But I would need to also know how far the shock mounts are ahead or behind the axle centerline as this also affects the motion ratios.

[save the racers]

I figured someone with CAD or something similiar could do it pretty easy. I will get that other measurement tonight. I have a couple other things to throw in i'm curious about if you have a program and dont mind me pm'ing you.

And to the person who tried to red rep me for trying to keep this one topic please get real.

[JustAddDirt]

I would have to say a 5/5, or 6/6 shock would do the trick. based off the theory that I use a 4/4 valve shock on RR.


I am thinking this because your shock may movement is decreased by 1/3 or more with the indexing. so in theory you would bump up 1-2 steps in valving to compensate for the lack of indexing.


just my opinion.

[Matt49]

I figured someone with CAD or something similiar could do it pretty easy. I will get that other measurement tonight. I have a couple other things to throw in i'm curious about if you have a program and dont mind me pm'ing you.

And to the person who tried to red rep me for trying to keep this one topic please get real.

I actually use a free program called SketchUp to do this kind of stuff. I know there is software out there but I enjoy working with this program. It takes some tedious work to figure it out but I have most of it down pat.

[Matt49]

I'm going to go with the assumption that your shock eyes are 4 inches ahead of and behind the centerline of the axle.
You said 6 inches below centerline.
I'm assuming birdcage radius rods pickups are at the standard 6 inches from centerline.
17" upper bar at 18 degrees static angle (uphill)
15" lower bar at 5 degrees static angle (uphill)
At 3 degrees of axle bump, the front shock mount will have traveled 3.234" and the rear shock mount will have only traveled 2.766".

[MasterSbilt_Racer]

I'm going to go with the assumption that your shock eyes are 4 inches ahead of and behind the centerline of the axle.
You said 6 inches below centerline.
I'm assuming birdcage radius rods pickups are at the standard 6 inches from centerline.
17" upper bar at 18 degrees static angle (uphill)
15" lower bar at 5 degrees static angle (uphill)
At 3 degrees of axle bump, the front shock mount will have traveled 3.234" and the rear shock mount will have only traveled 2.766".
Aren't rod distances from centerline normally 4.5"?

[DirtRacer9x]

Just getting back on here and reading what you guys are doing. I haven't had time to throw the number down and see what to start or go with either. Haven't seen this discussed much or really known about by many shock builders and they're somewhat shocked haha when I ask for a certain valvin in rr when I tried it. But like I said shock builders I've talked with and very big companies haven't got a clue. Not naming names but they're calling around trying to figure it out themself when I'm there and ask. Kinda a disappointment if you ask me but I guess as the racer we have to figure it out and they just build em right. Anyways if anyone has some decent info to share I'd appreciate a PM. BTW threw a green rep thing back at someone. Get outta here stocky stay loose after 5 laps Cali boy and keep that 1/2 second slower on the clock too 👍. Bet that's news to you haha. You're welcome.

[powerslide]

Matt when you say 3 degrees of axle bump is that inches of travel at the wheel?

[Matt49]

3 degrees should have said 3 inches of bump.
Birdcage pickups at 6 inches above and below centerline? Oops...ran that using the last birdcage model I was using...which I guess kind of gives away something I was working on...
I'll re-run that with the "standard" 9 inch tall (4.5 inches from centerline) birdcage.

[save the racers]

Your pm folder is full Matt. let me know when you get it rerun closer to "standard" and then what you come up with on what we discussed. Thanks

[Matt49]

I cleared out some inbox space.

[DirtRacer9x]

Bumping this thread back up. Seeing if anyone can inbox me some info they may have. Worked with 2 shock guys on the phone regarding the changes needed to compensate and wanting to compare notes.

[hucktyson]

9x the reason they don't have an answer for you is that don't have a model of your specific suspension so calculate the percentage of difference that would be need to achieve the same dampening. The entire purpose of going shock behind on the RR is to
Make the car react differently than with shock in front . With that being the case put a shock on and see how it feels and adjust from there . There's no saying the standard valving for a front shock is ideal or correct for your driving style or tracks you race on.