Dirt setup help

[stroutmail]

While there are those who like big caster "splits" as they make the car easier to turn.  You are at the high range with 5.5 difference.   At the high end caster can make the car a bit unstable and twitchy---here's why.... positive caster on the right front lifts the spindle when turning left.  This takes weight off the right front and reduces "cross" making the car looser. When the steering is turned right to "correct", the opposite happens--the spindle is pushed down, increasing "cross" and making the car less loose.  This process is beneficial, but if carried to an extreme it makes the car feel unstable as it changes from one tendency to another.  

Good that you are checking your springs.  Did I understand you to write that you could make the left front move 4 inches by standing on the bumper?  That's a lot more deflection than should be possible with a 180 lb spring IMHO.

One piece of data I maintain is the "static" compression of each spring.  Since you know the length of the spring, measure the spring with a tape measure or rule with the car on the ground and the difference is the deflection. (10-8.62=1.38)  1.38 " on a 180# spring is 250# of "sprung" weight on the spring.  (My data indicates that is in the range for the "sprung weight" on each front wheel.) This information can be correlated to your actual scale readings (affected by  "motion ratio" and "unsprung" weight) and you can get a better feel for changes to "cross" made at the track when putting in or taking out wedge.   (Motion ratio is simply the ratio of shock movement to wheel movement.)

[canabl]

in relation to the front travel, I have set the ride height at 4" and adjusted up from there. the front left has around 2 3/4" travel at that ride height and with the 185 spring  i can get it to bottom out with a good stomp on the front bar.
The springs have all come back and they are only 5-8pnd softer than the ratings printed on them.
is there two types of Bilstein shocks as the part numbers on my old chrome ones were different to the new painted units in this car? Think its only one number at the end different.

[stroutmail]

Sorry, can't help you on the shock ID--need to get info from someone with more specific information as to part numbers. 

I would be less concerned with the compression of the left front (although it does not seem very "stiff" judging by the crude "stomp on it" method)--I would be more concerned if the right front or right rear "bottomed out".  My experience is that very weird things happen when suspensions suddenly become "solid"--many times leading to loss of control. The loads on the right side are more likely to cause that problem yet I have not heard of anybody experiencing suspension "bottoming out" with the spring/ride height combinations you indicate.

If the shocks have insufficient compression damping or "bump" resistance, you should be able to tell that by testing them off the car.  I can say that my new shocks from INEX this year are quite stiff in compression. 

In the past, I've worked with Penske in configuring custom shocks.  I've not seen shock dyno data, but I think the Legends car shocks from INEX have sufficient compression damping for dirt cars---I wish they had a bit more damping under rebound.  IMHO as an engineer, their rebound setting allows a relatively rapid rate of weight transfer in roll and contributes to the car having a bit of twitchy feeling.

The handling complaints you have could possibly be caused by weak shocks--but I think they would have to be so weak that you would notice by a crude manual off the car push by hand  test.

Here's a crude method that has worked for me:  I lean over and (I weigh 200 pounds) push the shock down as hard as I can against a bathroom scale. (Use a block of wood to distribute the weight so it is not a "point load" and so it does not damage the scale.)  For my Legend shocks, the shock will compress at a consistent speed and the scale will read between 100 and 110 pounds. It will take just about a second to fully compress. ) If the shock is worn out, the scale reading would be much lower and the speed higher. If the shock is bent or the cylinder is damaged, the reading would be higher and the speed lower or jerky--you would sense it binding up.

[stroutmail]

After more reflection and your comments about: "new stiff shocks" and "not responsive to wedge changes" and since you found nothing binding in the suspension (not including the shock), logic to me says something is wrong with the right front shock-bent or damaged in some way making it not move freely.

[justfreaky]

I got that the springs were weak out of Adams comments. Just got back from vacation, so haven't had much time to catch up on things. I'll do some re-reading and see what I am missing.

Steve

[canabl]

i took the shocks out of the car and did the crude push down and release to time the return rates, all were spot on the same. approx 2.5 seconds from compressed to extended.
They certainly are not worn out in any way as they have only turned 15 laps prior to me pulling them out.
I am fairly strong and with the shocks alone in the car (spring removed) i can bairly lift the wheel against just the shock compression, take out the shock and the car rolls through its entire travel no resistance. after every event i have a habbit of spraying a lanolin based spray in the heims and work them through their movements to make sure all the water or clay is out.
I do think that the front end alone is too stiff with the rate of compression in springs to the rear end. every video i have seen of the new car has the front atleast an inch higher in ride height. I am going to stiffen the rear right up a fraction and lower the front rates down down and see how we go.
Our rules have just been adjusted back to be more in line with INEX, and the grooving of tyres has been struck out for this season, we must run a stock 57 AR on right rear and the grooved tyres we have from last season can be run on the other three corners till they either need replacing or are damaged throughout the season.
we have a practice session in a few weeks so i will try it out and give it a good run even try with it setup loose to try adapt to a tail out style as there will be less chance of getting hit if i spin.

 

[VMS Motorsports]

Is it rolling over on the RR through the corners, or is it rolling over just on exit, ie, when on the gas?
What's your pinion angle?

[canabl]

seems to be rolling over on entry through to mid corner when im partially on the gas and on exit the rear squats equally.
I have been rolling off the throttle on turn in and then feathering power in through the corner then as soon as i can get it ligned up for a straight exit the pedal is floored.
pinion angle is about 5 degrees as any more seems to have the uni smacking on the cross bar on rutty entry to corner.
the diff is also set to the right of the chassis, but there is only around 1/2" travel left to right overall.

[Gimpster]

Re-balance the chassis for that style driving. Lower the front ride heights and raise the rears

[canabl]

If i am to fit a stiffer RR spring to help hold it up, then dial in a tighter cross? will this in effect act the same as a light RR with a lower cross?
I have squared the rear end up and set the front camber caster correctly with a gauge i bought from VMS (thanks again) and it now looks like it has good amounts of both now.

[stroutmail]

I submit humbly that relative changes in difference between right and left rear springs will affect the "dynamic" handling in "transition" from off throttle to on throttle, but will NOT ultimately affect the "static" cross weight.  Increasing the right rear spring rate while keeping the left spring rate the same causes the car to be more loose or less tight immediately upon application of the throttle as the car begins transfer of weight from the front to the rear--the transfer will happen faster on the stiffer spring. (mid corner) But once the weight is transferred, the cross weight in not determined by the spring rate but by the actual weight on the wheels equal to spring rate times deflection.  When you change the right rear spring, you will need to set "static" ride height and cross weight to compensate for the stiffer right spring.  I would set the cross the same as you had on your previous car (50%) that seemed to suit your driving style--then go out and drive it on different track conditions (tacky and slick) before making too many more changes.  On the other hand, theoretically, changing the right rear spring to a stiffer rate would generally call for more cross.