corner weight inconsistency

[Rick Kirchner]

So here's one that has Art and I perplexed. Car is a 94VD FC. Not a lot of stiction in the joints, as many are nearing end of life.

Recently had the car completely apart, measured the frame, made sure everything was jigged perfectly and as symmetrical right/left as possible. This time I'm setting the car up per the original 1995 setup sheets - no front pre-load. pushrod lengths were set at a length suggested by the guys who ran the car back in the day when it set the Willow Springs track record. This means front ride height is set with the lower spring perch location, as opposed to cranking the springs into pre-load and setting ride height using pushrod length. Perches are secured with setscrews.

roll car onto the scales and front left is ~50lb light compared to the right.

crank a half-turn of RR pushrod into it and get the left/right balance perfect. roll the car off, jounce the hell out of it, roll it back - and it's off again. Adjust it to perfect, repeat above, and it's off again. I'm not even converging on a solution. Near as I can tell everything is tight, and even if it wasn't these weight changes require somewhere between 0.050 and 0.100 of movement. There's nothing obvious going on. Went back to baseline and have repeated this non-repeatability 3 times now.

Thoughts?

Since this is the first time I've set a car up without front pre-load, it occurs to me that with pre-load, if you jack weight onto a corner, as long as the pre-load limit is not exceeded, the car will just teeter over on that corner and the weights will even out. Should I be equalizing the weights with front spring or rear spring instead by changing perch locations instead of goofing with the pushrods?

[Daryl DeArman]

watching to see what the experts say. . .

[CGOffroad]

There was an RF95 in AZ that had a problem which made it basically un-driveable during the session. It was found that one or more of the shocks were not rebounding properly and the car was essentially jacking itself down to the ground. Full shock rebuild and tune solved that problem.

How long has it been since your shocks were rebuilt and tuned?

[Lotus7]

Perhaps it isn't the car?
Try moving the corner weight scales around, see if the problem follows one particular pad.
Or maybe the setup platform is flexing due to old age?

[Rick Kirchner]

How long has it been since your shocks were rebuilt and tuned?

Shocks have about 10 events and a few thousand miles of road tow. All are set on "1" bump and "0" rebound. When I started doing this I had them de-gassed, had the same problem, gassed them up and that didn't solve it.

[Rick Kirchner]

Try moving the corner weight scales around, see if the problem follows one particular pad.
Or maybe the setup platform is flexing due to old age?

A possibility. I built what I call a "hillbilly platen". I used angle iron to make the levelers and boxes of OSB for the bridges. to set ride height I have two pieces of metal that I use to bridge the front and rear scales.

But usually, when the levelers are flexing you can hear it during the roll-on/roll-off.

[John Robinson II]

Are you rechecking ride heights every time? Maybe check heights from floor to see if pads are moving. Are you rolling back to the same spot on the pads? We found some inconsistencies from that once started running increased camber. Now position scales where the contact is centered not necessarily Whole tire centered.

FWIW, I do not change shock settings when scaling. I check pressure and reset to desired running levels. I also set tire pressure to cold settings just to keep everything even. Very rare that hot pressures are even after a session. I no longer worry about stagger as the radials are consistent sizes.

When rolling car car back on scales are you just using the tires, not anything else on car? Is it safe to assume you are scaling with you in car?

John

[Daryl DeArman]

I was going to just listen, as I have ZERO experience with a pushrod car.

However, if you are "jouncing the hell out of the car" and then roll it back on the scales and something has changed, then something is not at the same position as before.

Jounce the heck out of it, roll it back on and let it sit for a few minutes---see if the weights change appreciably, if so that should point you in the right direction in figuring out which corner is settling or rebounding.

Not sure how much caster you are running (I remember you posting before some really high numbers). The more caster you are running the more weight that will transfer from one corner to another with a given degree of steering angle. Ensure that the steering wheel is oriented exactly the same each time.

[Rick Kirchner]

my setup precludes checking ride heights once I start scaling - because the reference planes cover the scales.

The scales are a tad short for the wheelbase, so I'll spread them a tad, see if that makes a difference - but I've used this setup for 10 years with no problem.

While waiting for answers here I went ahead and tried checking caster and camber. When I got to the rear things started being inconsistent again, so I'm going to take the shocks off, move it through the whole range of motion, and see if I have anything hanging up.

[mark elder]

Do you have the rack stops in when you are doing this?

[Purple Frog]

I'm assuming the ARBs are disconnected.
And... you are doing this with new tires.

Might spin the wheels to make sure the tyres are really round and not egg shaped.

Every time you roll back on, is it always LF light?

Being a one man team I always had weights to place in the car to represent the driver's weight.
I always used projected hot pressures to set ride height. I would nick the rub strips on the out lap waiting on the tires to build up. But, then JRII has more championships.

I suspect the platforms...or their placement.

[Garey Guzman]

Yes, sounds like things aren't going back to where they were during the previous scale. I'd look at the shocks as you mentioned you were going to do.

Since this is your first time without preload, make sure the springs are seated properly when you put it back on the ground.

[Rick Kirchner]

just like you do froggy with one exception: I set weights with no added weight in the car. Its easy to get the weight bars off-center. since we're going for front-end equal I usually do the ride ht with weights, take 'em out, set corners, put 'em back in and do the alignment.

I just moved the scales around and same problem. LF is always low. So I suppose I ought to start there - maybe swap shocks L/R see if the problem follows, then do same at the back.

[problemchild]

I get such different results with driver/simulated driver in or out that I consider doing corner weights without driver to be a waste of time. I also do not jounce the car, but apply equal up and down forces to the car, in identical process before each measurement. You may not have to do this with a softly sprung outboard car. Perhaps the stiffer spring/shock settings of an inboard car, make this a requirement.

Unfortunately, I have found that my scales read differently across the scale surface. I spend extra time leveling my pad structure so that each scale is also level, but I still get slightly different crossweight values with the car 2" fore or aft of the scale center. This may never show up if your pad is perfect for your wheelbase, but if not, you need to make sure you have the car exactly placed each time. You also have to make the decision that this variance is within an acceptable tolerance. If not, you need to rework your pad surface and/or get your scales recalibrated/repaired. On an ideal pad structure, the results will be the same with the car anywhere on the pad. It already sounds like you need to rework your simulated driver weight (or do your padwork with an actual driver.)

[Rick Kirchner]

just before this weekend I took all 24 of my weights, weighed 'em, and struck a centerline down the middle of each. At least I can get more consistency that way.

That's why I set weights without the driver though. at that point looking for things to be even, values are't as important.

interesting points on wheelbase and placement. That might be part of the problem. I also level each pad separately, then try to get pairs level side-by-side, and then all 4 front to back. I mark the floor when finished so I can put the pads back in the same place.

If I ever have a dedicated shop I'm going to bring someone in that can grind the concrete flat so as to minimize this mickey-mouse BS.

I have reb-co scales, and they are pretty robust. After looking them over I have to ask myself why bother with the leveling frames - they are just a complication. Each scale pad can be drilled and tapped for levelers and the top pads drilled for allen key access to do the adjustment, and not change the integrity or operation of the assembly in any way.

[Daryl DeArman]

just like you do froggy with one exception: I set weights with no added weight in the car.

But your right foot and right side of your brain are probably heavier than their left side counterparts

If you have a good baseline set up to copy, do that with you in the car. Then get out of the car, make zero changes, and record everything as the unladen set-up. Then, if a one-man show later you can duplicate the set up and arrive at the same point, barring no other changes have been made.

Changes to set up are made during the course of an event chasing performance at almost every level. All that work you are doing on the scales and with the alignment is just a starting point so that you unload off the trailer with a set up that worked best the last time you were there.

As to identifying your problem in the LF, it could also be the RR as the weight transfers diagonally.

[John Robinson II]

A few other points, I have a rack lock that I made out of pvc pipe to keep the wheels at the same degree. When I scale by myself, I have a rope attached to the wall that I use to pull up on the scales with. I figure getting back in the car settles it enough.

you made an interesting comment that your platen isn’t the same size as the car. Need to resolve that first. Biggest thing I got out of the ICP seminars was to keep everything consistent. I use a machinist level to level the pad, then place it on an I-beam on 1-2-3 blocks to level from side to side and front to rear. I have everything marked with arrows and placement to insure consistency. Once I think I have everything level, I go back and check the pads for level. I have been known to do this process several times a day to ensure everything stays the same. It was more important with the bias ply tires because no 2were ever the same size.

John

[sccadsr31]

The rack stops should take out any change due to steering centering and caster. Think about making 4 fake shocks out of square tube and use those and roll the car on and off the scales, if it is not consistent then it is some type of hysteresis in the suspension.
For leveling the scales I still like using a water level, it just cannot be beat in terms of getting all 4 scales exactly level and you can move it around on the scale and see how much the level changes.
Please let us know what the end is.

David

[Rick Kirchner]

I use a machinist level to level the pad, then place it on an I-beam on 1-2-3 blocks to level from side to side and front to rear. I have everything marked with arrows and placement to insure consistency. Once I think I have everything level, I go back and check the pads for level.

Exactly the process I used. I use a machinist's level (+/- .0005/10" ) on top of a long steel tube on top of 123 blocks. A stack of feelers under the low end tells you how many turns of the adjusters to make based on the thread. Once I got it right I marked the floor with stickers so the scales go back in the right spots. Since the pads are all completely independent, I used the bridges as spacers and ended about 3" short of the perfect wheelbase.

[Rick Kirchner]

Took the shocks off tonight. All corners move freely.

Since before I bought the car, it's run pre-load at the front. Since I'm trying the "old school" setup without pre-load and a specific pushrod length, the piston neutral position is now an inch further back, running on surfaces that haven't been run on much.

Maybe that's the source of the "stiction".

Also finally confirmed something weird. For years, at the rear, when measuring toe, I noticed that the right side stuck out a tad farther and took more camber shims. Now that I have all the arms jigged more precisely I measured this offset at the hub centerline to be 1/8". And, it takes 1/8 less camber sims to get the same amount of camber. The only way this can happen is if the mounting pads on the gearbox case at the bottom were machined too far. Anybody run into this on a LD before?

Actually measuring it without taking a lot of stuff apart will be a bitch.

[mark elder]

I think your problem of a lack of repeatable corner weight numbers has to be the front shocks not repeatably releasing to some constant static position.

If you have so much preload that there is no front droop (I would have thought a common setup for a RF94), then the shocks are not moving at all under normal setup circumstances (they never droop anyway and the small adjustments to equalise corner weights probably do not compress the springs much if at all).

On this basis (your old set up), you have reliable, repeatable weights. With your minimal preload approach, the springs and shocks move up and down depending on what you are adjusting. If the shocks 'stick' (either up or down), then your corner weights are all out.

I can see no other explanation. You have no stiction elsewhere.

The experiment is to either make some 1"square dummy shocks or put your old amount of preload back in (actual ride height does not matter for this experiment). Check corner weights and assess repeatability. Then report your homework back to the group.

As an aside, I think the (almost) zero droop school of thought post dated your cars original set up. Much clever people than me can comment as to which is the best approach. I would guess (maybe wrongly) that a minimal preload set up might need much stiffer front springs or a bigger ARB etc to reduce it falling over at the front.

Just my 0.02c worth.

[mark elder]

And Hewland machining is all over the place when I measure some of the LD200's. No surprise with what you found.

[SEComposites]

Could you replace the front shocks with dummy ridged rods? That way you might be able to confirm once and for all it is the front dampers.

[problemchild]

Perhaps, since you cannot find anything obviously wrong, you should modify your process and find a way to get consistent readings. That really is the end game. It is not like you are going to re & re shocks between sessions so you can check corner weights when putting on a new set of tires.

[Rick Kirchner]

I think your problem of a lack of repeatable corner weight numbers has to be the front shocks not repeatably releasing to some constant static position.

If you have so much preload that there is no front droop (I would have thought a common setup for a RF94), then the shocks are not moving at all under normal setup circumstances (they never droop anyway and the small adjustments to equalise corner weights probably do not compress the springs much if at all).

On this basis (your old set up), you have reliable, repeatable weights. With your minimal preload approach, the springs and shocks move up and down depending on what you are adjusting. If the shocks 'stick' (either up or down), then your corner weights are all out.

I can see no other explanation. You have no stiction elsewhere.

The experiment is to either make some 1"square dummy shocks or put your old amount of preload back in (actual ride height does not matter for this experiment). Check corner weights and assess repeatability. Then report your homework back to the group.

As an aside, I think the (almost) zero droop school of thought post dated your cars original set up. Much clever people than me can comment as to which is the best approach. I would guess (maybe wrongly) that a minimal preload set up might need much stiffer front springs or a bigger ARB etc to reduce it falling over at the front.

Just my 0.02c worth.

That's my current thinking and exactly how I saw the previous pre-load method affecting the static setup. The only reason I'm going back to the 1995 era setup is that this was what was used when the car set records. Not that it's likely to make ME a lot faster, just something I want to experiment with. It has 400 lb front springs and a big bar already.

I think tonight I'll de-gas them, remove the springs and actuate them, see if I can feel a difference (likely not I'd guess). Then I'll swap them side to side and see if the problem switches sides.

I wonder if just suffering through a test day (it has about a 60 lb offset) would get things moving into the "repeatable" zone.

Greg - I don't go to the extent of checking weights after a tire change. My platform setup is not very portable. I use avons, they're amazingly consistent, never had any issues to speak of.

[stonebridge20]

In 96 with a 94/95 VanD FC, we had the Runoffs won if not for an MSD box taking a piss with 600 fronts and no preload.

[R. Pare]

One more thing to check - the springs themselves.

Assuming that you have not already swapped the front springs side to side, it is possible that the LF spring is creating a lot more side load on the shock than the RF spring. This would happen if one end (or both) of that spring is way out of square in the wrong direction, causing the spring to thrust sideways on the damper than would be normal.

So, visually check the springs, and if in doubt, try swapping the springs side to side and repeat your scaling to see if it follows the spring.

One last possibility - do the bellcranks rotate freely?

[tstrong]

Rick, i may get ridiculed but after bouncing the car to settle it off the scales i roll up check weights and then grab the roll bar and “jiggle” it side to side to help release any stiction. Once jiggled the results are very consistent. Without jiggle sometimes up to 10 lbs different. Obviously my car is outboard suspension and not nearly as stiff as yours but it works for me. Todd

[Rick Kirchner]

Problem identified (but not yet verified).

So I owe everybody on this thread a beer. You see me, I'm buyin'.

Measured springs - perfectly square. perch heights - absolutely identical.

Took the springs off the shocks to see if I could actuate them and feel the "stiction". Fat chance.

Looked for a shock dyno here in town (several Baja/Mint guys here) - no joy

Stuck the left shock in a drill press vise and use the press to actuate it. Nothing obvious.

Did the same with the right shock. shock stayed compressed when I took the load off. Looked carefully in the shock eye to find some dumbass named Kirchner had adjusted one shock all the out, and the other all the way IN.

Yes indeed Gray Fowler/Mark Elder, I owe you a case. It wasn't the left but the right jacking. The right is a little less responsive than the left, so I'll probably be seeing Brenner in the off season, but then again, there wasn't 400lb of spring pushing it back.

Where's the popcorn emoji?

[xmazdatracy]

[Daryl DeArman]

Next time, dumbassery aside, if you get some funky-non repeatable numbers let the car sit on the scales for a bit and go inside and pour yourself a cold one. Go back outside and read the scales without touching a thing. If things have changed/equalized a bit then things moved while you were enjoying a cold one.

You could then go to chasing which shock is holding things down a little longer than the rest. The corner that got heavier and the corner opposite would be the ones to start with.