corner weight conundrum

[swiftdrivr]

I set up my car [Swift DB-1] yesterday, and found my corner weights were 80 lb off. Never been more than 20-25 before. My ride height is even as best I can tell. The car handled well at Atlanta last outing.The only thing I did different was I disconnected the rear sway bar at the rocker, as removing the bolt at the tail of the blade seemed inadequate. This could be the cause, but before, I never felt the sway bar was bound when I set it up, so not sure. One other thing of note. I adjust the corner weights by the drop links at the front bellcranks, and the Right link is shortened to the stops, but the left is the opposite. I couldn't balance the car any other way, and that was how it was before also.Any ideas of what else I need to check before hitting the track again?

Thanks for your ideas,

[BeerBudgetRacing]

Bad end link? Failing spring?
Did you have an off? Something bent?

[Will Velkoff]

Make sure the following are done before you dive into the chassis:

Step 1: Are your scales leveled? How do you level them?
Step 2: Make sure tire pressures are set (I use target hot pressures for setup purposes)
Step 3: Detension/disconnect anti-roll bars
Step 4: Stand on one scale at a time, confirm correct readings on all.

[swiftdrivr]

Springs are relatively new. No off. Haven't thought to check the scales individually. They are in pre-marked spots, so should have been level, One tire was down 3 lb an hour after, but I set them to specs [cold temp] an hour before doing the weights.
I'll check the scales individually when I get home.
Thanks

[SteveG]

Also, if you don't have the adjustable front sway bar set up*, make sure your front spring lengths are exactly the same. Any front sway bar preload will screw up corner weights.

* Even if you have an adjustable front bar it is a really good idea to make the front spring lengths the same.

Steve

[problemchild]

What is the conundrum? You have imperfect parts ...... probably bent rockers. Identify, remove, repair, and/or replace. Standard race car prep for rocker arm cars. Putting adjustable eyelets on the shocks and modified ARBs gives you much more adjustability without replacement, but if replacing bent components is too much trouble, then those mods are likely too much trouble as well.

[DaveW]

Springs are relatively new. No off. Haven't thought to check the scales individually. They are in pre-marked spots, so should have been level, One tire was down 3 lb an hour after, but I set them to specs [cold temp] an hour before doing the weights.
I'll check the scales individually when I get home.
Thanks

Make sure the tires are equal diameters side-to-side - bias tires can vary wildly from one to the other even at the same pressures.

[BorkRacing]

Make sure the tires are equal diameters side-to-side - bias tires can vary wildly from one to the other even at the same pressures.

As per Carroll Smith put a 1x1 steel bar and digital level across each wheel pair and check if they are level to each other.

Also did you turn down the bump and rebound on your shocks?

And...some scales can accidently zero out at a weight other than zero and give a false weight so every time you check corner weights you need to lift the car up and zero them to zero


Robby

[Steve Demeter]

All of the above and make darn sure to unhook the bars at both ends.
When you have the weights set hook the bars up with NO preload. Adjust them by however means but make sure there is no twist in them whatsoever.

[David Ferguson]

I've seen a spring perch not seat correctly on a damper and jack a bunch of weight. And make sure the tires are fully on the scale pads.

[EricP]

I had EVERYTHING set up right, level, etc etc and had troubles... until I realized I hadn’t checked damper pressure. Worth checking too...

[swiftdrivr]

What is the conundrum? You have imperfect parts ...... probably bent rockers. Identify, remove, repair, and/or replace. Standard race car prep for rocker arm cars. Putting adjustable eyelets on the shocks and modified ARBs gives you much more adjustability without replacement, but if replacing bent components is too much trouble, then those mods are likely too much trouble as well.

I guess the problem then would be to identify the bent part. Nothing obvious noted but I'll look again. Scales are showing same weight on each when I stand on them, BTW, so not likely the solution. It may be that my method of detaching the rear roll bar in the past was inadequate, and this is the first time I've ever done it "right." [It's amazing what I get away with when I don't know I'm doing it wrong!] The weird thing is that the difference in the drop link lengths is not new, so that pre-existed the technique change. Thanks for all the ideas of things to check. Hopefully I'll get some more quality time in the shop tomorrow. If I find a definite etiology, I'll post it when I do.

[Offcamber1]

Is the frame true? They can look straight until you pull everything off and put them on a flat surface.

[Lotus7]

Sorry to be late to the discussion, but the RF81 I bought recently has rocker-arm front suspension similar to your DB-1 (but without the bell cranks).
I couldn't get the corner weights right with the ARB's disconnected no matter what I tried.

I eventually discovered that the PO had the rocker pivot bolts so tight that the suspension didn't droop when lifted !
(worth a quick check)

[problemchild]

I guess the problem then would be to identify the bent part. Nothing obvious noted but I'll look again. Scales are showing same weight on each when I stand on them, BTW, so not likely the solution. It may be that my method of detaching the rear roll bar in the past was inadequate, and this is the first time I've ever done it "right." [It's amazing what I get away with when I don't know I'm doing it wrong!] The weird thing is that the difference in the drop link lengths is not new, so that pre-existed the technique change. Thanks for all the ideas of things to check. Hopefully I'll get some more quality time in the shop tomorrow. If I find a definite etiology, I'll post it when I do.

With a rocker arm car, you need to start with matched rockers. You take rockers off and put them on a flat table facing each other. You will immediately see how the shock holes line up. If they don't match exactly, then you use a press to tweek them so they match. If they are too badly bent, then cut/weld, send for repair, or replace. A well-driven rocker car will likely require some rocker tweeking after many weekends, or even sessions, based on curb-hopping, etc. A poorly driven rocker car more often, based on curb-smashing.

One trick, I learned from Ralph Firman and Trevor Hagurty, was to file out the shock holes and weld on AN washers to relocate the holes to match on a pair of rockers. That was standard procedure for VD in the mid-80s as the QC on their parts was so poor. Ralph would bring new rockers over in his carry-on luggage then end up filing holes and welding washers on them before we could put them on cars.

I make custom adjustable shock eyelets for my Reynard so I can keep up with my continually tweeking rockers. I run lots of spring, droop limiters, and hit curbs, and am too lazy to keep fixing tweeked rockers. I find it better to just adjust the shock length while maintaining shock preload, etc.

[RussMcB]

Jim, what are your four corners' weights (LF, RF, RR, LR)? That might (or might not) influence responses.

[Rick Iverson]

Jim, what are your four corners' weights (LF, RF, RR, LR)? That might (or might not) influence responses.

Yep

[924RACR]

With a rocker arm car, you need to start with matched rockers...

Awesome points, Greg, thanks for writing that all out.

I have the same goofy DB1 rocker front end on my car - for the moment - thanks to it's SE1 heritage, and do have some small asymmetry on the front end. Never thought to check the front rockers for evenness like this. Definitely will be doing so after the next outing, since I know the car was crashed at least once, likely more.

All the more motivation to get around to redesigning the front end and eliminating the rockers... it's just time... LOL

[problemchild]

All the more motivation to get around to redesigning the front end and eliminating the rockers... it's just time... LOL

You don't need perfect parts. You just need to know that they are not perfect, and adjust in the correct location. As pointed out here, the springs, shocks, and bellcranks (on Db1 or SE1) all need to be symmetrical and the FARB unloaded. So adjusting the links is your only place that you can compensate for rocker imperfection. Note that I am referencing cornerweights and not ride height.

My personal starting point, on any configuration of car, is jacking the car in the middle of the diff and balancing the front suspension so the chassis is level. At that point, I go up and down on the front for ride height, but all cornerweight adjustment is done from the rear. Sometimes, with cars that have internal ARBs you may have to break this principle, but I think it is a good objective to follow.

[Bob Wright]

To add to the discussion of rocker arm matching- I have pictures of my old SE3 in the middle of a corner taken from straight on and you can not believe the deflection of the front rockers. Its no wonder they bend. I remember Ray Gemaine (PROTEC) used to put strengthening ribs in his replacement arms.

[swiftdrivr]

Well, I put the car in the air, checked the spring perches, looked for cracks and bends, confirmed air pressure, shock notrogen, and no issues found. I also put on a different set of tires. [Numbers below] I looked at my old data, and found that I never documented weights before I set them, just where I ended up. So, as best I can tell, either my memory is bad [no news there] and this is nothing new, or my old technique wasn't unloading the rear sway bar. I'm going to run it as it is and see if anything feels different.

these are End results.[ Again, I failed to document what I started at]

LF 236 RF 234
LR 303 RR 309

Different set of spec radials

LF 229 RF 239
LR 308 RR 303

[Daryl DeArman]

Well, I put the car in the air, checked the spring perches, looked for cracks and bends, confirmed air pressure, shock notrogen, and no issues found. I also put on a different set of tires. [Numbers below] I looked at my old data, and found that I never documented weights before I set them, just where I ended up. So, as best I can tell, either my memory is bad [no news there] and this is nothing new, or my old technique wasn't unloading the rear sway bar. I'm going to run it as it is and see if anything feels different.

these are End results.[ Again, I failed to document what I started at]

LF 236 RF 234
LR 303 RR 309

2# across the front, 6# across the rear. 50.37%cross weight. Looks pretty damn good to me, except being 18# under weight per your scales.

[swiftdrivr]

2# across the front, 6# across the rear. 50.37%cross weight. Looks pretty damn good to me, except being 18# under weight per your scales.

I never had an issue getting the weights to come right, they were just much farther out to start with than I recall them being in the past. I am pretty happy with the end results, and so far, I've never been underweight on track scales. Hopefully, my old technique for disconnecting the rear bar was inadequate, and now that I've got that fixed, I'll be FAST! [One must have their fantasies]

[glenn cooper]

Send that chit, mayne!!!!

well, i put the car in the air, checked the spring perches, looked for cracks and bends, confirmed air pressure, shock notrogen, and no issues found. I also put on a different set of tires. [numbers below] i looked at my old data, and found that i never documented weights before i set them, just where i ended up. So, as best i can tell, either my memory is bad [no news there] and this is nothing new, or my old technique wasn't unloading the rear sway bar. I'm going to run it as it is and see if anything feels different.

These are end results.[ again, i failed to document what i started at]

lf 236 rf 234
lr 303 rr 309

different set of spec radials

lf 229 rf 239
lr 308 rr 303

[billtebbutt]

I guess the problem then would be to identify the bent part. Nothing obvious noted but I'll look again. Scales are showing same weight on each when I stand on them, BTW, so not likely the solution. It may be that my method of detaching the rear roll bar in the past was inadequate, and this is the first time I've ever done it "right." [It's amazing what I get away with when I don't know I'm doing it wrong!] The weird thing is that the difference in the drop link lengths is not new, so that pre-existed the technique change. Thanks for all the ideas of things to check. Hopefully I'll get some more quality time in the shop tomorrow. If I find a definite etiology, I'll post it when I do.

As Greg said earlier, check the rockers! Curbing a rocker car will put bends into front rockers VERY easily. Ask me how I know (former owner of and RF 82 and a DB1)!

best,
Bill Tebbutt

[DaveW]

The 1st step, IMO, alluding to what Greg said, is to jack at the front and then at the rear at the lateral center of the chassis with everything set to a nominally "equal" position side-to-side.

Then, assuming the chassis is not twisted, if the front is jacked at the lateral center and the car is noticeably off level, the issue is in the rear. If jacked in the rear and the car is off level, the issue is in the front.

[gearwire]

On Swift front roll bars that are not adjustable you need to take the bar out to disconnect it. Then set the corner weights. The trick is to time the bell cranks so the bar will go back in without preloading at ride height. It is a tricky adjustment to get the corners correct and the bar installed in a neutral position. There are some modified bell crank systems out there to adjust the arb null position to eliminate this complication.

[swiftdrivr]

With covid and then a crash, likely caused by not figuring this out the easy way, I got very few races in between posting this question, and finding the answer the hard way. I think this is where my left rear shock died, and that's what caused the corner weights to be off.
I just thought I'd add the solution in case someone had a similar problem in the future.

[t walgamuth]

Just a general note: If your garage is not heated frost heave could make your corner weights different than when not frozen as the ground expands and raises it all up.

[swiftdrivr]

Pretty sure it was the shock. The shop has heat, and never gets below 50, and with the shocks repaired, the link lengths are now much more even with good corner weights