ride height: fender lip vs. t-bar centers way out of wack

[Thrasher]

I got some time last night to check the centering of my rear torsion bar covers in relation to the spring plate covers as Wil mentioned. They are nearly perfectly centered, unlike prior to replacing the OE bushings with Neatrix. I think I can eliminate that as a problem source.

I also jacked up the front end by the central point on the crossmember, removed the front wheels, and then lowered the front end until my jack bottomed out. This left my front end up about 1 ½” higher than my target height of 25 ½ at the fender. Not ideal, but close enough to measure the rears using the tripod method.

The LR cam in at 24” and the RR came in at 24 ¾. The RR’s eccentric is at full “up”, and would require reindexing the torsion bar to attain 25” using the tripod method. As another option, I think I could get the LR up to 24 ¾” using the eccentric, because it’s not maxed out yet. That is an easier option than reindexing the rears.

But, that will almost certainly leave my front end up way high when all the tires are on the ground. That’s the dilemma I’m facing – do I let the fronts down to match the rears, even though I know that this would make the fronts very low when measured using the tripod method.

Or, do I go thru the pain of reindexing the rears up a notch to see if that causes the fronts to drop while at their proper tripod-measured height?

With the current indexing, I’ve been able to attain 25 ½ rear and 25 front, but only at the expense of being way off on the tripod measurements (on the low side). So, even if I get the ride height to look good, there’s a good chance that my corner balance will be way off.

I’m really amazed how I can get the fender lip measurements to vary so greatly by adjusting the opposite end. Maybe this is why some have had more success with the factory t-bar/wheel center diff spec than others?

If I were to reindex the rears, that would bring my t-bar centers up, and I suspect I could get the fender lip measurement to the desired heights with some tweaking. I think this is the better solution, because my corner balance may be better.

Thoughts???

[Wil Ferch]

Thrasher:
This might be the ticket.....

You say...
"With the current indexing, I’ve been able to attain 25 ½ rear and 25 front, but only at the expense of being way off on the tripod measurements (on the low side). So, even if I get the ride height to look good, there’s a good chance that my corner balance will be way off."

OK....get it back to 25.5 front/25 rear, even if the tripod method looks bad. Then....get it corner weighted. Find out the corner weights.

If corner weights look good, and the ride heights look good....screw the tripod !

At least with corner weights established...we can take educated stabs at next steps... if any.

Wil

[Thrasher]

screw the tripod

Really? Wil, I'm surprised to hear you abandon the tripod concept so quickly. If I do that, my chassis will be sitting very low. Much lower, I think, than other 911's set to Euro spec using the fender measurements.

I guess I was thinking I could set the heights using the tripod measurement and be very close on corner balance. I don't know anyplace locally that will corner-weigh the car, but I'll start asking around. Anyone in St. Louis area listening?

I was noticing bump steer and heavy steering at the lower height. I assume these are side effects of the very low height. I have a bump steer kit on the way from Pelican, though. I will also be more susceptible to scraping on curbs, etc.

I guess the upside is lower center of gravity should corner better.

What other downsides to having such a low center of gravity?

[echrisconnor]

I think what Will is saying, is get it close using the tripod method, then have the weights checked, and go with what the scales say not the tripod. These car's aren't pefectly weighted from side to side and even after I corner balanced mine, when I jacked up the car from the rear one tire would lift before the other.

Also, with proper corner weighting your're just making sure that the diagonal ratios are correct, you'll never get a perfect 50/50 side split.

Here were my weights:

LF 496 RF 416

LR 744 RR 656


As you can see my left side was considerably heavier (given, this is with me in the car).

So, use the tripod to get yourself close side to side. If you're still high in the front, turn the screws equal times to drop the front down equally. Adjust the rear then so the car stays close to flat up front and you get the height you need. Tweak as necessary. As you know, drive it at least a few blocks after every adjustment. Last of all, your toe will be off after it's lowered, have a full alignment done before you pronounce it done.

[Wil Ferch]

Maybe I'm being misunderstood ( or perhaps I don't understand ! ....very possible, stay with me on this..)

I'm just saying that most people use (only) the fender height measurement as a guide to target ride height ( and don't use the torsion bar-to-ground-to-chassis method). Fine. Do this. Get to your desired height. 25.5" front and 25" rear with 24.9" diameter ( stock) tires/wheels gets you close to stock Euro spec.....on RIDE HEIGHT.

Then....check corner wieghts. If ( at this resulting ride height, approx Euro), you have a fairly good match with desired corner weights ...then STOP..and don't worry about what the tripod method would have given you as an answer.

BTW....Chuck Mooreland and I have had numerous discussions on proper corner weighting, and he came up with an elegant "short-form" to the lengthy process I used ( see archives). Still...the same answer, and to be critically correct....is NOT equal diagonals ( although this method may get you close in many cases). The proper Chuck Mooreland short form would be to check if LF/LR = RF/RR , say....both ratios within 1% ( for example...LF/LR= 59% and RF/RR=60%). You'd be doin' good.

When you're all done, see if the lower front A-arms are angled slightly downward toward the balljoint ( as you found out is good), or at least "parallel" to ground. Not lower at the inboard pivot. The steering arms ( for bump steer) are typically taken to also be parallel to ground....but I've found out through this board a better parameter is to match it to the angle of the lower A-arm ( ends up being parallel to ground ---by default---only if the lower A-arm is parallel, too).

You then set bump steer by adding spacers , but you only have about 1/2" of room for spacers. If you need more...you either have to heat and re-shape the steering arm ( of the shock body where the tie-rod ends attach) , or use the spendy ERP bump steer kit ( $300 or so).

Whew!

Wil

[Thrasher]

Thanks guys for the ideas.

I'm going to follow Wil's plan and just set the fender heights to "Euro" and see where that lands me on balance. I can do preliminary checks on side-side balance by using the tripod method, even if I do fall way off on the actual heights. If these are way off, I know the corner balance will fail also. In that case, I'll do some more tweaking before trying to get it on scales.

I dropped the fronts last night and I'm going to see what I can do about getting the rears up, then re-tweak the fronts again when the rears are close. The A-arms are just slightly pointing downwards from parallel toward the balljoint, and the tie rods are sloping upwards.

My bump steer kit should arrive by the weekend. Hopefully that'll give me enough lift to minimize bump steer.

I have an alignment "party" set up at Hunter Engineering with my former co-workers. They are the engineers and product managers who design and market the alignment equipment, so I want to have the ride height/corner balance pretty close before I show up.

[Thrasher]

It's looking like I'll be able to get the rear back up to 25 on the RR now. It had mysteriously sunk to sub-24 with the adjuster seemingly in the "full-up" position.

Seems that the eccentric lobe had gotten pushed behind the adjuster slot and wasn't raising the car at all. So, I really loosened it, then centered the eccentric lobe in the slot with no tension. Then I tightened the nut enough to keep it in the slot, and when I adjusted it, the suspension resisted and raised the body.

When I dropped the car, it was back near 25" (from the sub-24 I was seeing with the eccentric in the same position before).

That explains why I wasn't able to raise that corner and I feel a whole lot better now. Couldn't get out to settle things in because of storms, but hopefully this weekend I can get it dialed in and check the balances.

[Thrasher]

I spent yet another nice day working on my rear suspension. The good news is that I got my new rust-free shiny spring plate covers on.

While I had everything off, I measured the angles on my spring plates. Before I raised it up, the rear was sitting at 25" Left and 24 1/2 Right with the adjuster centered on the left and fully up on the right.

The left side measured at only 29 degrees (24.5 degrees at the spring plus the 4.5 degrees jackstand rake at the door sill). Wil/Widebody Thom's spring angle calculator pointed me to 34-35 degrees assuming 24.1mm t-bars, 2760 lbs and Euro height. Maybe something was sticking, but it didnt 'relax' as much as I would've expected. My original plan was to duplicate the left angle on the right, but not now.

I bolted the left side all together and proceeded to the right side (which has been sitting low for a while now). It actually measured in at 34 degrees ( 29.5 degrees at spring plate plus 4.5 degrees jackstand rate at door sill) with the eccentric adjuster at full "up". I centered the adjuster before removing the spring plate and re-checked - that moved it down to 30.5 degrees (26 at spring plate + 4.5 rake). So, I reindexed the right bar to gain 3 degrees - which should equate to approx. 1 inch in ride height. Then I re-measured the spring plate angle after assembly, and before lowering. Now, with my eccentric centered and the reindexed bar, I have 35 degrees (29.5 spring plate + 4.5 rake) - just as the calculator recommends. I put on the wheel and lower the car, bounce the bumper and I'm sitting at 24.75" fender lip - about as you'd expect relative to the "before" measure (24.5" at 34 degrees), but the eccentric is still centered, so I can probably bump it up 1/4".

I didn't check, but I'm thinking the only way this might be possible is that I have a bigger t-bar on the left than on the right. The unloaded numbers from the calculator correspond on the right side to a 24.1 mm bar, but on the left they correspond more to a 26mm bar. Surely, having two different sized t-bars cannot make for good corner balance.

I need to drive the car around a bit and then measure the spring plate angles while they're loaded. If the numbers match up, then I probably just had sticky bushings holding things up. But, if they're significantly different L-R, then I'll have to conclude that the left t-bar is bigger than the right.

Fun, fun, fun.

[Thrasher]

ok, so I drove it around a bit and now my right rear settled back down to 24 1/4 - even lower than before the reindex. The loaded spring plate angles are 11 degrees on the left (now sitting at 25 1/2") and 9 degrees on the right (at 24 1/4). I guess I'm shooting for 10 or 10.5 degrees to get to 25 inches at the fender.

So, I guess it's time to add another 2 or 3 degrees to the right side and see where the height comes in. That'll put the right spring plate at 36 or 37 degrees (unloaded), while the calculator recommends 34-35. Maybe my car is heavier than the 2760 I'm going by, since I've never weighed it.

Anyone have any other ideas?

[RoninLB]

Originally posted by Thrasher

Anyone have any other ideas?

you did change out the stock spring plate bushings ?

[dickster]

thrasher

if the bars are an unknown quantity you might want to pull them if you're going to reindex again.

i used the same method as you and only did it once

good luck.

[Thrasher]

Originally posted by RoninLB
you did change out the stock spring plate bushings ?

Yes, that's sort of what started all of this. KurtV performed the operation and I assisted. Immediately after we did it, my heights were close enough that I thought I could dial things in with the eccentrics. We measured the before and after spring plate angles and things were looking ok. But, the front heights were not set even close.

I guess things settled in after driving it awhile, and my right rear has been sitting low since. In the time since changing the bushings, I also changed out the rear shocks (with the same Boge model that was in there before) and disconnected the sway bars. I have also been tweaking front and rear heights ever since trying to get this resolved so I can get an alignment.

Originally posted by dickster
if the bars are an unknown quantity you might want to pull them if you're going to reindex again.

I agree - but since the driver side (left) height is ok I was thinking I'll just reindex the right (passenger) once more to get the height equal in the rear.

Since Kurt and I measured the driver side angle when replacing the bushings, and set it to around 36 degrees, it must've just been the bushings hanging it up when I had it free the other night. That's the simplest and most likely explanation. It's not like the splines slipped.

After the rears are dialed in, I'll try setting the front heights equal to each other, with the rear jacked up from the center (tripod). If the torsion bar spring rates are significantly different, when I lower the rear back down, my front end will be off left-right by a large margin if I'm dealing with different spring rates, right?

If that happens I'll pull the driver side t-bar and measure it. If it doesn't, then I'm probably set.

[randywebb]

Pull & inspect - it won't be pretty if it let's go suddenly.

[Thrasher]

Originally posted by randywebb
Pull & inspect - it won't be pretty if it let's go suddenly.

I assume you're referring to the torsion bar? Are you suggesting that the reason my right side is low is that the bar is cracked/failing?

That thought hadn't crossed my mind, but I will pull it and inspect, since I'll be in there to reindex anyhow.

[randywebb]

no, but it just makes sense to inspect these things periodically

search for the posts with pics and comments from people on the board

[hobieboy]

I glanced thru this trail but don't think anyone mentions this so here goes...

My understanding of the factory method is that you are measuring for the difference in ride height between centre of your wheel & centre of torsion bar. So, it doesn't "really matter" if the ground is level - you are measuring difference.

With the factory method, the tire size is also irrelevant though depending on the diamater, your fender to ground height will be different; which might be your case as you are running non-standard tire size.

[Wil Ferch]

His tire choices are within tolerance spec for factory size tires...( around 24.8"....+/- .... overall rolling diameter).

Wil

[Rot 911]

Craig, your problem has me baffled. I know your torsion bars are good because I checked them. I just assumed they were stock and were the same on both sides, but I didn't measure or check the numbers. If I recall, you seem to believe that the rear shocks had been changed sometime in the past. I am now starting to wonder if the shocks are leaking(?) which would give you a correct ride height after adjustment, but over the course of several days sink down.

[Wil Ferch]

Especially if they are high pressure "gas" shocks....

even the factory "drop" settings for the spring arm are 1 degree different between hydraulic-shock cars and gas pressure shock cars....all with the same torsion bar size.

If you have one "duff" shock...this can explain a lot...

Wil

[Rot 911]

Crap Wil, I just read back and saw the Craig changed out the rear shocks since the time we replaced his bushings. I wonder if one of the new shocks could be bad???