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Nothing like doing things twice.
Seems to be a theme here LOL. This is NOT the way a trailing arm bushing is supposed to look. The sleeve was SPLIT. It was done at a Corvette shop too, but they had a young guy do these, and the shop foreman said the kid used too much pressure on the press, or even the wrong tool. And neither they or I double-checked. I found it when my alignment kept changing weirdly b/c the shims would not slide in. They sheepishly redid them for me on the spot and apologized profusely. Excellent customer service. Corvette Paramedics.
This is the way the pressed sleeve flange is supposed to look.
I also carefully checked the sleeve lip for flatness and dressed it down a little with a file. I need those shims to slide in & out of there smoothly.
All done the second time around. Second assembly & alignment went much faster. Practice makes perfect.
My H.D. IRS is complete and waiting for my BB. All built by me, but the diff with GTR1999's guidance.
The rear diff always looks awesome on these cars.
I’m so glad it can’t been seen once the body and spare tire tub is on, otherwise I’d be spending a fortune on chrome.
I’m starting the front suspension today.
I autocrossed an LT-1 Z28 for 27 years, so alignment is very important to me.
The last couple of days I measured the camber curve on the IRS.
At all 3 of the positions on the VBP Smart Strut Bracket.
Tomorrow I will put the stock bracket back on and do that one also.
My plan is to post all 4 charts up here for others to use.
Today's project was building a Bump Steer Gauge so I can measure the toe-in changes (bump steer) on all 4 of the rear strut rod heights.
I can't stand spending $300 for something I will only use once and could make it, so $35 and a couple hours later it is built.
3/4 MDF board is stiff enough and smooth enough for light use for me, and I don't mind throwing it away when I'm done.
I know that changing the strut rod inner mounting height will change both the camber gain curve and the bump steer (toe-in).
I want to wind up with the best setup for me, but I figured I may as well post the 4 choices here when done.
The charts really should not change from one car to another.
Last edited by leigh1322; Sep 16, 2023 at 09:43 PM.
I have my static rear alignment set now, and am trying to determine my optimum ride height and strut rod settings.
Those are the only 2 things you can really alter in the rear IRS.
So the bumpsteer numbers are in!
The home-made bump steer gauge worked great.
I really built this thing to dial in the front, but the back is easier, and good practice for me.
I had previously looked at the rear camber curve in this thread: Suspension Analysis
The curves were all theory, but based on actual suspension location point measurements.
I did not trust that to analyze the bump steer.
So all of these latest graphs are based on actual alignment measurements:
These camber alignment measurements pretty well matched the prior ones. The OEM C3 strut bracket causes the greatest change in camber. The Van Steel Smart Strut has 3 positions, and 3 curves.
I have previously concluded that a C3 with Gymkhana bars and F41 springs will roll 0.7" or 1.5 degrees in a 1G turn, and 1.2" or 1.5 degrees on brakes or gas.
This chart calculates the Camber in mid-corner:
So just looking at the Camber situation, it would seem the C3 bracket would be the best for cornering power, keeping the tire the flatest in a turn.
But I just measured the rear Bump Steer (Toe-In changes)
Ouch! THAT IS A LOT OF BUMP STEER!
A good street car should have .040-.060" , and a race car under .030, even under .010" And Toe-out is preferred under bump.
With a stock C3 strut rod bracket setting the IRS has 0.125" of toe out bump steer per side per inch in compression (turn or gas) .
And under braking the rear rises, and the C3 has .201" of toe-in change there. That is per side.
So .400" total. Over 3/8" additional. That makes the car very squirrelly under braking.
It overwhelms the 1/16" static setting you had it aligned to.
In a turn, adding up the right and left side, you have .080" toe-in bump steer, that makes it roll understeer. In the back. At least that one is minor compared to the prior 2 issues.
The bottom line is I am going to select the Van Steel Strut Mid-point setting for my street car.
I feel the Bump Steer curve overpowers the Camber curve.
It should have these effects:
VBP bracket cuts the total toe-in change on brakes by 25%, from .400 to .300”, eliminating some of the squirrelly feel on the brakes.
Toe-in change on a single wheel bump drops by 2/3 from .123 to .045” toe-out on bump. The car will steer itself less on a one-wheeled bump just driving down the road. Again just feeling less squirrelly.
The highly loaded outer tire in a turn has 2/3 less toe-out, and the inner one has 25% less toe-in, so the alignment just changes less rolling into a turn, feeling more stable as it takes a set in a turn.
Total Roll steer (understeer) is not changed too much.
The camber will change less on the throttle, giving my BB and fat tires more traction.
Yeah the tire will have a little more positive camber in a turn, but at least it is close to 0.5 degrees. Radials are OK with being that close.
Last edited by leigh1322; Sep 18, 2023 at 10:32 PM.
Thanks Leigh for this great info. Having a physics master on the forum is a great thing. Since I am pulling and replacing the differential on my 77, maybe I should consider the Vansteel bracket. I do have concerns about the clearance with the exhaust though. My 2-1/2 inch mandrel bent exhaust are already close (like 1/8 inch or less) from the stock bracket. They are not flattened like factory exhaust, so it would mean they had to be dropped down. I think they could be "manipulated" to be lower, but its something I would have to experiment with.
I measured this VS Smart Strut Bracket because of exhaust clearance concerns.
The center section is 1" deep and exactly as deep as the OEM C3 bracket. That section is 6-1/2 inches wide, then there is a taper.
Then the two ends, where the strut rod bolts on, drop down another 1-1/4"
So as long as your (and my) 2-1/2" mandrel bent exhaust pipes almost touch each other in the middle, they should fit like factory.
Unfortunately my 2-1/2" mandrel bent PYPES exhaust does not do that. It runs basically right under the strut rod ends, where this bracket would interfere.
Today's project was building a Bump Steer Gauge so I can measure the toe-in changes (bump steer) on all 4 of the rear strut rod heights.
Leigh, this is a great thread, thank you for posting it. My '67 has a lot of rear camber that I'd like to take out and perhaps your method with the angle gauge may be just the ticket.
Can you explain how your toe-in jig works? What are you measuring with the indicator? I see the bolt at the back, but without the base secured in position on the floor, how are measurements taken consistently? Maybe walking me through the setup of the jig could help with my understanding.
This jig is only for measuring bump steer, not actual toe-in.
My real Camber and real toe-in were previously set using the method in prior posts.
See post #222 for a link to the whole thread on alignment. With $35 worth of special tools. This pic shows the camber setting.
This pic shows the real toe-in setup, using a magnetic laser, pointing at a ruler 10 feet away at the front of the car. That ruler was set to zero precisely on the cars centerline.
I suppose I could have rigged something up to measure real toe-in with the dial indicator, but that is not what I did here.
The good news on alignments is that when you change the rear camber on this car, it has virtually zero impact on the toe-in setting! I was very surprised the first time I saw that. So you can "fix" your camber and not worry about the toe-in!
For this Bump Steer Gauge I copied the setup of a $300 all aluminum bump steer unit online.
Only I used 3/4" MDF board, which is easily cut, and has a really flat smooth surface.
I got by with one $30 2x4 ft sheet of MDF.
I used 2 cabinet door hinges, you could use a piano hinge as well, but I could not find one the right length.
I stabilized the bottom board with (2) 10 lb weights, I did not want it to move.
I still had to be very careful not to bump/touch it while measuring.
The hub plate needs to be at least 6-8" high to handle the suspension travel.
The hub plate got bolted to the hub with a couple fender washers and wheel studs. They did not make this pic.
The hub already had 1/16" toe-in as set earlier against the car's centerline.
The stud and hub holes were easy to mark and cutout, by just placing it against the hub and giving it a dead blow hammer love tap.
The rear bolt means all the movement happens at the front dial indicator.
My one inch dial indicator just barely had enough travel, I used it all, and another .030" over.
The pivoting action of the hinges is very necessary because there is a lot of travel in/out as the suspension moves thru an arc.
I kept my bolt and dial indicator pretty close to 24", so I got a direct readout of toe-in, basically at tread level.
(A little short I guess but I did not want to run any corrections)
I could have made the boards 27" wide but then it would have cost me double, I would have needed 2 MDF boards, and my dial indicator would not be long enough.
So the real toe-in or out is 12% higher.
I made the vertical pivot board very tall so I could get it to work even up on my jack stands. It was so high my hub centerline to ground was 23"! The T/A arm mounting bolt was 3/4" lower than the hub C/L at ride height. The H/S had a 2 degree down tilt.
But the frame was dead level right/left.
I moved the T/A with a floor jack, and measured the ride height with a tape measure between the bump stop pad mounting bracket on the frame, and it's flat landing pad on the arm.
Factory ride height is 3.5" there where I measured it.
I removed the bump stop for this.
The 1-3/4" bump stop was only 1.75" above the T/A.
I went 3" higher and 3" lower than my ride height baseline. That's about where the shocks top out, or it hits the frame metal/metal. Real suspension movement is less, around 2".
It was very stable, and very repeatable, as long as I kept my toes away from the base LOL!
Last edited by leigh1322; Sep 24, 2023 at 03:00 PM.
I aligned the Camber pretty quickly and then set the caster. Had to set the Z-height to 2.5" Camber Setting tool
Here is my Caster tool set-up. I read it directly off the grease fittings on the ball-joints. Gives me a direct Caster measurement.
I wanted to see how my shims stacks looked now after I cut off the one a-arm bracket and moved it.
This is where I was assembly wise 2 years ago now, before the stroke and the new A-Arm Bracket.
I guess my welder and I did a better job than GM did 50 years ago because the shims stacks are pretty even now, right vs left side.
This shim stack currently has 0.5 Neg Camber, and 5 degree Pos Caster.
Pretty much what I am looking for.
To get the 5 degree pos Caster. I have 1/4" offset Moog upper control arms, which really just give me a greater range of shim adjustment. The driver side originally had Zero shims with a decent alignment.
But I have also slotted those arms 1/4" on the mill to slide the upper ball joint back 1/4".
(If you look close you can see how the nuts are not centered on the square shoulders of the Moog shafts)
Procedure:
I had 2.1 & 2.6 pos Caster (R&L) now with zero shims.
Then I added roughly 4 shims on each spot to set the Camber to -0.5 degrees. That did not change the Caster.
Sliding the shafts back in their slots added 1.5 Caster and took the Caster numbers to 3.6 Pos.
Then I moved one shim from back to front, and got 5.0 Degrees Caster.
Perfect! Just what I was looking for!
I guess I have a few threads left over for a strut support bar in the future, but not much.
Note that I have a strut to strut distance that is 3/8" too wide. Mine is at 26.75" vs 26.375" being supposedly correct.
Tolerances were not so hot back then! At least it hasn't sagged. But that is also causing me some problems with thick shim stacks to get some neg camber. The MOOG arms helped with that. Since my alignment "dialed-in" OK it means I do not need another trip to the frame shop to fix the "reverse-sag". I am just gong to leave it as-is.
It also meant that I could run these stock A-Arms with their Ridetech Delrin bushings and I would not need to purchase the SPC threaded adjustable upper a-arms to fix my alignment issues. They cost $900 now, so I saved a few bucks, but this way definately took longer. The SPC arms would have been faster and easier. But I wanted to keep the car as closely as possible to my "stealthy mods but stock looking" theme.
Mine will "look stock" unless you look really close, but I will still have the high performance mods.
Win-Win!
And I do not need a trip to the frame shop or the SPC arms so my frame build-up can continue uninterrupted. Yay! Another win!
Last edited by leigh1322; Jan 12, 2024 at 01:44 PM.
Now the real fun began.
Lets call this section: "The effects of Positive Caster" or just:" The Advanced Section"
I always knew adding pos Caster helped the car gain Neg Camber, but I never knew how much.
Even when I was racing, we just "did it".
My inquiring mind wanted to understand this, so I measured it. I have never seen a chart for this.
No time like the present with a bare frame!
So I had to design something to measure the Camber as the Spindle turns, and it had to be accurate.
So here is my measuring rig, to measure Camber as the wheel turns. The laser and the pointer kept things very repeatable.
The spindle will turn 55 degrees to the inside and then hit the a-arm stop. Going to the outside this happens at 35 degrees.
So I forced my alignment to 0 degrees Caster, and measured the Camber thru the spindle's range of motion.
Repeated at 2.5 degrees Pos Caster, about stock level. And what my frame gave "naturally" with no shims.
Repeated again at 5.0 Degrees Pos Caster.
Here is the result:
Because of the kingpin angle, there is an arc involved. The upper ball joint is an inch or more inboard of the lower one, yet the spindle is level.
One thing in particular became apparent. Most of this does not matter. The spindle only turns about 5-10 degrees during normal driving. 1G on a 100 ft radius skidpad would be about a 5 degree spindle angle, and only 40 mph. 60mph would be only a 2 degree spindle angle and on a 250 radius skidpad. Only on a 50 ft radius turn, at 1G, would the spindle turn 10 degrees.
So here I re-scaled the graph to the important section:
The little Green Circle is the important part. It shows that if you change the Caster on your car, from 2.5 Pos to 5.0 Pos, when you turn the spindle only 5 Degrees, you gain Neg Camber. About 0.2 Neg at 5 degrees or 40 mph, and as maybe half that at 2 degrees Spindle, at say 60 mph.
It is not a lot, but it is very important to keep the heavily loaded outside tire vertical in a turn for maximum cornering power. Since the car rolls around 2 to 2.5 to 3.0 degrees Positive in a corner. it tilts the tire positive as well. Every mod you can make to keep the Camber on the Negative side helps the car go faster. It is probably hard to go too far with this.
One other thing that positive caster helps with is "steering feel". Tire feedback at the steering wheel is greatly improved. I have experienced this first hand. But why? And how can I explain why?
I measured the spindle height from the ground, as it goes thru it's say 30 degrees of motion, like above.
The spindle tip moves up and then down!
The movement is like 1/4"
This is due to the arcs above.
So say you turn right, the right spindle moves back, and down, while the left one moves forward and actually up.
The weight of the car tries to turn both spindle "inward".
This helps the car go straight.
Changing the Castor to more Positive, makes the spindles move up & down even more than they did before.
The change affects the inboard spindle more than the outboard. The inboard spindle moves Down more than the Outboard moves up.
You can see how the line gets more and more tilted as you add Positive Caster. But the spindle having pos change is greater than the one getting a Neg change.
What all this does it you are actually trying to lift the front end of the car 1/4" when you turn the wheel.
Obviously gravity tries to turn it back straight again.
The more Caster you have the greater this effect, because the graph lines are tilted more.
So the bottom line is the steering wheel feels twice as firm to the driver, and the tires push back twice as hard, and you can feel what the tires are doing at their limit, twice as well as before.
To me as a driver trying to go fast, and hold the tires at the limit, this was the biggest impact of Positive Caster.
As you hit the "limit", a tire started to slide, the steering wheel suddenly got lighter, and you knew you better back off!
That is why you see the Nascar drivers on TV sawing their steering wheel back-n-forth, an inch or two. They are testing how close they are to the "edge". They want to see how it feels! Is the tire responding better? Or is it worse?
Woah! Enough measuring for today.
Next time: Front Bump Steer!
Last edited by leigh1322; Sep 28, 2023 at 05:37 PM.
Today's coup! I was going to buy one of these anyway, new, and I just happened to check Craigslist first, and find one 2 miles away for $900 vs $1600 new.
Win-win!
