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Suspension Tech / Motion Ratio / Unsprung Weight


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Tried to redo my experiment and it went like a Buster Keaton movie. I tried a different workbench with a stiffer surface and as soon as I put leverage on the bar it started cracking like the lip of the bench was going to snap off. So I looked around for something more sturdy... my extra cherry picker should do. So I welded onto the back of the cherry picker and as soon as I put pressure on the bar the thing tipped. Better move to the front legs. That was better, but when I put enough pressure on the bar to move it 1", the rear was lifting up. Added some weights from an old barbell set, still lifted. So I'm going to wait until my wife gets home and have her stand on the thing and only move the bar 1/2" instead of the full inch. Preliminary results look like my previous experiment was wrong (must have been due to the bench flexing), and the rate looks to be pretty similar to what Dan had found in his experiment. I'll report back with a final number.

 

This will probably make a really big difference in my spring rate choice, so thanks for checking my work Dan.

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Jon,

 

Go to the following link:

http://www.fromsteve.net/carstuff/suspension/SuspensionCalc.htm

 

After the page opens, select the Sway Bar Rate Calculator tab at the bottom of the page. On that page is a figure of a sway bar with dimensions A, B, C, and D. Would you measure those dimensions for me on your sway bar and post them here?

 

Ignore the extra bend in the sway bar. The extra bend in the Z sway bar provides clearance for the L6 oil pump and really only has a small affect on the stiffness.

 

Dan

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A: 8.25, 9.25, 10.25

B: 30

C: 9 3/8, 10 5/16, 11 1/4

D: 25.4

 

A and C are given for all available adjustments.

 

EDIT--Based on that guy's spreadsheet the rates should be:

474.7761 in/lbs

373.7793 in/lbs

301.2005 in/lbs respectively.

 

I'm still a bit confused about the weight transfer worksheet though. You say it shows 2" of wheel movement, isn't that right? So then that makes the required front spring quite a bit smaller than what it would be for 1" of movement. Just trying to get all my ducks in a row before I order springs...

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The bench stiffness was tested with the same mounts. I still disagree with you on that one though and don't think the rate would be much different if at all and in actuality might be lower due to the lack of stiction. What we will be able to do is check the measured rate vs the calculated rate, compare to Dan's result and see if there is a significant difference. In the interest of getting as much info as possible I'll try to get the biggest measurement that I can in terms of deflection. I might be able to get 1" out of it the way I have it rigged...

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I did the test again and I'm still not satisfied with the results. I could see the cherry picker lifting ever so slightly as I put weight on the bar. What I measured for 1/2" of movement at the end of the bar was 110, 135, and 152 lbs respectively for the different adjustment holes on one side of the bar, with the side on the scale in the last or softest setting. When I tried to go for a full inch of deflection I was lifting the wheels of the cherry picker off the ground even with my wife standing on it.

 

I think I may just back off and rely on the spreadsheet at this point.

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  • 3 weeks later...

In going through all of this process, I have installed 425 lb/in rear springs, 450 lb/in front springs, replaced my front 1" sway bar with a modified stock (18 mm) sway bar, and lowered the heck out of my car. The car is about 1.5 " lower than when I started this process and about 3.5" lower than stock with 245/45/16 tires.

 

The car is much better in transitions than it was, but I have been having trouble putting down the power since going to the higher spring rates. I have not aligned of or corner balanced the car since starting this process. Just looking at the car though, it was obvious that the rear tires had acquired a bunch of negative camber. At the old ride height with the 250 lb/in rear springs, I ran -2.0 degrees of rear camber. At the new ride height, I found that I had -3.7 degrees of rear camber.:shock: Perhaps this accounts for some of my lost rear forward traction.

 

So, I decided to try and set my rear camber to a more reasonable number. That begs the question: What is a good rear camber setting for my new suspension setup that will optimize forward traction without letting the camber go positive with body roll? To answer that question, I needed to know two things: First, how much will my suspension compress in roll? Second, how much camber gain will my rear suspension give me per inch of wheel travel?

 

To answer the first question, I did the suspension analysis as outlined in RCVD and as implemented in the weight transfer worksheet. The weight transfer worksheet predicts 1.6 degrees of roll per G of lateral acceleration.

 

To answer the second question, I spent today measuring camber curves for various positions of my rear camber plates. What I ended up with was setting the plates as far towards positive camber as the slots would allow. This yielded -1.3 degrees of camber at static ride height. The camber gain at this position is -0.74 degree/ inch in the range centered around my static ride height (The camber curve is not really linear, but can be approximated as linear over small ranges).

 

 

If we assume that the suspension rolls about the center (big assumption), that amount of roll corresponds to approximately 0.8" of bump on the outside wheels and 0.8" of rebound on the inside wheels. If we also assume that we want the tire vertical at 1 G lateral acceleration, then we choose a static camber that will yield -1.6 degrees at 0.8" of bump. For my case, the most positive that I was able to set my camber was -1.3 degrees at static ride height. At 0.8" of bump (1.6 degrees of roll), the camber should be -1.3 deg - (0.74 deg/in x 0.8 in) = -1.89 degrees. This value is more than the 1.6 degrees of body roll, so I should have a little camber more than the minimum required to keep the outside tire from going positive.

 

I'm sure that I have oversimplified some of this, but I am quite sure that the car will be a bit happier than it was with -3.7 degrees of static rear camber.

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great information! This brings up an issue that I've observed in myself as well as others, and that's the assumption what when camber plates are installed, and the car lowered, that the middle setting of the plates will equate to zero camber. I've found that for zero camber on a lowered car (depending on how low you go), usually you end up on the outer limit of the slot range.

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If we assume that the suspension rolls about the center (big assumption), that amount of roll corresponds to approximately 0.8" of bump on the outside wheels and 0.8" of rebound on the inside wheels. If we also assume that we want the tire vertical at 1 G lateral acceleration, then we choose a static camber that will yield -1.6 degrees at 0.8" of bump. For my case, the most positive that I was able to set my camber was -1.3 degrees at static ride height. At 0.8" of bump (1.6 degrees of roll), the camber should be -1.3 deg - (0.74 deg/in x 0.8 in) = -1.89 degrees. This value is more than the 1.6 degrees of body roll, so I should have a little camber more than the minimum required to keep the outside tire from going positive.

 

I'm sure that I have oversimplified some of this, but I am quite sure that the car will be a bit happier than it was with -3.7 degrees of static rear camber.

 

I really like the analysis you did and I think that will really help to get you into the ballpark. The next step, which maybe should be a new thread, is now how do you tune this at the track. There are a couple of things to keep in mind. Your car should be generating 1.3 to 1.5 gs of lateral acceleration and the tire will deform a good amount (see photo below).

 

IMG_4107.JPG

 

What I've found is that you actually need more camber than just upright for max grip. Each tire will be a little different and you'll need to experiment to find that window.

 

Cary

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In my previous post, I stated that my car would roll 1.6 degrees per G and that I had set the static camber to -1.3 degrees. The static setting of -1.3 degrees would give me -1.89 degrees on the outside tire in a 1 G turn. I was happy with myself because I had stood the tire up a little bit more vertical and convinced myself that I would have enough camber at max lateral acceleration. Then Cary (Tube80Z) had to go and inject a little reality: "I should be generating 1.3 to 1.5 gs of lateral acceleration."

 

At 1.5 G my car should roll 2.76 degrees, the outside tires should rise (relative to the body) 1.2", and the inside tire should droop 1.2" from their static ride position. With -1.3 degrees statics camber, I will have about -2.2 degrees of camber in a 1.5 G turn while the body will roll 2.76 degrees. The tires will go 0.5 degrees positive in camber. I want at least enough camber to offset the body roll.

 

I'm looking for the least amount of static camber to improve my forward traction, but I want enough camber to prevent the tire from going positive due to body roll. So, I generated some camber curves for several positions of my camber plates. For each of these positions, the static ride height was held constant (5.75" at rear of the rocker panel, or 7.125" at the center of the LCA inner pivot).

 

Camber_curves.JPG

 

On this graph 0 on the X axis represents my static ride height. Displacements to the right represent bump of the tire and while displacements to the left represent droop. Each of the curves represent a different setting of the camber plate. The bottom curve is what I have settled on (for now). I have set the rear tires to have -1.88 of static camber. This will give -2.8 degrees of camber on the ouside wheel and 0.88 degrees on the inside wheel in a 1.5 G turn.

 

If you analyze the graph, you can see that there are two things that affect your camber. First, If you hold your ride height constant, you can increase/ decrease camber by adjusting the camber plate. Second, with the camber plate adjustment held constant, you can increase/ decrease camber by lowering/raising the ride height of the car. If you look at the curves, raising the ride height 1" makes the camber become ~.74 degrees more positive. So as Terry noted: the Ground Control camber plates will be set at or near there maximum outboard position on a severly lowered car.

 

BTW: Cary what wheels / tires are on that car? I'm trying to decide which tires are going on mine next.

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BTW: Cary what wheels / tires are on that car? I'm trying to decide which tires are going on mine next.

 

We have six local cars all running formula atlantic tires. Two are on 13s and 4 are on 15s. Here's a few pics from our last event (my car is currently not running). As you can see there's a lot of variation on how the cars are using their tires. We'll use this to make changes to try and get the tires in a better orientation to the ground, which will usually make the car faster.

 

IMG_4064.JPG

IMG_4075.JPG

IMG_4088.JPG

IMG_4146.JPG

IMG_4172.JPG

 

One thing to keep in mind when looking at all this data is that you'll need to compromise in the real world. Our low buck acquisition system is a camera, stop watch, radar gun, pyrometer, and driver feedback. We look at the data and the lap times and then look at the pics to see if the car looks good or has obvious problems. We often see that the tires are struggling to maintain a good orientation to the road but often find that when we get this right the lap times may suffer (less braking or drive off the corner). Looking at pyrometer data most of the tires (hoosier, GY, yokohama) all will run about ten degrees hotter on the inside when they are working well.

 

It was looking at pics like this that lead to using droop limiters to try and keep the car jacking up and lifting the inside of the tire off the ground. In these pics only the yellow car is running limiters. Look at it's tires compared to the others. Sometimes crutches work :-)

 

Cary

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Cary,

 

Thanks for the reply. All of those cars look like some serious hardware. I love the big rear quarter panels. Makes me wish that I had flared mine a bit further (but alas).

 

Hoosier lists several FA tires on their web site. These tires range in size from 22.0 x 9.0-13 to 23.5 x 11.0-15 in Bias Ply slick and 23 x 9.5R-13 to 23.5 x 13.0R-15 in radial slicks. The big tires for both are supposed to be mounted on 14" wide wheels:shock:. Are you guys using the Bias ply or radial tires?

 

Because of my brakes, I cannot use the 13" at all, and the available FA 15" sizes are incompatable with my bodywork. With the right selection of backspace, I could fit these Hooiser Bias Ply tires on all four corners of my car.

 

P/N________Size_______App________TW_____Dia______Circ_____ Rim Width

43470__22.0 x 10.0-16__GT2_______ 9.7"___ 22.9"____72.0"_____10"

 

Other options are to use the R6 DOT radial tires from Hoosier. The following could be made to work on my car:

 

P/N________Size________TW_____Dia______Circ_____ Rim Width

46535___P275/35ZR15___10.1"___ 23.0"___ 72.2"______ 9.5"

46630___P275/45ZR16___10.3"___ 25.6"___ 80.5"______ 9.0"

46730___P275/40ZR17___10.3"___ 25.5"___ 80.1"______ 9.5"

 

I have been trying to decide whether to stick with a DOT radial or to try a bias ply slick. Which lasts longer? I am tired of tires that "go away" after only a few events.

 

I love the pictures as a means of troubleshooting the car. I will try and get someone to start getting some pictures of my car at work.

 

You mentioned that one of the crew is using droop limiters. Is he using them on the front and back, or just on the front? I have been toying with droop limiting my front suspension.

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One option is to run the FA 15" front tires on all 4 corners, and there is nothing wrong with that. I ran them on my car with no flares at all, although I did run them on way too skinny 8" rims. The 9.5" wide tires fit correctly on a 10" rim, and that should fit under your subtle z flares.

 

The Yokohama radials would last me a full season of autox plus a couple track days. They do get hard and need replacing for that reason, rather than wearing to the cords, although I think your suspension will work them a lot harder than mine did and you may actually be able to wear them out. I was starting with used slicks that came from the pro Toyota Atlantic teams (the Yokohama is their spec tire). I think the Goodyear radial is supposed to be a faster tire than either the Yokohama or the Hoosier.

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Thanks Jon,

 

Were you using the bias ply or radials?

 

Bias Ply

P/N________Size_______App______TW_____Dia______Circ_____ Rim Width

 

43361__22.5 x 9.5-15___FA______9.2"_____22.9"_____72.0"_____10"

 

Radial

P/N________Size_______App______TW_____Dia______Circ_____ Rim Width

 

43570__23 x 9.5R-15____FA______9.4"____22.9"_____72.0"_____10"

 

These would both be good options for my car.

 

Regardless of which of these tires that I select, I am going to need some different wheels.

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