Jump to content
HybridZ
tholt

Suspension Tech / Motion Ratio / Unsprung Weight

Recommended Posts

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.

Share this post


Link to post
Share on other sites

This is about the point where I'd close Excel and shut down the computer and start taking tire temps at the track. I'm betting (and if I'm wrong it wouldn't be the first time) that you'll need more neg camber than that to optimize the tire temps.

Share this post


Link to post
Share on other sites
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

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites
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?

 

We're using them all. Jeff (white and blue) and myself are on 13 inch bias ply hoosiers, the Chandlers (light blue custom bodywork are running 15 inch hoosier bias plies), Morgan (red and yellow cars) is mostly on 15 inch GY radials, and Dave (grey and black) is running yoko radials. Past testing on a L6 powered car indicated the GY radials were quickest. The yokos are a medium compound and take a while to heat up and may work better in the heat we'll get this summer. We plan to do a big tire test this summer and see how it works between a few cars.

 

 

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"

 

We chose the slicks because they are easy to find used and cheap. The atlantic tires are very light and heat up quick, which is good for autox. The new hoosiers are now all using compounds similar to the A6 radial street cars (designated B compound). Supposedly in response to crappy performance on the nationals "dune" course.

 

You can run the fronts all round but can't do the same with the rears. And when buying used you need to be careful to get fronts and not rears in similar sizes as they won't turn very well. By going with the same size tire all round you'll lose half to three quarters a second on a 45 to 50 second course. At least that's been our experience. The larger rears allow much earlier throttle application while still maintaining a lot of cornering force. Gives a whole new meaning to point and squirt.

 

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.

 

We have a tire management routine for the slicks that lets us run them to the cords with decent performance. We use formula V tire treatment and bag the tires between events. The performance drop we see is a lot less than our buddies running the kumhos and hoosier radial street tires, of which hoosier seems to fair worse.

 

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.

 

The yellow car above is my old car and ran front and rear limiters. Currently it only has rear installed. You'll find using the limiters will make the car react quicker. It will be similar to when you jumped spring rates. Also keep in mind you need to go stiffer to make the atlantics work if you chose to o down the route. All our cars are running at least 500 lb-in springs. And a few of the heavier V8 cars will probably be going up to the 650 to 700 range this year.

 

Cary

Share this post


Link to post
Share on other sites
You can run the fronts all round but can't do the same with the rears. And when buying used you need to be careful to get fronts and not rears in similar sizes as they won't turn very well. By going with the same size tire all round you'll lose half to three quarters a second on a 45 to 50 second course. At least that's been our experience. The larger rears allow much earlier throttle application while still maintaining a lot of cornering force. Gives a whole new meaning to point and squirt.

Steve and Ian in Fresno are running the GY rear radial slicks front and back on their Z. They have them stuffed on 15x10 wheels. I asked Steve about them and he gave me some feedback, but I've since replaced my computer and I don't have access to those old emails anymore. I seem to remember that he was pretty pleased with their performance.

 

They have to have some funky construction though. The 11.5 treadwidth is recommended for a 14" rim...

Share this post


Link to post
Share on other sites
Steve and Ian in Fresno are running the GY rear radial slicks front and back on their Z. They have them stuffed on 15x10 wheels. I asked Steve about them and he gave me some feedback, but I've since replaced my computer and I don't have access to those old emails anymore. I seem to remember that he was pretty pleased with their performance.

 

They have to have some funky construction though. The 11.5 treadwidth is recommended for a 14" rim...

 

Maybe they'll chime in but my experience was a ten inch rim gave up too much on these tires. And watching one of the locals run them you could see the tread move before the car would ever start to turn. Are you sure they aren't running the GT-2 radials that are a similar size?

Share this post


Link to post
Share on other sites
Referencing the yellow car using rear droop limiters, would not a rear swaybar accomplish some "droop limiting"?

 

Technically yes, practically no. What I mean by that is you can jack up one side of the car and the spring will come completely unseated. With the limiters this isn't allowed and there's a small amount of pre-load still left on it.

Share this post


Link to post
Share on other sites
Maybe they'll chime in but my experience was a ten inch rim gave up too much on these tires. And watching one of the locals run them you could see the tread move before the car would ever start to turn. Are you sure they aren't running the GT-2 radials that are a similar size?

I'm sure they are using the FA Goodyears. I can say that running the Yokohama fronts on an 8" rim didn't work particularly well either. You could really feel the tire fold over before the car would take a set. Slaloms were SLOPPY. But it was faster than the DOTs that fit the 14x7's that I had used previously by a long shot.

Share this post


Link to post
Share on other sites

Saturday, I finally got the car aligned for the first time since redoing my suspension. Here are the alignment settings that I used:

 

Front: 0 toe, -2.5 degrees camber, and 5.6 degrees caster.

 

Rear: 0.10 degrees toe in, -2.1 degrees camber.

 

Sunday, I ran an autocross and the car did extremely well considering that I have some seriously dead Hoosier A3S04 tires. At the limit, the car had just a little understeer, and the rear traction was much improved from the last couple of events. I attribute this to using less rear camber and less toe in at the rear.

 

There was a photographer present, so I asked him to get some pictures of the car in various parts of the track. The following picture surprized the heck out of me.

 

BodyRoll.jpg

 

The picture was taken at the end of a fast sweeper with a decreasing radius exit. The body roll in the picture is about 3 degrees. This is more roll than I expected the car to achieve with the tires and suspension that are in place (Hoosier 245/45/16, 450 lb/in front springs and 425 lb/in rear springs). The weight transfer worksheet predicts 1.6 degrees of roll per gee of lateral acceleration (for a steady state 1 gee turn). I guess I overlooked the additional roll that will occur in transient manuevers.

 

Also from the pictures, it looks as if the rear tires have sufficient camber to compensate for body roll, and it looks like the front tires could use a little more negative camber. Unfortunately, in order to get more negative camber on the front I have to either lengthen the lower control arms or give up a little bit of caster. I will probably make some new lower control arms (and outer tie rods) to widen the front track, and allow me to run more static negative camber.

 

Before I do make new control arms, I plan to do the following to add to this thread:

 

Measure front camber gain.

Measure front bump/roll steer.

 

BCC_050408_2.jpg

Share this post


Link to post
Share on other sites

 

BodyRoll.jpg

 

The picture was taken at the end of a fast sweeper with a decreasing radius exit. The body roll in the picture is about 3 degrees. This is more roll than I expected the car to achieve with the tires and suspension that are in place (Hoosier 245/45/16, 450 lb/in front springs and 425 lb/in rear springs). The weight transfer worksheet predicts 1.6 degrees of roll per gee of lateral acceleration (for a steady state 1 gee turn). I guess I overlooked the additional roll that will occur in transient manuevers.

 

When you figure 1.5/1.6 Gs for transients you'd be close to 2.5 degrees and that doesn't take into account tire deflection. So that's about what I'd expect to see. It does look like more camber could be used but I'd check temps to see how they look first. More camber will help in turns but you may lose too much braking for it to translate to a laptime decrease.

 

Your picture is good but it's hard to tell how much the inside is jacking up (if at all). I've found in the past that sometimes I can't control roll as much as I'd like with bars and spring when the inside of the car lifts up. This is where a droop limiter can help to reduce roll. While it will reduce the lateral contribution of the tire that's limited keeping the outside tire flatter do to reduced roll may give you a net gain. And limiters are a lot quicker to add then longer lower control arms. It will also make the car change direction quicker in transitions.

 

Just a thought,

 

Cary

Share this post


Link to post
Share on other sites

Thanks for the responses.

 

Jon you are correct, I am neglecting the tire contribution to roll. Also, I lowered my tire pressures for this event. I have previously been running 40 psi front and 36 rear (cold), for this event I lowered the front to 32 and the rear to 28. I am just shooting in the dark until I get a tire pyrometer. That will be my next purchase.

 

Cary, I have been wanting to try droop limiters. My next event is in 2 weeks in Gainesville, Fl (05/18/08). I hope to have droop limiters on the front by that time. From my picture, it appears to me that the inside tires are fully drooped (limited by the struts) and that the outside tires are on the Koni bump rubbers.

Share this post


Link to post
Share on other sites

A little update:

 

I ended up replacing my tires with the same size (245/45/16) Hoosier A6. I wanted to go up a size, but my old tires became corded before I could acquire some larger wheels.

 

Even though I couldn't go to the next larger size, I have to say this: New Hoosiers are MAGIC! My old tires had died from age and heat cycling, so I had enough time to forget what they should feel like. The car does what I tell it to, when I tell it to almost without exception. I just wish the driver was a little better.:rolleyesg

 

Additionally, I have installed the front droop limiters. They are simply cables that prevent the strut from drooping past the point where the spring comes loose. In my case they stop about 1" of droop travel. I have not installed them on the rear because my rear springs do not unload at full droop.

 

The front droop limiters do reduce body roll. Here is some video of the car from an autocross at sebring two weeks ago:

 

 

 

I am extremely happy with the way the car is handling, but I still plan to get some bigger wheels before the next set of tires.

Share this post


Link to post
Share on other sites

Wow. That looks like it transitions amazingly well and is running super flat. I noticed in your second video that you appeared to pull the RF front tire at :37 and the RR rear at :41. It's a bit hard to tell for sure because the concrete is so washed out with the sunlight. Anyway just thought I'd mention it and see if you had noticed or cared.

 

Pics of droop limiters??? Please.

Share this post


Link to post
Share on other sites

Thanks Jon, I am really happy with it. I love the way the car now wiggles through the slalom sections. I noticed the front tire coming up but not the rear. I'll have to look a little closer at the video.

 

The concrete looks so washed out because the temperature was 97*F that day.

 

I'll borrow a camera soon and get some pictures of the limiters.

Share this post


Link to post
Share on other sites

So the stupid question. If you hadn't done this would you believe a combo like you have would work as well as it does? I still have people to this day that tell me droop limiters will not work and all I'll do is pull tires off the ground.

 

Cary

Share this post


Link to post
Share on other sites

I'm sold on the limiters.

 

Since finishing the suspension mods and getting new tires, I have run two autocross events. The first was at Gainesville on an asphalt track with no limiters, and the second was at sebring on concrete with the droop limiters installed. The transient response at Sebring seemed faster. I meant to disconnect the droop limiters at Sebring to get a more apples to apples comparison, but I was having so much fun that I forgot.

 

I still haven't taken the time to fully understand why they work (at least not formally). Subjectively, I like the way the car handles with the limiters installed. Objectively, I plan to do back to back runs with and without them installed at the next practice event.

Share this post


Link to post
Share on other sites

×