Suspension Tech / Motion Ratio / Unsprung Weight

[tube80z]

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

[JMortensen]

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...

[cygnusx1]

Referencing the yellow car using rear droop limiters, would not a rear swaybar accomplish some "droop limiting"?

[tube80z]

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?

[tube80z]

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.

[JMortensen]

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.

[74_5.0L_Z]

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.

 

 

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.

 

[JMortensen]

Looks like some of that roll is from the tires and not the suspension.

[tube80z]

 

 

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

[74_5.0L_Z]

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.

[74_5.0L_Z]

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.

 

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.

[JMortensen]

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.

[74_5.0L_Z]

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.

[tube80z]

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

[74_5.0L_Z]

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.

[74_5.0L_Z]

I've run a couple of events since Sebring. At Sebring, the surface was concrete and the traction was amazing. The traction was so good that the car was easy to catch if it started to oversteer or understeer. The other surfaces that I have to run on are not nearly as good. BCC in Palm Bay is smooth asphalt and has probably 80% of the grip as Sebring. The surface at the Deland airport is old deteriorated asphalt and the traction is probably 50% of Sebring.

 

The car as I had it set-up in Sebring and most recently at Deland had a Magic Number of 2.92 per the Weight Transfer Worksheet. The car set up with a number this low should be very prone to oversteer when the power is applied coming out of the corners. At Sebring, the traction was so good that it was easy to compensate, and the car wiggled through the slalom sections amazingly well. At Deland, the car was nearly undrivable. The car was always on the hairy edge of wanting to spin, and forget accelerating out of the corners. It was not fun at all.

 

To try and make the car more drivable on the surfaces with less traction, I am putting together a new front sway bar. The current bar is a modified stock sway bar from a coupe (18mm). I am replacing it with a modified bar from a 2+2 (20mm). According to the WTW, the magic number with the new bar should be about 4.95. The higher number should give the car a little more understeer tendency and allow me to get out of the corners a little better.

 

In addition to the two sway bars that I've mentioned, I have a 25mm front sway bar at my disposal. The problem with a bar that big is that my Magic number would be 12.44%. A number this big would mean that the car has a strong tendency towards understeer.

 

Anyways, with three sway bars at my disposal, I need to do some testing at the next practice day. The next practice day is coming up August 16. I plan to arrive with the 20mm bar installed, and then based on the car's performance switch to either the 18mm bar or the 25mm bar.

[tube80z]

How much of a change can you get in your magic number by changing chassis rake?

[74_5.0L_Z]

Cary,

 

I looked at the effect of chassis rake. I can raise the magic number by about half a point by lowering the rear roll center ~.75 inch. Unfortunately, my rear tires get pretty darn close to the wheel housing as it is. Additionally, going lower also forces me to have more static negative camber in the rear (I am at the limit of adjustment of my camber plates). So I decided not to lower the rear any further and to make a new front sway bar.

 

Additionally, I think the stiffer front sway bar will help keep the inside rear tire on the ground going into the corners.

[74_5.0L_Z]

I installed the 20mm sway bar and ran autocrosses on Saturday and Sunday. Both events were at BCC: Saturday was practice, and Sunday was competition.

 

I made the first two runs Saturday by myself. The first impressions of the new sway bar was that the car felt a little more stable, but was still a little loose in the rear. I felt that I had made some improvement, but I was thinking that I needed to do something more.

 

For the next run on Saturday, I took a passenger. I strapped my friend Gary, who is approximately my same weight (185 lbs), into the passenger seat and away we went. The car felt incredible. All looseness in the rear end had disappeared, and I ran the course almost two seconds faster than my previous runs.

 

For the fourth run on Saturday, I ran by myself again. The car felt extremely loose compared to the previous run when I had a passenger. I was so floored by the difference that I decided to run the entire event on Sunday with a passenger.

 

So, how did I do on Sunday? I beat the other two guys in my class (EM)by almost four seconds on a 35 second course. I had the fourth fastest overall time out of 80 plus entrants. The only people who beat me were in two cars that will be at nationals and one in a F125 shifter cart. One of the cars that was faster than I was a Super Stock Corvette, and the other was a A Prepared S2000. The Corvette beat me by ~0.5 second, the S2000 beat me by 0.004 seconds, and the F125 beat me by 1.2 seconds.

 

Here is the riddle: Why did adding 185 lbs to my car help so much?

 

Before we can answer that, we must look at the effects of adding a 185 pound passenger to the car:

 

First, The additional weight balances the car left to right and improves the rear weight distribution. With just me in the car the rear weight percentage is 51.6% and the left side percentage is also 51.6%. Adding the passenger increases the rear percentage to 52.6% and balances the car left to right.

 

Second, The additional weight lowers the suspension natural frequencies all the way around especially in the rear (125 pounds of the passenger goes on the rear and 60 pounds in the front). Aside from lower the natural frquency, the additional weight also helps match the frequencies left to right.

 

Third, the Magic Number (from the WTW) for the car with just me in it is 5.24. Adding the passenger increases the Magic Number number to 6.22. A higher number indicates a greater tendency toward understeer and less toward oversteer.

 

My guess is that the improvement is a result of all of the above. It would be nice to determine which has the strongest influence, so that I can optimize the handling without the additional weight.

 

So, what is giving me the most benefit? Is it the balanced weight left to right? Is it the increased rear weight percentage? Is it the higher Magic Number? Is it something that I have completely overlooked?

 

Man, I really love this stuff.

[JMortensen]

I don't think its the left/right balance. I've seen really fast cars that were extremely unbalanced left to right. Like Gian Bowles's 510, which was really underweight, probably weighed 1600 lbs, and Gian himself is 220, 230 lbs. There is just no way the car was balanced side to side, but it was amazingly quick. My buddy Larry Butler's 510 is much the same way. It is a fast car that probably weighs 1800, Larry is also generally between 215 and 240 lbs.

 

I'd be tempted to move your magic number up and see what happens there. I think you should be able to raise the front ride height a bit to get that accomplished. That 5% is not a hard fast rule. It's more of a general guideline and you need to fine tune it in a bit. Sounds like maybe your driving style and your car like 6% better...

 

Sounds like a fun weekend! 4 seconds faster on a 35 second track is incredible.