Suspension Tech / Motion Ratio / Unsprung Weight

[tube80z]

My vote is more rear percentage. What I've seen is the more power you have the more static weight on the rear helps. I had to run 200 lbs. of ballast to be legal in FP and when I moved it from the passenger side to hanging off the diff the car was quicker. But you're right that a number of things change and it's hard to pin it down to any one item.

 

The other question I'd ask is whether the car performs transitions better with a passenger. That's also very important. And I agree that this is all really fun. I had a good weekend too and found out how well a splitter can work.

 

Cary

[74_5.0L_Z]

I am leaning a bit toward the rear weight bias as the major player, but that is just my gut feeling. Unfortunately, there is not much left that I can move toward the rear or remove from the front. In the mean time, I'll just plan on having a passenger along. Its not a bad thing. I like the company. I was thinking about charging admission. The price being the use of a video camera.

 

My plans for the next year or so include wider wheels and tires and getting the new engine in the car. When the new engine goes in, I plan to offset it and the transmission ~3/4" to the right (currently they are centered). This should help even out the left to right balance.

 

Jon- I could raise the front a bit, but I really like the way the car is sitting. I will probably explore some other options first. Some of them are to go a little stiffer on the sway bar, install stiffer front springs, or install softer rear springs. The easiest of these would be a stiffer front sway bar. Changing springs forces me to re-corner balance the car. Changing ride height forces me to re-corner balance and realign the car. A sway bar change stands alone. I've had my eye on one of those Speedway engineering tubular sway bars....

 

Cary- I also want to start playing with some aero downforce. There is currently no spoiler or wing on the rear, and I may be making a bit of downforce with my front end. A rear wing may help balance the car a bit during the faster sections. I would really be interested in what you are doing with a splitter. Do you plan to write it up in the aero section?

[Diomedes]

I don't have much experience with heavy cars, but a performance increase resulting from adding weight could also be a sign of your tires being too big, or too hard a compound, right?

[74_5.0L_Z]

The car is just a lot easier to drive with a passenger. The improvement manifests itself in the transitions. By that, I mean that the car is easier to drive into and out of the corners.

 

I ran the car at Sebring in October, and I had an in-car camera on board for the first time. During the two day event, I made 14 runs (8 with a passenger on board and 6 by myself). Looking at the videos, it seems that the car is just a lot more prone to oversteer when I am alone in the car (both at corner entry and exit). I did weight transfer worksheets for both conditions, and the worksheet predicts more bias toward understeer when a passenger is on board.

 

What this is telling me is that I need to do some combination of the following:

1. Bigger rear tires (the downside would be the inability to rotate tires between events).

 

2. Stiffer front springs (just slightly).

 

3. Softer rear springs (again just slightly)

 

4. Slightly stiffer front sway bar, or increased sway bar motion ratio.

 

I plan to go with bigger tires next year. This year I had 245/45/16. Next year, I will probably run 275/35/15. I am on the prowl for some light weight 15x10 wheels.

[JustinOlson]

Can you position the strut inserts so they have only say 1.5"-2" of droop travel. Wouldn't this perform the same as have droop limiters?

 

Justin

[74_5.0L_Z]

To some degree yes. In my front setup, the koni 8610-1437 struts are all the way in the bottom of the strut housing, and the strut housing has been sectioned to fit. Now, with car sitting on the ground at nominal ride height, this places the strut at about mid-stroke (~3" exposed). With my 450 #/inch springs, they will be compressed about 1.5" while sitting there. So, when I jack up the font of the car, the strut will extend 1.5" before the spring comes off its seat, and then another 1.5" before the strut limits travel.

 

If I had softer springs, lets say 225#/inch, then the spring would compress about 3 inches under the weight of the car, and the spring would not come loose when the car is jacked up.

 

For my application, I want the 450 lb/inch springs, and I want to have 3 inches available for bump travel (even though I will only use about 2"). So in order to keep the springs from coming loose when I jack up the car, I have straps on my front suspension that limit the droop travel such that the springs remain lightly loaded at full droop.

 

On the rear of the car, the struts were sectioned a little more than they should have been. The result is that I lost about an inch of droop travel. So at nominal ride height, about 4 inches of the strut shaft is exposed. I have more weight on the rear and 425# springs. The result is that the rear springs are lightly loaded at full droop, so I do not need a rear limit strap. If I were to up the spring rates, or lower the nominal ride height, I would need rear limit straps.

 

There are other ways that I could have accomplished this. Namely, I could use a light weight tender spring.

[johnc]

Can you position the strut inserts so they have only say 1.5"-2" of droop travel.

 

Yes but make sure your specific shock is designed to function this way. Some shocks are not designed to be repeatedly topped out.

[74_5.0L_Z]

If I remember correctly, the koni 8610's are self limiting in droop. I don't think any shock is designed to bottom out in bump.

[JMortensen]

If I remember correctly, the koni 8610's are self limiting in droop. I don't think any shock is designed to bottom out in bump.

You could argue that Bilsteins are designed to bottom in bump since they have internal bumpstops.

[TensorWorld]

I ran the car with the 450 lb/in rear and 425 lb/in front with the 1" sway bar. The weight transfer worksheet gave me a magic number of 9.4% which predicts understeer (which it did). With the new sway bar and swapping the springs front to rear, the weight transfer worksheet gives me a magic number of 3.2% which is much closer to neutral.

 

If you don't mind me asking, what is this weight transfer worksheet you speak of? And do you have a copy of it or book that would have it?

[74_5.0L_Z]

The weight transfer worksheet is an implementation of the Total Lateral Load Transfer Distribution (TLLTD) material presented in Race Car Vehicle Dynamics (RCVD) by Milliken.

 

In the text (chapters 16 and 18) Milliken states the basic relationships between spring rates, motion ratios, static weight distribution, track width, and other vehicle variables to the TLLTD in a steady state corner.

 

Within the text, are some suggested starting points for setting up a car that will be driven on a road course type circuit.

 

On page 605, the following recommendations were given: (begin quote)

 

Ride Frequencies:

 

Non-Aero Sedan________________1.6 to 2.0 Hz (with front higher)

Aero Cars _____________________3.0 to 5.0 Hz (with front higher)

 

Roll Gains:

 

Sedans________________________1.0 - 1.8 deg / g

Aero Cars______________________0.25-0.5 deg / g

 

TLLTD's

To insure initial understeer, calculate the TLLTD to be 5% more than the weight distribution at the front.

 

(end quote)

 

What this is telling you is that if you have 50% of your weight on the front tires, then the front suspension should supply ~55% of the total roll stiffness. Remember, these are only suggested starting points for road racing cars. Cars intended for other purposes (like autocross) will use slightly different values (My car is set up with 2.5Hz frequencies, and Cary and others are exceeding 3 Hz frequencies).

 

There are a couple of weight transfer worksheets available. The one that we keep referring to is marketed by Smithees out of Australia. As far as I can tell, this worksheet directly implements the gospel according the Milliken. The WTW provides a very user friendly worksheet to input the vehicle parameters. The worksheet then provides a "magic number", which is the difference between the static front weight percentage and the percentage of the roll resistance provided by the front suspension. If the number is greater than 5%, the prediction is that the car will tend more toward understeer, and if the number is less than 5% then the car will tend more toward oversteer.

 

Although this tool isn't exact (because of inaccuracies in user measurements), it is a great tool for visualizing the effect of suspension changes like spring rates and roll centers.

[TensorWorld]

WOW! That is so much more helpful than the vague generalities I have usually found in car books. I will have to pick up a copy of that. I may be getting off topic here, but is there a list somewhere of books with descriptions like the one you just gave (say for aerodynamics or engine design)? I did my undergrad in physics but it was mostly theoretical so I don't know the books to read for the more mechanical engineering based stuff.

[Leon]

Race Car Vehicle Dynamics by Milliken & Milliken is very good.

 

Also take a look at:

Tires, Suspension and Handling by Dixon

Aerodynamics of Road Vehicles by Hucho

 

Great thread! I need to find the time to sit down and read it all the way through some time.

[heavy85]

Any update after another year of tuning?

[zredbaron]

Great thread! I have a couple of questions.

 

 

Why has limiting droop to 1" been mentioned more often than zero droop? I thought that zero droop was ideal, but I'm getting the sense that allowing a controlled amount of droop is more practical. Which is it and why?

 

 

74_5.0L_Z,

 

You recognized the lack of comparison, but I didn't see where you came back and compared no droop limiters to droop limiters back to back on the same course on the same day. Did you? If so, I would think you would also want to compare zero droop to 1/2" droop to 1" droop, etc.

 

 

Has anyone "tuned" the amount of allowable droop? I would think that's something you'd want to dial in just like every other suspension component...

[74_5.0L_Z]

For me the droop limiter was more for keeping the springs seated when the car is jacked up or when I lift a front wheel in a turn. My limiters are set so that the springs are "almost" loose at full droop. Without the limiters, the struts will allow the front wheel to droop more than an inch below the point where the springs are in their seats.

 

I've been wanting to come back and do some more with this topic, but I have been busy lately. I am working on my Masters Degree in Mechanical engineering and that eats up all of my time. I have only run the car at two events this year, so it is mostly collecting dust in the garage. I plan to take the summer off from school, so I plan to do some serious playing with the car.

 

I have a brand new set of Hoosier 275-35-15 tires that need to be used and abused.

 

It would be interesting to play with the droop thing some more, and I am also contemplating replacing the droop limiters with helper springs to allow the tire to stay on the pavement without changing my ride frequency. I'll update this later in the summer.

 

Dan

[JMortensen]

Strut suspension has a tendency to extend the inside suspension more than it otherwise would as the sprung mass slides up the canted strut, causing body roll. Body roll in a strut car is also camber loss for the outside tire. Some other suspensions like SLA don't respond to the angle of the spring and shock, so they aren't negatively affected in the same way. Struts are not the best suspension, and what we're really doing here is working around the limitations of the strut geometry.

 

If the Z was designed for much stiffer springs, Nissan wouldn't have used so much droop in the suspension. So really what we're trying to do first and foremost is get rid of the extra droop that allows the body to roll excessively which is only there because we've upped the spring rate by a huge amount. That means setting the limiter to stop the suspension when the spring comes loose.

 

There is a potential benefit to limiting the droop even further, but this is where you get to the fine tuning bit, and I would start with your limiters set as described above, and then reduce the front droop until you get the turn in you want. I would bet that you find that zero droop doesn't work that great, as it causes its own problems. I drove in a Toyota truck that had the front torsion bars cranked up to zero droop and it was very uncomfortable and the front end would lose contact with the ground a lot. It might be OK for a Formula Ford that drives only on smooth tracks, but not a good idea for a Z car that has to drive on bumpy autox courses or tracks. I wouldn't limit front droop on the front of a street car beyond the loose spring scenario.

Edited April 24, 2011 by JMortensen

[zredbaron]

It would be interesting to play with the droop thing some more, and I am also contemplating replacing the droop limiters with helper springs to allow the tire to stay on the pavement without changing my ride frequency. I'll update this later in the summer.

 

By ditching the droop limiters for the helper springs, wouldn't you lose the anti-roll benefits of the droop limiters completely? Forgive my lack of having read the referenced suspension books, but why would you consider intentionally letting your car roll (and giving up camber) so that you could have a better frequency?

 

From what little I understand of ride frequency, a faster frequency means the suspension will more quickly dampen any movement encountered. What I'm getting hung up on, is that in a steady state sweeping turn that frequency is less relevant and the loss of anti-roll is brutal due to camber loss. Obviously there is a trade-off?

 

Your car was quite flat and very responsive to steering changes in the external videos you posted. What is it about your handling that you don't like / leads you to contemplate ditching the droop limiters for helpers?

Edited April 25, 2011 by zredbaron

[JMortensen]

Wasn't sure where to put this, but I think this is the best single suspension and related thread on the site, so here goes: 

 

I found a little gizmo that I think a lot of you might be interested in, especially if you run wide front wheels. It's an adjustable front wheel bearing spacer. What this little thing does is it fixes the distance between the inner and outer wheel bearings. By having the bearing space fixed, the two bearings share the vertical loads more, so it should make the bearings run cooler, less chance of spinning a race in a hub, etc. Also reduces friction a lot and rolling resistance is a lot better. I was looking at getting some of these made, but found that the Legends car spacer fits out of the box like it was made for a Z, as the same set 2 and set 6 bearings and the spacer adjustment is right in there, doesn't hit the taper on the spindle, etc.

 

 

They also sell REM'd wheel bearings and low friction seals if you're interested. Here is their page on spacers:

http://store.drpperformance.com/bearing-spacers/