Front suspension changes

Can't find where I first started this thread, so.. At nationals someone suggested my front was too stiff on my 90 Van Diemen if I was not getting some rear wheel spin. I tried disconnecting the front bar at an event 2 wks ago on asphalt and was impressed by the increased turn in and less push, but the rear behaved so badly including bouncing oversteer that I was not thrilled. Today I tried it on concrete and again better turn in and less push, but pretty high on the rear wheel spin and oversteer even after I softened the rear bar. So, it seems I could use a less stiff front bar for a compromise between improved front and messed up rear. Anyone know what is ava for front bars on Van Diemens or know where to look?
I will post link to video soon.
Mark

A decent fabricator and make anything you need. Remember, roll-stiffness is the combination of ARB and spring rate, so you could change springs as well. Let me know if I can help you make anything.

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Originally Posted by David Ferguson

A decent fabricator and make anything you need. Remember, roll-stiffness is the combination of ARB and spring rate, so you could change springs as well. Let me know if I can help you make anything.

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David,
I am running 350 lb front springs. They make 300 lb; do you think that would that help?
I know anything can be fabricated and I may need to go that route.
Mark

Here is video from the run with disconnected front bar. Not as fast as with bar due to loose rear end.
http://youtu.be/yk1JkI_ICpk
Mark

I would suggest half that front spring rate...about 175. Original 1991 Van Diemens had 120 lbs fronts and 300 lbs rears for the Dunlop FF tire. The FARB was .688" dia and non-adjustable.
Just my $.02

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Originally Posted by mwizard

David,
I am running 350 lb front springs. They make 300 lb; do you think that would that help?
Mark

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Well, that's in the right direction (assuming you re-connected the ARB). Before I can advise how much you should change, I would calculate the wheel rates of the existing ARB, and the motion ratio (damper movement/suspension movement). It looks like there is some good advice above, too...

Don't be afraid to try changing things. For any change there are only three results -- better, worse or the same. You just have to figure out which result you ended up with.

I would probably start by removing the rear ARB altogether, to maximize rear grip, and then try to stabilize the platform with front spring and front ARB. Springs stiff enough to keep the car off the ground under maximum braking at the speeds you achieve and the ride height you want, then front ARB to achieve the roll reduction you want, steering stability, front grip, etc. I'm not sure why spinning the rear tires under acceleration is considered a good thing, I would be looking for maximum hook-up and being able to get on the gas as early in a corner as possible, hence the no rear ARB and stabilizing the platform with the front to maximize rear grip.

Brian

Mark:

The over-steer you encountered after you removed your front ARB might have been solved by simply lowering the rear ride height and 1/8" or so.

You need to do a little home work. Take David's advice and plot out the motion ratios and wheel rates for your ARB.

Read on only if you want to get a head ache.

You can pick your spring rates based on the ratio of the spring rate to the load at the wheel. Example: your car weighs somewhere around 230 lbs. at each front corner. Maybe you start with a wheel rate of 150 lbs./in. or 150/230 = .65. If you motion ratio is 1 inch of wheel to 0.6 inches of shock travel, you will need a 355 # spring. Another way to look at this is that a 120 pound spring will give a wheel rate of 50 lbs./in. That means to support the car; you will have to compress the suspension 4.5 inches to just to get ride height. Your suspension won't travel that far so you will have to just compress the spring on the shock.

Next you need to add the ARB spring rate to your calculations. And this is very tricky. As long as the car just rolls, that is the inside wheel goes up and the out side wheel goes down, the ARB rate is one thing but as soon as the inside wheel stops going down, the ARB rate at the outside wheel falls in half. This is usually about the time the front is getting really loaded. Now if you are ready to really go nuts, you have to calculate the steering angle because the actual roll of the front suspension is compounded by the steering geometry. The inside wheel goes down and the outside wheel goes up as you turn the steering wheel. And it you are sawing on the wheel, the spring rates of the wheels are constantly changing. Are we having fun yet?

My bet is you will want a spring to weight ratio somewhere between 1.5 and 2. And for sway bars, I plan on my stiffest ARB option being equal to my spring rate at the wheel.

The VD front spring rates can be very deceptive because preloading the spring and limiting wheel droop is much more improtant than the wheel rate proper. Talk to someone who is really knowledgable in setting a car up with droop limiting and prelaoading springs. Given the way the VD front suspension is designed, the setups are very logical and obviously work well when they are done right.

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Originally Posted by S Lathrop

Talk to someone who is really knowledgable in setting a car up with droop limiting and prelaoading springs.

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That person is clearly not me. There are a lot of threads that have drifted into this topic on ApexSpeed, but is there a really good one that covers why instead of how?

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Originally Posted by Neil_Roberts

That person is clearly not me. There are a lot of threads that have drifted into this topic on ApexSpeed, but is there a really good one that covers why instead of how?

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I have almost never used droop limiting at the front of a car that I was setting up. I can understand the logic of how and why it works. I just never put myself into a position that I needed to do it.

However I have used droop limiting at the rear of a few cars that I have engineered.

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Originally Posted by Neil_Roberts

That person is clearly not me.

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I have noticed you imply this before and was perplexed. Why would you lack knowledge on this subject with all your race car design experience? I would think that you would be one of the most knowledgeable members of this forum.

Brian

I'm thinking he doesn't buy into the concept?

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Originally Posted by old 59

I'm thinking he doesn't buy into the concept?

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Correct, but I'm always hoping to learn. Bump limiting makes good sense, especially for ground effect cars, and especially if it's done with a 3rd spring. Droop limiting only makes sense if the gain in aero performance exceeds the loss of mechanical grip. Only flat bottom cars with big diffusers are in that category, but I see and read about a lot of non ground effect cars using droop limiters. Why wreck your mechanical balance and grip for no gain?

Here is my best explanation and as I reread it I don't think it is very good:

In an effort to gain grip at the front, you decide to run softer springs. The problem with softer springs is that when you unload the front by say 50#, the front rises more than it did originally. Also you need to run the car higher to keep it off the ground.

Neither of these effects are wanted. So you preload the springs on the shocks. Preloading the shock also droop limits the shock. As you raise the front of a car with preloaded springs, it does not go up as much to reduce the load on the front tires to zero. You can increase the preload to any amount. One condition is the point where the tires do not go down at all but the tires will go up and the car will go down if the load on the car is increased by any amount. This is called zero droop.

Droop limiting simply limits how much the front suspension can move and the amount of preload sets a platform to support the car at some ride height. The suspension will go down only when the load exceeds the preloaded amount and only goes up as much as the shock will allow. The spring rate on the bump side is the spring rate of the tire until the loads exceed the amount preloaded by the spring, and then the spring rate is whatever the spring on the shock combined with the spring rate of the tire gives.

Also, when the inboard shock tops out, the inside wheel stops moving down, all kinds of things change. For one, the ARB rate drops in half on the outside front wheel. The front roll center is now the inside front tire contact point on the ground. Load transfer from the inside front tire to the outside rear tire is disrupted.

Take the suggestion of a 175 pound front spring. For a motion ratio of say .6 (shock/wheel movement) you are going to have a front wheel rate of 63 pounds per inch. The car weighs 230 pounds so it will take 365 pounds of spring force to support the front corner of the car. Or spring will be compressed 2.1 inches to hold the car up.

If you aren't confused now, it does get worse when you try to analyze the effect of preloading on weight transfer to all four wheels before and after you hit the droop limit at one corner. Add to this the fact that the demands on you shock damping changes as you preload the springs. You need to control the movement of the spring as it approaches the upper limits (the droop limiting point) otherwise the front tires will be lifted off the ground as the chassis rebounds from a bump and the suspension hits the upper limit of the shocks.

Now you know why I don't use front droop limiting.

I ordered an adjustable front bar. Hope I can get it in for an event this Sat.
Mark

I never thought droop limiting or preload made any sense on cars at this level, just another variable that is very difficult to control and almost impossible to model. It also introduces a discontinuity in the roll rate when you hit the droop stop.

Preload often makes a car handle differently because you're moving to a different motion ratio (with rising rate rockers) not because of the preload itself. It effectively becomes a way to changing the wheel rate and heave ratio.

I know this is the autocross forum, but it gets worse for cars that need to handle in both low and high speed corners. An FF car that usually has net lift at speed will have a different ride height (or be at the droop limit) in a high speed corner. An FC car with downforce will have enough spring compression from aero load that the suspension position with respect to the droop limit will be completely different in high speed than low speed corners.

I'm not smart enough to design a suspension with droop limiting that works well in both low and high speed corners.

Bump limiting is a complete different ball of wax. Especially if it's a third (heave) spring as opposed to individual bump limits.

Nathan

Got adj front bar installed, required some welding. Got to try it out yesterday along with a change of rear camber from -3/4 to -1.25. It did make a difference; the car is "agile" now.:D And no rear wheel spin, ya! It may take some getting use to, less push and the rear feels like it is just waiting to come around(hopefully to help out). I now have a sports race car. Another event coming up this wkend.
Mark