Understeer, is that a common problem with these cars?

[chetcpo]

It is on mine, and on another similar homebuilt I drove this past weekend. What do most of these cars do when pushed to the limit on street radials?

[yellowss7]

Guess it depends on how it's set up.

 

EvilRome0's car seemed to be prone to oversteer at some of the first autox's he attended.

 

On street tires I tended to use too much steering input and caused understeering. With the same set up on slicks, the car handles really neutral.

 

Questions, what size front and rear antiroll bars are you using?

 

What is the front to back Rake?

 

Have you changed spring rates? I believe these can all have and impact on your handling.

 

Tom

 

[chetcpo]

Guess it depends on how it's set up.

 

EvilRome0's car seemed to be prone to oversteer at some of the first autox's he attended.

 

On street tires I tended to use too much steering input and caused understeering. With the same set up on slicks, the car handles really neutral.

 

Questions, what size front and rear antiroll bars are you using?

 

What is the front to back Rake?

 

Have you changed spring rates? I believe these can all have and impact on your handling.

 

Tom

I designed and built it myself so I realize it won't be the same as a production based car. I did however carefully design the suspension utilizing computerized suspension simulators/calculators to establish the geometry. The spring rates are 450 front and 300 rear, which works out to wheel rates around 190# front and 230# rear which is really close to 50% of the unsprung weight of the relative corners. I don't have swaybars on the car right now. I wanted to get it as close as I could to being "balanced" and then add swaybars as needed to correct any lasting weight transfer problems rather than make a guess.

 

The car has a Miata 1.8 engine and tranny. (well most everything on it came from a Miata)

[yellowss7]

Chet, just curious, what does your car weigh?

 

For comparison, mine weighs 1164 pounds, and the front springs are 150#ers. I have a set of 200# ones that I haven't had the need to use yet.

 

I'm not an engineer, and I just assembled my Caterham using the parts supplied so I'm not making any assumptions, I'm just giving you a point of comparison.

 

Tom

[WestTexasS2K]

Be careful comparing spring rates. Rate needed will vary widley from car to car depending on shock angle, shock location, Control arm lengths etc.

 

If you car is pushing it can be a few things. Tire pressure can greatly affect understeer. If I run my car at 21 psi on the front it understeers badly, If I drop it to 19lbs it drives like a different car. So play with tire pressures.

 

Does the car understeer as you accelerate thru a corner? If so you might need to increase rear spring rate. You could be getting to much transfer to the rear which unloads the front.

[chetcpo]

Chet, just curious, what does your car weigh?

 

For comparison, mine weighs 1164 pounds, and the front springs are 150#ers. I have a set of 200# ones that I haven't had the need to use yet.

 

I'm not an engineer, and I just assembled my Caterham using the parts supplied so I'm not making any assumptions, I'm just giving you a point of comparison.

 

Tom

 

With me in it probably around 1400lbs. I realize the front rates are a bit high, but so are the rears. I did this to control body roll without swaybars. I figured since the front to rear wheel rates were high, yet still in equal proportion to the cornerweights, it would be somewhat balanced.

 

 

[pi7ot]

Here are some things to check:

1, What are the camber specs for each corner?

 

You may have too little or excessive front camber neg.

 

 

2, What tyre pressures are in setting?

 

You may need to test 'n' fiddle with tyre pressures. If you carry ballast in the passenger seat, you'll have to either raise the rear tyre pressures or lower the fronts by 2/3 psi once you have discovered the optimum tyre pressures for driver only operation.

 

3, What kind of tyres do you use?

 

I have recently removed the Rcompound rubber I had been using on the car and put some narrower cheapo Firestones on the car. Now it understeers badly, without alignment or any other change.

 

4,What degree of rake is the suspension set at?

 

The differential between front and rear right height affects the balance of grip between front and rear.

 

m

[shiva]

Here are some great books that might help you:

 

How to Make Your Car Handle

 

and a more recent (up to date) one:

 

High-Performance Handling Handbook

 

Those two were highly recommended to me.

[Earley Motorsports]

Chet. Mine weighs 1320 wet in street trim. I have 200lb springs in the front with a sway bar. I used to have 180s but they were too light for our rough roads. Mine used to understeer a little when pushed so I lifted it up in the back about an inch and that made it a lot better. Now it only understeers when I get in the marbles (with slicks).

Graham.

[JohnK]

... I did however carefully design the suspension utilizing computerized suspension simulators/calculators to establish the geometry. ...

 

chetcpo,

What software did you use and how'd you like working with it?