Advice on curing understeer

[PhillipM]

An autograsser is probably setup to throw almost 100% of it's weight over the back wheels on the throttle, with a massive rear weight bias, can't steer if the fronts aren't on the floor

[kosingcurry]

Thanks for the comments so far, useful but somewhat varied but maybe the following info will help.

I could do with some decisive info.

Front track = 67inch

Rear track = 67inch

Wheelbase = 75inch

Static toe rear = -1 deg

Static camber = -1 deg

Static toe front = + 2 deg

Static camber front = -3 deg

Front tyre = 13inch 200 soft slick

Rear tyre = 13inch 200 med slick

Diff is a quafe lsd. Dont know much about it but it doesnt seem to be set too stiff i would say.

[kosingcurry]

The weight distribution i dont know, however it is nost definitely very bias towards the rear because the engine is mounted behind the rear wheels and there is just no weight at the front.

i ran 500lb springs on the rear this weekend but the rear was still soft and lofting the front under acceleration.

So do i add stiffer springs, or ballast the front, or both?

what about rear anti roll bar? loose it, stiffen it, put one on the front?

[kosingcurry]

http://i428.photobucket.com/albums/qq8/ ... 1429649042

http://i428.photobucket.com/albums/qq8/ ... 1429649047

http://i428.photobucket.com/albums/qq8/ ... 1429649047

[WilO]

One piece of info that I don't see is the motion ratios of the front and rear suspensions. Shouldn't be too difficult to measure. I think knowledge of this, and the weight distribution, might help guide you. Maybe a friend or fellow racer has some scales you could put the car on?
Sounds like the car is pitching quite a bit.....

[Tim.Wright]

Holy toe settings batman, how did you arrive at those numbers?

[garygph]

I should have said it before..buy and read Carol Smith's " Tune to win" or " "prepare to Win" and " Drive To Win". It will save you a lot of money and heartache. Yes I have included the one about driving as that has an effect on the cars handling as well

I have been to and watched a few Autograss races and have know that the suspension is very compliant for the rough surfaces but way too soft all around for asphalt so there is one starting point. What I would strongly suggest is get to know yourr car the following way which I believe suites a weekend racer who does not have a fancy workshop and engineeers for his car. Take the springs off and reconnect the shocks. Then with jacks and spacers position the chasssis into the angles it would be in braking , accelerating and cornering. Whilst it is stagnant in those positions remeasure your toe,camber etc. You may get a shock at the camber and toe changes that are taking place and help give you direction in what static settings you should use. By stiffening the car up you will automatically limit thos problems as well.

But read those books!

ps. those tyre on the video look stone cold to me and a stone cold slick is pretty useless

[Uniracer]

From watching your video, it looks like there is some issue with the steering geometry. It looks to me like it may have negative (anti) ackerman geometry. When you add steering angle, it generates toe-in. This promotes understeer as the outside wheel gets overloaded and starts to slide.

[Facts Only]

Autograss cars are designed to go round corners like this:

http://www.autograss.net/AutograssRevie ... -J1064.gif

So anti-Ackerman may be the desired steering geometry, certainly it won't have been designed to go arond corners with the back end following the front without sliding as you are now using it.

Have you got any good pictures of the front upright?

[garygph]

Have a look at the car in the backround of your pic Factsonly Now that is some serious understeer as well. There is so much droop under braking I wonder if the front shocks are not bottoming out and therefore instant massive roll and bump stiffness. I have always wanted to do a third shock/coilover on one of these cars as I cant help but think it would help enormously.

[kosingcurry]

How can i go about tackling the raised ackerman issue? Also given your comments about cambet and toe, what would you suggest?

Just weighed it front and rear and currently its 60% rear 30%front.

Ballast?

[Tim.Wright]

I think the main problem is that its the wrong car to race on tarmac. How much you can fix the handling will depend on how "forgiving" the suspension was designed. I've seen some off-road cars with very questionable kinematics which perhaps don't matter off road but once you put on "laterally stiff" road slicks and run on asphalt they ruin your handling.

I think fitting much stiffer springs could help patch over these problems. Also, stiffer springs will let you run the chassis lower to the ground which will reduce the CG height and that's generally better for grip.

The toe settings are huge. A road racing slick might have its peak force at 4-6deg of slip and you are effectively "preloading" the tyres to run at 20-30% of this value just from the static toe settings. Generally speaking you want toe out on the front and toe in on the rear - so I'd at least start there as a baseline. Try maybe 2mm out on the front and 2mm in on the rear. Play with the front toe, in both directions to see if this helps. This will "approximate" different ackermann settings.

Longer term you might want to think about reducing the size of the front tyres so that the tyre size distribution is closer to the mass distribution. This will help even out the warm up time for the front and rear. One of your problems now might be that the rears warm up quick due to their large weight and traction forces and the fronts lag behind so the balance is understeer due to lack of front tyre temp. Try a test with ballast in the front (but fix the static toe first!) and see if this helps. If yes then smaller tyres could be the answer. Ballast should only be a temporary measure just for testing.

Regarding the springs its quite difficult to say much without knowing a lot more detailed info about the suspension geometry but I think the first things to sort out are the tyres, mass (ballast) and the toe settings.

What is the total mass by the way?

[Uniracer]

How can i go about tackling the raised ackerman issue? Also given your comments about cambet and toe, what would you suggest?

I haven't driven this style of go-kart like vehicle, so I am guessing. Maybe to help figure out what's going on, you should set up zero toe in the front. 2.0 degree toe-in is far too much. I'd have thought that 0.1 or 0.2 degree per side would plenty if you wan to add stability. For the rear end maybe around 0.2 degree toe per side would be a starting point.

To avoid the anti-ackerman effect, you need to find some way to make the steering tie rods longer. That normally means re-designing the front uprights. Also the position of the inboard steering rack will affect the amount of ackerman. If you move the steering rack further back you can reduce the amount of anti-ackerman.

I am not really a car designer, but I believe positive ackerman can be useful to avoid understeer push in low speed corners. Adding front toe-out will also have a similar effect in low speed corners, but I would suggest to only use about 0.1 or 0.2 degree per side maximum.

[garygph]

How can i go about tackling the raised ackerman issue? Also given your comments about cambet and toe, what would you suggest?

Just weighed it front and rear and currently its 60% rear 30%front.

Ballast?

Ok I am confused..were has the other 10% gone?

[George-Jung]

How can i go about tackling the raised ackerman issue? Also given your comments about cambet and toe, what would you suggest?

Just weighed it front and rear and currently its 60% rear 30%front.

Ballast?

Ok I am confused..were has the other 10% gone?

Easy in the middle...