Unhealthy Understeer [Archive] - SaabCentral Forums

: Unhealthy Understeer


ChrisW
05-03-04, 07:47 AM
Help needed for my '93 2.0 16v Classic (3door)-or maybe for me?! The car has done 250k miles,has an 85l autogas tank in boot(mounted across car behind rear seats) and has had uprated rear springs fitted-I don't know the spring rating as all this was done by the previous owner.Whether it is my driving style or wear and tear but I find the car understeers excessively but the problem is less noticeable when driven with 'gusto'! What can be done to improve matters without spending a fortune on a car of this age and mileage?(It's overall condition is very good,so I feel it is worth trying to cure the problem)

Johann F
05-03-04, 08:34 AM
Improve the grip at the front - tyres, shocks and springs all affect this.

Willy Wonka
05-03-04, 06:15 PM
Since you asked how to reduce understeer rather than create oversteer, one assumes that a neutral car would be nice. To create oversteer usually one lifts the back end. To ballance it at optimal road cohesion a neutral drift should be possible at brisk driving as a test. So if the castor and camber of the front are correct and toe in/out is in line and the height of the front not too different from the back then the options are to raise the back.

I personally prefer ( and use) as close to 0 camber as possible but one could decrease it at the cost of tire wear. Lower the castor as well. This usually creates lively conversations with the front end guy. I assume all responsiblity and he tunes it to my specs.

I've got some home brew stuff on how to tell if it's time to see the front end guy but they're unneeded here.

GPT27
06-03-04, 07:02 PM
Having recently attended a skid-pan session at Goodwood (which was awesome) the only modifications to the 2 cars we used were tire pressures, one was set up to understeer hugely and one to oversteer ridiculously, by the end I could hold the oversteer car in a perfect powerslide right the way round each end of a figure of 8 circuit.

Anyway the point I'm making is that it might be worth playing with the pressures a bit to see what happens.

(The oversteer car was running 75psi in the rear tires, maybe not reccomended for road use)

Johann F
08-03-04, 05:38 AM
The oversteer car was running 75psi in the rear tires, maybe not recomended for road use - 1 inch contact patch on each tyre :o FWD cars are designed to have a bit of understeer because the method of pulling in a breakaway rear end is not something the average driver is able to cope with. A breakaway front end - understeer - is corrected by instinctivly lifting off the throttle - do this with an oversteering car and you are in big trouble. My C900 is very neutral - no front ARB - but I'm always very carefull when pushing it round bends at speed and in the wet I'm super cautious at the moment because my front tyres are much better than my rears. Even on a track you need quite a bit of space to correct a rear end trying to overtake you - not something you have on a road. I've only ever lost the rear in the snow and by he time it started to go it's was near impossible to bring it back - thankfully this was on a big open area of tarmac - not the road. On the other hand a badly understeering car is not very predictable or enjoyable - you have to make bigger steering inputs to correct the drift and as these are when you are putting power down (changing a wheels direction at the same time as delivering power through it is an sure way to loose grip)

Alex
08-03-04, 06:29 AM
The fact that you have a large LPG tank in the boot won't help. Having this weight at the rear of the car (its probably just outside the wheelbase) will increase understeer, until the rear breaks away which will then happen suddenly.

First step would be to get the geometry checked out (make sure you check castor and camber as well) before starting to look at other problems.

ChrisW
08-03-04, 12:37 PM
Thanks for the replies.Looking at comments about Abbott uprated springs,elsewhere on the site,and that my car definately sits rear low,it stikes me that some means of raising rear ride height(any suggestions as to where to obtain a set of these elusive rubber spacers?)and checking castor and camber +/- playing with tire pressures is the direction I should be heading in- thanks to all.
On a completely different subject:-can anybody tell me how long the radiator cooling fans should run for because mine only seem to run for about 10 seconds at a time,even in slow moving traffic.Even when the engine is stopped hot the fans still only run for about ten seconds.This on going hot running problem is getting expensive!I have replaced the spurious themostate with a Saab one-a big improvement,replaced the water pump and I am now awaiting a replacement radiator.I just have a horrid feeling that that might not be the end of it!!
Essay finished-although I could go on...

Johann F
08-03-04, 01:04 PM
Rear sag is common on old Saab's Abbor springs may not be the best option unless combined with better shocks. The best solution is to get some Heavy duty load springs from German and Swedish or Eurocarparts - these are stiffer and progressive - just what you need for the extra rear weight.

Jezzadee
08-03-04, 02:36 PM
On a completely different subject:-can anybody tell me how long the radiator cooling fans should run for because mine only seem to run for about 10 seconds at a time,even in slow moving traffic.Even when the engine is stopped hot the fans still only run for about ten seconds.

There is a timer relay that runs the fan for up to 10 minutes. If it goes wrong it would mean the radiator thermo sensor would not function accurately. My fan only kicks in at 2/3 on the temp gauge or thereabouts. When I had a different thermostat in it came in at 1/2, and sometimes would repeatedly only run for a few seconds to get the temperature down to below it's activation level. How hot is the engine getting when this is happening? At this time of year anyway it might not have to run for any length of time.

ChrisW
10-03-04, 09:08 AM
Thanks Johann and Jezzadee.
I will inquire about the springs,the slighly maddening thing is that the previous owner had 'uprated' springs and gas shockers fitted inorder to cope with extra weight of LPG tank. Unfortunately I'm unable to find out what they are.Maybe a bit more research is needed so as to avoid buying the same things again!
As regards the fans;They come on at about 2/3rds temp which generally only seems to occur in slow moving traffic.Where does the timer relay live-is the'fan relay'in the main fuse box or does it hide else where?
Have now fitted replacement radiator so hopefully temp will remain more constant now-looking at the state of the old one things can only be better...!

Eric van Spelde
15-03-04, 05:15 AM
Stupid thing is - if the rear spings and shocks are uprated and the fronts aren't the balance of the car should have shifted to oversteer! Same with the extra weight of the LPG tank in the back.

Something is definitely awry here...

Johann F
15-03-04, 10:23 AM
If the rear has been uprated would not this lead to understeer? - better shocks and springs = better grip therefore less drift outwards at the rear. Worn springs and shocks at the front = less grip therefore more drift out in a turn i.e understeer. Or am I missing something?

Eric van Spelde
15-03-04, 10:38 AM
No, not if the suspension components are functioning properly. If you increase the stiffness on one end versus the other, the car will lose grip on the 'stiff' end sooner than on the 'soft' end. If you want to improve RWD traction, you make the rear end soft. If you want to improve FWD traction, you remove the front sway bar ;) .
FWD racers are generally set up with a very stiff rear and to make the back end 'adjustable', i.e. you can provoke a degree of oversteer by lifting the throttle to aid turn in and to make the back end help the front not to kiss the outer armco barrier in an emergency understeer situation (i.e. driver's error of judgement leading to too high an entry speed for a given corner).

As for the weight balance of the car, the balance is simply where the weight is. An Audi with the engine entirely in front of the front axle line will have a nose heavy balance, no matter which and how many wheels are driven. A 911 on the other hand, well, you get the picture. Of course, you can cancel out the effect by applying rather too much motive force through the wheels of the opposite end...
The more of the weight is concentrated close to a car's center of gravity, the harder it is to spin. A car with a big fat engine in front of the front axle line and a big fat LPG gas tank behind the rear axle line may have a quite neutral balance, but it takes less lateral force to spin the car than if it were to have both items inside the wheelbase.

Johann F
15-03-04, 10:43 AM
That makes sense but if the shocks are worn they won't let the wheels track as well and this would loose traction on anything other than the smoothest road surface.

Eric van Spelde
15-03-04, 10:48 AM
That's why I said something's awry. Could well be worn front suspension components, although I would check alignment and tyre pressures front/rear first. Could also be an odd mix of tyres (pattern/compound as well as sizes) or a combination of factors...

Alex
15-03-04, 02:05 PM
A big fat LPG tank outside of the wheel base of the car will tend to unweight the front wheels though, increasing understeer.

On cars like the Hillman Imp with a rear engine and fuel tank over the front wheels, the amount of understeer increased as the fuel tank emptied, and the weight balanced shifted rearwards. However, once the rear tyres have let go, the back of the car would come round very quickly.

ChrisW
16-03-04, 03:58 AM
Seems like I've stimulated an interesting discussion!
Temperature 1st;-The temp gauge runs at 1/2 until you get into slow traffic,it then creeps upto about 2/3 to3/4 at which point the fans cut in(air-con and rad fan run together),but they only run for about 10sec a time and if you turn the engine off they still only run for about 10sec. I have checked the rely pt nos and my car has a none delay relay no8522310.It appears that the delay relay pt no 9563339 is now dicontinued but Hella have replaced it with pt no 9558073.At the mo I'm trying to find one of these-I'm told it costs 25-just need to find a local Hella stockist.

On the steering issue it seems like I really do need to find out what the previous owner did(I think he just had uprated springs and gas shocks fitted to the rear but nothing done to the front),get the camber checked and take it from there. The tyres are Pirelli 6000's and are not old/worn.
I have played about with tyre pressures and have settled for 38psi in rear and 33 in front.
The lpg tank sits pretty much over the back axle.
Thanks for help and suggestions.