Understeering Datsun 260Z

[260DET]

This is a registered car but its mainly used for sprints at Queensland Raceway. R tyres are used, the anti roll bars are around standard (22F 18R), - 3.5 camber, +5 caster, 2 mm toe in on the front. Coil over springs are 275 all round. The rear grips really well with great drive out of the corners and the car is generally nice to handle.

However the front is reluctant to turn in and understeers quite badly mid corner. The front tyres are showing excessive wear towards the outside of the tyre but not on the outside edge. I'm getting double adjustable shocks for the front in the belief that more bump is required. Increasing bump on the rear improved grip there significantly. The car will also be lowered a bit more on the front to get some rake.

My trail braking ability is not the best, the front feels light when turning, it feels like it needs more weight on it. The car does not flip flop very well either. The rear bar size was changed from a standard 20 to an 18 which made it worse if anything last time although I may have been pushing the car harder.

Comments and suggestions?

[260DET]

Is this 'forum' just a place to flog Whiteline gear?

[Jakub]

Hello Richard,

We welcome all technical issues in this forum, some people specifcally ask questions about our products, and products available for their cars, we can answer these relatively easily.

I apologise greatly for the delay in our response, we will be working to reduce this in the future.

From what you have explained, It really seems that during pure cornering the general nature of the vehicle is to understeer. With some kind of anti-squat effect happening under power, which effectively stiffens the rear end and balances the handling back to nuetral. Also another valid reason is for the rear end giving up a little cornering capacity for the traction out of the corner - leveling out the balance again, this unfortunatly is physics, you cant corner 100% and accelerate 100% at the same time. This effect was also increased with the replacement of the rear bar from a 20mm to an 18mm unit.

Ideally the vehicle, through a corner, should start in the understeer region at entry and then slowly shift to the oversteeer region upon corner exit, due to the traction forces during braking and accelerating.

I would be aiming to get the mid corner, zero brake and zero throttle application to be balanced, so a larger rear bar would be called for, alternatively a softer front bar - especially if the track is considerably uneven and rough. Or more balanced for which ever part your car is better at (braking or accelerating).
You mentioned the car does not flip flop well, as in quick left right corners, is this because its slow to react? then the stiffer rear bar would be the go to increase the overall stiffness. This would also be the case to minimise the outside tyre wear.

One thing to remember when inducing some rake by lowering the front, there will be some castor loss, id say roughly 1/4 degree, per 10mm drop.

Dampers for racing situations require much higher damping than road, both due to higher stiffness and also damping ratios are higher - comfort gets a back seat here. Id say anywhere between 50% and 100% increase in damping values compared to road. Im not exacly sure how much, as im not too familiar with stock values for the Datsun.

If theres anything I could go into further, or you would like to ask anything else, please dont hesistate.

Hope this helps in some way.

Regards
Jakub Zawada B.E. (Mech)
jakub@whiteline.com.au

[260DET]

Thanks Jakub, the funny thing is that these cars often run softer rear bars than mine does which is why I'm perplexed. I understand what you are saying, basically put more load on the rear tyres. So the stock 20mm bar will go back on to replace the 18.

To explain further, there are two main aspects of the problem. One, it is relatively reluctant to turn in, two, it understeers badly mid corner. This shows up through flip flops with the car lacking agility and responsiveness in quick changes of direction.

Then in fast corners if the throttle is backed off slightly or held steady the car will understeer. I have to keep accelerating slightly to keep it on the track, which can be a bit scarey. In this situation you can feel the front fighting for grip.

Anyway, I'm waiting for the double adjustable front shocks (strut inserts) to arrive, this car does seem to be very responsive to more compression.

[Jakub]

Hi Richard,

In terms of handling, it can be achieved in a number of ways, perhaps others are using softer rear bars, but stiffer rear springs to compensate? As long as your not getting any wheel lifting you can keep adjusting your handling characteristic.

It does look predominantly understeering the nature. Turn in understeer can also be attacked with softer front damping or harder rear damping, as in a perfect corner dampers will only effect your turn in and exit. As said before I would definatley go with a stiffer rear bar as well.

Another thing you can do to improve your turning ability, is to reduce the cars yaw inertia. Involves bundling any parts of the car as close as possible to the mass center. For example moving the battery from the engine bay to say behind the drivers/passangers seat (will need to be in a cover then).

Regards
Jakub