Getting power down out of corners.

[johnc]

Adding caster does help with rear traction in a turn. Due to the jacking effect on the front you transfer more load to the inside rear tire. A friend who autocrossed a car very similar to yours ran 8 degrees of positive caster in his 240Z and put 1/2" wheel spacers on the front. Both of those changes helped a bunch with rear traction, although his steering effort and scrub was huge. I wouldn't recommend those nubmers for a track car but often in the autocross world we need to do strange things to get quicker runs.

 

I also agree with the rear toe comments above.

[burninator]

Thanks for explaining that. I never would have guessed the jacking effect from lots of camber was enough to affect rear traction, but the spacers probly magnify that. It's definitely true that autocrossers do some interesting stuff sometimes that wouldn't work well on a track or on the street.

[Drax240z]

Caster, not camber.

[burninator]

Caster, not camber.

Right, typo.

[JMortensen]

Caster jacks the front end up by changing camber. Just sayin...

[JMortensen]

This was itching the back of my brain. Seems to me in front that the outside tire gets leaned over farther making the corner to corner height taller which would load up the outside front and take weight OFF of the inside rear. The inside front stands up straighter and hence lowers, adding weight to the outside rear. So caster in and of itself should make inside tire spin worse. Searched "caster weight transfer diagonal racing" on Google and came up with these:

 

From: http://fsae.com/eve/forums/a/tpc/f/125607348/m/551104293

Running a car with lots of Caster will result in significant diagonal weight transfer when turning into a corner or at any time steering lock is applied. On turn in, this diagonal weight jacking will unload the inside rear wheel so a team running without a locking differential of some sort may suffer traction difficulties.

 

Seems to prove my point.

 

From: http://racingarticles.com/article_racing-24.html

 

Scrub radius (also called 'kingpin offset') and caster angle work together to produce a diagonal mechanical weight transfer from the inside rear tyre and the outside front tyre, to the outside rear tyre and the inside front tyre. This weight transfer causes the inside rear tyre to be physically lifted from the track surface at turn in. If this weight transfer is not great enough, the combined grip of the rear tyres can simply push the front wheels straight ahead.

 

This one doesn't make sense. I had to read it about 4 times and then draw a diagram, but he basically says that the inside front and outside rear both gain weight and the inside rear and outside front lose weight. I think he meant to say "produce a diagonal mechanical weight transfer from the inside rear tyre TO the outside front tyre, to the outside rear tyre and FROM the inside front tyre". This makes more sense to me. I do remember once reading somewhere about a guy who had his car on scales and turned the wheel and documented the weight change, and it wasn't good for the inside rear tire as I recall.

 

Anyway, adding caster was one of the best things I did to get grip out of the front end of my Z, but it isn't going to help that inside tire spin.

[johnc]

This makes more sense to me. I do remember once reading somewhere about a guy who had his car on scales and turned the wheel and documented the weight change, and it wasn't good for the inside rear tire as I recall.

 

Well... I just had my 350Z on the scales and it comes with +9 front caster. Turning the wheel to the right saw a increase in total load on the LF and RR of .5% of that cross weight (80.9 lbs). Turning the wheel to the left saw a increase in total load on the RF and LR of .5% of that cross weight (79.9 lbs.).

 

Remember the fundamentals - if you raise any corner of the car the weight on that corner and its diagonal corner increase and the weight on the other two corners decrease. Think of a four leg chair.

[jt1]

If you're going into a left hand corner, and the steering wheel input is to the left, the caster unloads the RF and LR. If the car gets loose, and the wheel input is to the right, it unloads the LF and RR, which generally helps the loose condition. So it depends on the attitude of the car and the steering input.

 

Edit: Assuming you have a positive scrub radius.

 

Edit#2: I bet John's 350Z has a negative SR, while S30's have a positive.

 

jt

Edited July 14, 2010 by jt1

[burninator]

This is getting beyond my knowledge, but I've been doing some research and a lot of thinking. It seems like with given caster (more caster exaggerates the effect) a negative scrub radius will cause more jacking on the outside front tire (increasing weight on the rear inside). Where a positive scrub radius will cause more jacking on the inside front tire (decreasing weight on the inside rear). I think that's what jt was getting at.

 

JMortensen pointed out an article on kart racing which is interesting because kart racers have positive scrub radii and they use caster weight jacking to try and lift the inside rear tire.

 

I don't know how this relates to JohnC's example of a friend with the wheel spacers, because the increased scrub from the spacers combined with the large caster should decrease the weight on the rear inside. Maybe there is more to that story.

Edited July 15, 2010 by burninator

[jt1]

A key point is that the effect of the scrub radius changes depending on which way the steering wheel is turned.

 

jt

[johnc]

Ahhhhh... (the lightbulb comes on). Forgot about scrub radius. It would be needed (in a negative way) to make positive caster work as I described. I need to call someone smarter then me.

 

EDIT: Scrub radius has an effect but not as much as I thought. The Go kart example is an extreme because it has no suspension and has to go on three wheels to turn.