924RACR

100% ditto on everything fatbillybob says; just adding a more direct correlation for you. I run about 4.5 deg caster in my 924, always been quite happy with feel and all in my tail-heavy (2%) car. Sure, it's heavy in the paddock - but I prefer to race on the track, at speed. When I'm going at it with the rest of the crowd out there - the steering effort is just one on a long list of things that I ignore.

Larry Herman

Once again John, to your technical information...clear & concise.

Bob Rouleau

Here's another perspective on what the Larry and John show has demonstrated. Increased caster = increased self centering action i.e. the car wants to go straight - excellent for high speed stability. As L and J have explained, the reason the car is more stable is that the tire is being twisted. That 'twist' which creates the self centering force is scrubbing the tire. How much? That's what John showed in his scrub radius post. So, more caster = more self centering = more scrub. A GT3 calls for about 8 degrees of caster. At 9, the tire rubs on the fender liner- at least mine does.

DGaunt

Just one small point, Larry--with great respect!

King Pin Inclination and Caster are not the same thing. KPI is the inward or outward tilt of the steering axis when viewed from the front. Caster is the forward or rearward tilt of the steering axis when viewed from the side.

Another downside of excess caster is toe change. Caster changes camber (as already mentioned). Any time you change camber, you change toe, hence tire wear can be a problem. This is why toe is always set last, every other adjustment you make will affect toe to some extent.

Scrub radius is the distance between the centre of the tire and the place where an imaginary line drawn through the steering axis would meet the ground. Typically in stock steups, this line extended outside of the centre of the tire. This makes the steering more stable during bumps as there is a smaller moment on the steering. Increasing the offset moves this line to the inside of the tire centre making handling more responsive and turn in crisper.

RedlineMan

And...

King pin Inclination is more universally refered to as Steering Axis Inclination. SAI is the lateral angle formed by a line drawn through the upper and lower ball joints (McPherson strut through the strut bearing and ball joint). It is yet another way engineers have discovered to increase stability at speed. It also has the concurrent effect of reducing steering effort by reducing scrub radius.

Larry Herman

Thanks Dave, I appreciate the correction.

Thanks also, John.

superloaf

Does anyone know the maximum caster which can be set on a stock '86 944/951? I know 2.5 to 3.0 is what it calls for--I'm just wondering the max if both eccentrics are set to max positive. Also, where would the adjusting tab on the eccentric be positioned for this max?

Just curious since I recently converted my951 to manual steering and thought some of the hard steering effort was due to caster being too much but when I measured it I came up with +5 or so. Just want to know if that's possible or if my measuring was off?

Thanks--

BrandonH

And incidentally, this is why caster is actually the most important suspension setting for karts. Since they have no differential, you want to unweight the inside rear wheel in a turn, preferably in a controlled but decisive way as you turn in. Lot's of caster achieves this: the inside front tire pushes down, the outside front lifts up, and voila the inside rear lifts and the kart will turn in.

Interesting discussion. I've speculated that your comment must be true also for a car chassis, but don't recall ever seeing it discussed by the Smiths and Van Valkenburghs etc.

On my son's small cadet chassis kart, the rear tire will be an inch off the pavement at a rest when the wheel is at full lock...

DGaunt

John, you are right. KPI was the original term used when vehicles had solid axles and actual kingpins. When I was trained in the late 80's KPI and SAI was pretty much used interchagably in SLR (short-long arm) suspensions. On Mcpherson struts, SAI is the preferred term and as SAI functionally serves a similar role to caster (like, corner jacking and camber change in turns) and is a necessary part of Mcpherson packaging, caster has been reduced in strut cars, giving over some of its duties to SAI.

Time to shut up.

Larry Herman

Thanks Dave, that's even more helpful information. I just ran across an excellent article written by Roger Jackman on Scrub Radius on the H&R website. Click here to read. I thought that it explains even more than we discussed, and is worth reading.