weight transfer via kpi

[golfpin]

thought this may be of interest, got involved with the "remanufacture" of quite a successful clubmans car, golf 1800 pwr, alfa five speed g.box engine orientation north south Toyota back axle. For sometime I had been chewing over the issue of kpi of which there is a huge amount in most Mcpherson strut set ups this leading to unwanted weight transfer on turn in and camber loss on turn [this was my opinion]. Because there was a lot of design freedom we removed the kpi altogether introduced a reasonable amount of caster, result was a very good turn in, reduced understeer and a very happy owner driver.comments

 

[BrianPetersen]

How did you manage to completely remove kingpin inclination in front view - presumably while preserving general MacPherson strut layout and operation - or if you changed something in the way a MacPherson strut system operates, what did you change?

The upper strut mount is the upper joint of the steering axis. It has to be inboard of the top of the tire. The lower joint of the steering axis is the outer ball joint of the control arm. It has to be somewhere within the tire in order to have scrub radius that is acceptable for a front-drive car ... but you're not using front drive. Did you move the lower ball joint substantially inboard, thus operating with substantial positive scrub radius? How did that work out? What does the steering do if you are braking on uneven surfaces?

Kingpin inclination doesn't directly lead to weight transfer ... cornering G-force, center of gravity height, track width lead to side-to-side weight transfer, and this is essentially irrelevant of the steering geometry. Steering feel is sure affected, though, and it does have an effect on the camber, but it's only significant during low speed cornering. The steering angles during high speed cornering - unless you are going sideways - aren't enough for this to be significant.

There have been a couple of variations of MacPherson strut lately that separate the steering axis from the upper strut mount - GM Revo strut, and Ford has a version also, but I've forgotten what they call it. Strut stays stationary when the steering turns and there's another set of pivots for the steering so that the steering axis can be more vertical while still preserving near zero or slightly negative scrub radius, which is necessary on a front-drive. Given that the applications of those struts are on higher performance vehicles, your objective of getting less kingpin inclination is probably in the right direction ...

 

[golfpin]

Brian thank you for the reply and the interest shown, I have to be so careful making bland statements to a forum of this calibre. To my simplistic mind the strut was mounted in a vice, the vertical was established, a wedge cut was made in the tube and the stub axle was bent to rt angles to the vertical, cut welded, reinforced and that was it. Seems to have worked car very competitve, now sold on sadly.

I cannot really add to this discussion at your level I am only a mechanic so I do appreciate your input and others who have bothered to reply, because I learn all the time. .Did not know of the Revo layout interesting, tried to keep the scrub radius as close to zero as possible but forgot to clarify that this car was for track use only.

Did you ever see adaptation made by Peugeot on their top of range rally car; they interposed a wishbone [upper] onto the suspension that then freed off the Mc srtrut to act as shock only, very clever I thought and within the rules of course.

Your paragraph 3 has given me a lot to think about. Could I ask another question at this point... when having an understeer problem, frt engine rwd layout, why does adding ridiculous amounts[to me 8mm] of toe-out seem to help with understeer this seems to tie in somewhere with the amount of ackerman angle used, this comment supported by Carrol Smith in his book Engineer to win? Are there slip angles at play here that is not too clear to me. Still use a drawing board and chalk on the wall/floor!!
WITH THANKS
DENZIL Golfpin RSA

 

[JayMaechtlen]

Hi, Golfpin
Do you have access to any photos of the setup?


Jay Maechtlen

http://www.laserpubs.com/techcomm

 

[golfpin]

Hullo Jay'
the short answer is no, however what is there that i can clarify? I am too computer and IT illiterate to show drwgs or photos of which I do not have anyway. the car in question has been sold and I believe seriously "hacked about" the original owner/ builder is a very good friend but lives about 1000 kms away [I moved to the coast] So my input is now via pc and phone The alteration was simple. The car was a space framed golf 1 as I said in the post; I have always been irritated by the fact that when using a Mc P. as in the golf, one was caught up with the kpi [which in my opinion leads to unwanted weight transfer] that is built into MCpher concept. because our clubmans rules allowed, it was simply mounted, the strut,vertically in a vice, put an eye to the droop that the spindle showed, and cut a wedge out of the outer Mcpher/shock tube. Bent the axlt stub/spindle up till it was at rt angles to the tube closed with a weld and a gusset to add braces to belt and assembled. because the top of the Mc joint was "free" we added about 4 degrees of caster by placing the pivot point to the rear and that was it.
Had more money been available for track time we feel that a few spring changes would have been beneficial. A small addendum that was incorporated a very simple but very effective rear wheel adjustment; the diff was locked, could not afford a L/S so added limiting straps that were adjustable from the drivers seat so that when the car rolled in the corner the outside wheel would lift, like the go carts, all the drive then coming from the inner wheel this helped hugely with understeer. Difficult to say how quick the car was because we have no comparative factors track wise but let me try, here in RSA we ran a series quite closely related to the American IMSA v8 series this golf put up times that would have put it at about 16th in a 22 car grid here at track called swartkop. Hope I haven,t overstepped the mark with regard to the site if I have I apologize and please feel to contact me direct on schultzp@telkomsa.net if you wish to or anyone else on this fantastic forum.
Cheers Golfpin

 

[golfpin]

sorry in the aforestated thread, should read inside wheel not outside
and then all drive coming from outerside wheel.
thks golfpin