3 replies to this topic

[Philip Whiteman]

First; credit where it's due - Doug Nye and David Weguelin's latest is marvelous. Doug: I especially enjoyed your film show-style voice over on the 1956 British Grand Prix 'rushes'. My late father was there, and always described Mike Hawthorn in the P25 BRM as looking like 'a man in a bath tub' - now I see quite what he meant!

To my question: part 1 of MFQ Ten show the design and building of the fascinating Cummins diesel Indianapolis car. This was said to be the first of the 'roadsters' that dominated Indy racing through to the 1960s (when first Cooper and then lotus finally made them look like dinosaurs). The Kurtis Kraft chassis of the Cummins car looks to have independent front suspension, using longitudinal torsion bars acting on the top track control arms. Later, Offy-powered machines from the same stable used a tubular dead axle at the front, as did the Watson roadsters that followed.

I have the Carousel 1 model of Rodger Ward's 1956 Kurtis, and a close look at the suspension raises as many questions as it answers: at both ends, longitudinal axle location is provided by a Watt linkage - a Z-shaped do-hickey consisting of a vertical link or member fixed rigidly the axle which is connected radius arms that go forwards to locate on the chassis at the top and backwards to meet the chassis at the bottom.

The model has one further arms on each 'corner', with a drop link to the axle. Were these the torsion bar links? (The otherwise finely detailed model seems to have no torsion bars.) Second, what effect on handling did the Watt linkage induce? I ask this, because it is plain that the axles would have to rotate about the wheel-axle axis (or twist) a the wheels rode up and down - and, at the front, this would alter the caster angle of the steering...

Anybody out there ever drive one of these Indy cars - or read an account of what their handling was like?

We all know that the wishbone set-up introduced by Cooper and Chapman was better, but that odd Watt-linkage dead axle can work well: you'll find one on the back of any old-model SAAB 900!

[Gerr]

G.E.White's Kurtis book may answer your questions. Ward's Kurtis in '56 was a KK500C or D. A diagram on page 108 shows that the front torsion bar placement is above and ahead of the axle and the rear is behind and below the axle. The same arrangement was used for years on Sprint and dirt cars. On the 500C/D, the front left radius rod attachment/bracket/axis rotates around the axle, the right side is welded solid to the axle.

I guess it would cause a caster change with weight transfer and make the car to turn-in or ease the steering effort of turning left. At an oval that would be a good thing.

[Gary C]

........................and I really enjoyed Doug's two 'commentary' pieces in MFQ 10.

[McGuire]

Actually, a Watt linkage translates the arc motion of its arms into true straightline motion at its central vertical element. So with a Watt linkage there will be no caster change through the axle's range of motion.

With paired leading or trailing arms -- the more common beam axle setup -- there is indeed caster change. (For example, on a front axle with trailing arms, caster gain in bump when the bars are parallel to ground or in ascending angle at normal ride height. The shorter the bars, the greater the gain.) That said, it's typically not a problem. And probably not why Kurtis selected the Watt linkage; more likely for packaging reasons.

With all due respect to Gordon White (which is to say the utmost) his description of the suspension on the later model Kurtis cars is incorrect, unfortunately...or at least confusing. The caption of his diagram states the front suspension location is via Watt's linkage while the rear location is via Jacob's Ladder. Actually, rear longitudinal location is by Watt's linkage as well. Lateral location at the rear is via Jacob's Ladder.

Earlier Kurtis Indy cars (Cummins, KK3000) employed independent front suspension via wishbones and torsion bars. Kurtis then sampled a number of beam axle setups using various combinations of leading and trailing arms front and rear, before essentially settling upon the Watt linkage setup front and rear for the 500C and beyond (with some variations). On the 500C's front axle, the upper front links also served as the torsion bar levers, while the lower rear links carried the torsion bars for the rear axle. (For the longest possible "spring base," in the vernacular of the era.)

I can't speak to the scale model as I have not seen it (and who knows about these things; probably safer to trust a period photograph) but the extra link may possibly be for a lever action damper. In that period either tubular or lever action dampers were fitted by individual teams...even both sometimes. Shock technology at that time was in a state of near-futility.