A-Frame Location, Anti-Squat, Anti-Lift?

[Catgraham]

Does anybody know what the claimed benefits (if any) were for the underslung A-Frame mounts, introduced, I believe, along with the revised (anti-dive) front wishbone geometry, at the mid-1996 major chassis update, and deleted again at the 2001.5 update?

I'm not sure whether all (road) chassis manufactured between these dates were fitted with the underslung mounts, as I haven't seen all that many. I don't think race chassis had them.

I would be interested to hear if anyone has first-hand experience of these vs. the "standard" mounts and noticed any significant difference. I suspect that the perceived benefits did not outweigh the disadvantages, as they didn't last for long....

Graham.

[SLR No.77]

My '99 car has the underslung mount which results in  the A-frame being longer, I believe it's the increased length that is a possible benefit rather than it being lower although it's maybe negligible. Some competition cars many years ago ran trailing arms external to the chassis sides to massively increase the radius arm length, top and bottom (live axle).

Did the reversion to the original location not occur when lowered floors became an option? The lower mounting point really messes up the lowered floor fitting and Arch modify the corner of the floor to accommodate the A-frame mounting boss, even then it makes it problematic to install a 6-point harness. I think there's a bolt-on part available to modify the chassis to fit the later type A-frame mounted in the original position.

Stu.

[Richard Price]

"I don't think race chassis had them."

My sons 2001 race chassis does have the underslung A frame. 

[Catgraham]

Thanks for your replies. I hadn't considered that it might be just the increased length that they were after. Of the two I have, the underslung A-frame is 2" longer, at 17" vs. 15", so around 13%, which is not insignificant. Interestingly, as an aside, the longer A-frame is also quite a bit lighter, at less than 1.6kg, whereas the short one is nearly 2kg!

I had assumed that the underslung mod was something to do with altering the squat characteristics, which I'm sure it will (just not sure how!), but maybe that wasn't the original intention.

I had heard that originally the underslung mounts were further back, so the A-frame length increase would be less, although there doesn't seem to be a specific A-frame available to support this. I had also heard that prior to that there was a bolt-on bracket available, but that it was to convert the standard pivots to underslung, and was used by racers.

I agree that it compromises the installation of the lowered floors. Arch box-out the floor to clear the A-frame bush and bolt, leaving a rectangular projection in the outside rear corner of the floor, which may interfere with the seat runner and the old seat mount (crutch strap mount) bracket. The flat floors are also slightly compromised, in that the corner of the floor panel is cut away, therefore the panel is not riveted and sealed to the steel right into the corner. Also, the seat support channel under the floor is cut down to half its width to clear the A-frame bush, losing one of its flanges and one of the two steel rivets holding it to the chassis tube.

Regarding race chassis, I was looking on the CC parts website and they list the short "standard" A-frame as "used 1991 to 1996, 2000 Race & updated 2001.5 onwards", so I assumed some race chassis were built without the underslung mounts, but evidently not all. I guess something  changed in 2000...?

Graham.

[SLR No.77]

IIRC it was approximately 2000 when the tunnel handbrake was adopted for race cars, together with a full cage. Prior to this race cars from '96 included the front chassis mods but retained the original non-handbrake tunnel which collapsed more easily in a side impact situation, when cages became mandatory for competition this wasn't necessary and the stiffer post-96 full chassis could be used.

Stu. 

[Matthew Willoughby]

2004 season Academy cars were the first to have a tunnel mounted handbrake.  That was also the first season that cages were mandatory.

[Wrightpayne]

Perhaps we should all post pictures of our rear ends so Graham can do some analysis?

[Richard Price]

"IIRC it was approximately 2000 when the tunnel handbrake was adopted for race cars"

"2004 season Academy cars were the first to have a tunnel mounted handbrake."

I'm pretty sure it was actually 2002 that was the first year academy cars had the tunnel mounted hand brake (the same year I originally assemble my car)

[SamC]

I'm not familiar with the exact changes between the under slung and side mounted wishbones, but if the only difference when looking in a front or side view is the distance of the inboard mounts from the chassis centreline, then the primary difference will be your camber change with bump. The longer lower wishbone will give you more negative camber with bump which is generally a desirable characteristic.

Your antis are varied by the inclination of a line drawn between the fwd and rwd mounts in side view, so would likely be largely unaffected by moving the points inboard (again, assuming that is all the change was).

[aerobod - near CYYC]

Re #9 - It is the A-frame to the deDion that is underslung in some cars. The length of the A-frame affects the amount of rotation of the deDion tube as it moves vertically and depending on whether the car uses Watts linkages or just radius arms, the change in wheelbase and the amount of rear steer as one wheel is deflected.

[SamC]

#10. Extremely good point - please ignore me, I'm being daft! I was fully in 'double wishbone' mode having spent the evening fiddling with my CSR!

What a plonker! 

[ECR]

Don't know if this is relevant but i'll throw it into the mix. My notes confirm that the Front A frame chassis mounting bushes were moved up and back from 1991 onwards. This would obviously impact the dimensions of the A frame.....

[Nigel Blandin]

My 1990 chassis and two others the same age had a non 'A'  A frame! The cross bar was deleted for that year only, something to do with anti squat. But it didn't work and reverted back to the traditional A frame from '91.

we welded cross bars back in on the advice of John from Redline. Can't say I ever noticed any difference either way.

[Mcalvert]

Yes, my 1991 car originally had the non 'A' A frame.  I reverted to a standard part when rebuilding the car a few years later and can't say I noticed any difference either!