Caterham A Frames - check them as part of your annual maintenance

[CarlB]

I haven't modeled the Mumforf link either, but it looks like the roll center is fixed to the two bolts on the car. The two bell cranks allow the differential to roll but not move laterally. It would be very interesting to have a suspension design that allowed equal lateral roll center movement towards the inside of a turn.  

[MV8]

The articles mention of "nil vertical loads" makes a good point. The Mumford does not generate a vertical lifting force on one rear corner the way a panhard does under load whenever the chassis end is higher than the axle end. I think any RC movement should be minimized (practically with the overall compromise) for more dynamic stability/feedback to the driver. Lateral RC migration typically increases the distance to the CG (RC shifts inboard/away from the corner and CG), decreasing roll resistance as the cornering load increases because the CG has more leverage to act on the RC.

[EdWills]

Hello Gents.

 

Although this design is perhaps not very practical for a Lotus Seven due to the cost and complexity of the system, this drawing of a De-Dion rear suspension by Len Terry would have complimented his other arrangement for the rear suspension of the Lotus Type Three Seven race car. The Terry design in the attached drawing is still in the  Lotus vein with the small diameter tubular frames for the De-Dion suspension (as opposed to the large 3 inch? diameter tube used on cars such as the Lotus 11 and the Seven Series IV Clubman car). Note: Regarding the drawing, unfortunately, my copier doesn't work that well, and although I tried to eliminate light, it didn't work - my apologies.  The 3rd picture should read 'FULL ROLL' and it looks like 5 degrees?  I really like the look of this layout, and it would be an interesting project if time and money allowed.  Cheers,  Bill

[MV8]

I've not seen that before; a telescopic upper link. I assume the thin diagonals off the lower uprights are the coilovers. The wheels would move laterally. Reminds me of a formula vee "zero roll" resistance system where the rates are just enough to keep it off the ground, though I'm not saying it would work that way.

Edited January 16, 2024 by MV8

[EdWills]

Hi MV8. Correct, the thin lines are the coil overs.  Len Terry was quite a character, and I had a few emails back and forth with him before he died. Regarding the layout shown in his book, he advised the following: "It comprises two simple frames, each forming a triangle with its 'upright', connected to a sliding joint. Each frame is located by a parallel-arm lower link, as shown, and by the usual top and bottom longitudinal radius arms which are not seen in this view.  In the drawings the body is depicted purely diagrammatically to indicate the attachment points.  This system keeps the wheels perpendicular to the road under any combination of bump, rebound and roll, and the lateral sliding travel at the central joint is less than half an inch.  Unsprung weight is clearly rather higher than for a conventional rear suspension, but the advantage of no camber change could well outweigh this, and the structure does fit surprisingly well into the layout of a normal open-wheel racing car".  Apparently, Len Terry did not incorporate this design into any of his cars, or those for Lotus (too bad!).  Just an aside.  Way back when Lotus was going up-market, they would sell off many of their sports and racing car spare parts at the factory.  They would bundle them into wooden tea chests, 2 feet x 2 feet x 2 feet or so, and you purchased the tea chests more or less items unseen, not knowing what was in the middle and bottom of the container (you were not encouraged to dump the contents out at the factory apparently!).  A friend of mine purchased one of the chests from the factory, and inside were a pair of new cast alloy rear uprights, drive shafts, brake parts, wishbones and lots of other assorted bits and pieces from their Formula Ford cars and other G.P. cars.  The price, according to him, was quite reasonable, and he planned on using some of them on a couple of Lotus formula cars that he owned, as well as a Seven.

Best, Bill.

[MV8]

I don't see any down side to Terry's design.

I like what I've seen with some Cats having a watts trailing arm system on the de dion for zero roll steer. For a live axle, I'd like to see three watts or two and a mumford with a sliding torque link next to the trans slip yoke, with the lower coilover attachments offset to the front and rear and lower for longer coilovers and springs, all attached to axle tubing with a hand full of bell cranks and a dozen sphericals. Definitely not as elegant and clean as the cortina trunion or spherical A frames though.

[CarlB]

Len Terry might not have gotten a chance to build this suspension system, but he did design the 1965 Indy 500 winning Lotus with Jim Clark driving. 

 

 

[mccasksl]

Pardon my being late to the table, but I'm still searching and trying to catch up on the A-frame issues as I refresh / upgrade my 2009 S3 suspension (street only). I had planned to upgrade to Powerflex bushings throughout, but reading about the A-frame issues as well studying the geometry / suspension it may not be the best idea here. I note several Caterham and Lotus Seven Club folks (courtesy of Neil - "7 wonders of the world") have gone to a spherical bearing in the A-frame de dion tube attachment point which looks like a very good improvement although it may not solve all problems for full on track cars w/ slicks etc. Does anyone have experience with this bearing kit or similar? I know some of the USA7s folks are also in the Caterham and Lotus Seven Club so thought I'd inquire.

 

@MV8 @EdWills Thanks for the Len Terry book reference, it brought back fond memories of many hours studying the rapidly evolving Formula Fords during the 70's at SCCA events and dreaming of fabricating / trying some of the Len Terry designs. 

 

Spherical A Frame bush set - Page 9 - For Sale - Caterham and Lotus Seven Club

Batch 2 - Spherical A Frame bush set - For Sale - Caterham and Lotus Seven Club

 

Cheers!

Steve

Austin

 

Edited February 23 by mccasksl

[Slonie]

Wondering if anyone here has gotten ahold of the spherical bushing as shown in that thread... With the second batch open, now would be the time to try and get in if anyone wanted one, or if we wanted to do a group shipment to get them over here...

Edited April 11 by Slonie

[IamScotticus]

I don't understand how, for the same reasons one would NOT use a Powerflex bush, that one would opt for a hard spherical or rose joint.

To harden one point to the detriment of the next?  Yes the A-frame to chassis mounts have bushes, but rather thin ones.  Do these suffer too much by the use of a spherical at the center?

Edited Saturday at 03:42 PM by IamScotticus

[williamwashere]

I’m on the UK forum thread, it seems like the explanation is it removes a wear item when you track a lot. It doesn’t seem to have an impact on handling in their explanation, but I can see where having some flex would be beneficial and a more fixed point would just move that stress elsewhere.

[KnifeySpoony]

I may be mistaken, but I thought the point of a spherical bushing swap was to create MORE flexibility/range of motion at that joint, not less. 

[williamwashere]

For those that don’t have access, this was the explanation on the UK forum:

For gentle road use, the original rubber bush and washers are fine. With sticky tyres for aggressive road driving and/or track use the rubber bush can tear, especially if the nylon washers become too worn and the sideways movement of the bush becomes aggressive. With track use, the nylon washers can be destroyed after a few track days due to the high lateral loads continually hammering the deDion mount into the A-frame and putting high pressure on the nylon washers as the rubber bush flexes.

As the rubber bushes for the A-frame and radius arms / Watts linkage for the deDion isolate the mounting points from the chassis, the deDion to A-frame bush doesn’t cause any noticeable deterioration in NVH when it is a solid spherical bearing, but the handling can be more precise when the car is under high lateral loads, besides the longevity issue of the standard bush and washers.

[Croc]

On 4/11/2026 at 1:47 AM, Slonie said:

Wondering if anyone here has gotten a hold of the spherical bushing as shown in that thread... 

 

Yes I got one of the original batch spherical bearings and put it in a 620.  I also installed powerflex poly bushings all round as I like their excellent durability more than the usual rubber plus suspension just feels tighter with better feel when using poly bushes.  Just trundling around on roads there is zero difference in feel.  Will see what it does when I get on track.  

 

Quote

With sticky tyres for aggressive road driving and/or track use the rubber bush can tear, especially if the nylon washers become too worn and the sideways movement of the bush becomes aggressive. With track use, the nylon washers can be destroyed after a few track days due to the high lateral loads continually hammering the deDion mount into the A-frame and putting high pressure on the nylon washers as the rubber bush flexes.

 

My experience is the rubber bushes tear readily with a lot of track driving (think 2000 miles level) but I have never seen the nylon washers wear out first or at the same time.  4 days at Spa-Francorchamps is circa 1000 miles for me to give perspective.  Anyway, I replace the washers when I replace the bushings.  My disappointing experience with the rubber bushings is why I have converted solely to poly bushes all round the suspension - far more durable and a slightly tighter feel to the car.

 

Just to be clear, I don't think bushes or bearings will change anything on the original reason I had for starting this thread.  A frames can break suddenly in a high stress track environment (or lots of bumpy rutted roads) and you should check them regularly and think of them as a consumable if you track Caterhams a lot.    

 

Edited to add: I will probably order a second set of the spherical bearing to put on my UK 420R since it lives on track and gave rise to this thread.  Probably upgraditis but hey!