Chassis question(s)

[theDreamer]

So I’m trying to catch up on some 25 years of Caterham design improvements and I’m not having any luck with one puzzle. Why the “modular interior” on some CSR’s while others have the more tradition look to them? All the added tubing must have come with a weight penalty. Was it to make the chassis more ridged? Or, was tubing sizes changed in other areas to offset the extra tubes in the interior? See attached photos.

[Croc]

 

I own CSRs with each dash design.  The facts were always hard to come by from Caterham but here is what I know:

 

- The CSR was intended to be the future high end evolution of the Caterham 7.  However, it was only produced by Arch, could not be produced in great numbers and could not be produced cost effectively.  So it was a design triumph but a dead end from a car evolution point of view.  

- The CSR can still be purchased in the USA and Europe today.  Its only the UK home market where it cannot and it was intimated to me by a factory source that they just could not get the necessary production volume out of Arch at the right cost to make the effort worthwhile. 

-  The dash is only part of the chassis changes.  There is a LOT more cross bracing in smaller tube sizes plus the suspension pick up points front and rear are very different as would be expected.

- The CSR weight penalty over an SV is claimed at 25 pounds in a roller chassis (no drive train) in the standard dash.  Supposedly the curvy dash is only 2-3 pounds heavier but I never verified this.  I do know my CSR with curvy dash is only 30 pounds heavier than my UK 420R when in full road mode (i.e. full tanks and no driver).  Not all of these data points correlate which makes me wonder about what Caterham claims. 

- But the CSR picks up a significant increase in torsional rigidity - 25% over the SV in standard flat dash format and supposedly 40% in curvy dash format.

 

The idea with the curvy dash was to emulate Donkervoort in some fashion and hopefully attract some of the premium coupe market purchasers.  Not sure that worked - pretty convinced it failed.  It was an extra cost option at the time.  

 

What worked?  The CSR is by far the quicker point to point car on any road compared with a regular de Dion chassis.  Bumps do not exist.  It is also quicker on track than a deDion SV with equivalent hp (which is quicker than the S3 de Dion) - I participated in an informal test at Donington and I was consistently a second a lap quicker in the CSR.  The suspension gives you the confidence to really take it to the edge where most non-professional drivers struggle to get there and stay there in a de Dion chassis.  90% of the drivers who participated that day were quickest in the CSR. 

 

What did not work?  The curvy dash tubes are way too good at transmitting heat into the cockpit which required me to make venting modifications to the undertray to ensure I did not burn my arm on the exposed tubing.  The dash bits and pieces are ex-Rover Metro so really crappy plastic quality.  Because of the desire to have turn signals etc, the steering column is slightly too low for my personal fussy liking and you cannot modify it.   The original design only had two mounting points for the diff which in 265hp CSR260 mode saw people regularly rip the mount points off the case.  The initial fix was the extra race struts to give 4 mount points but these extra 2 struts go up through the boot, reducing usability by a third over the regular SV.   The current fix as seen on my new build is a cage for the diff with 4 mount points which improves torsional rigidity further.  

 

 

[Vovchandr]

You won't get a more concise answer from anybody else ^!

 

Croc is one of the few people in the world other than maybe Caterham employees with that much CSR Cat experience. 

[IamScotticus]

Was it a product of the 21 program?

[Croc]

5 hours ago, IamScotticus said:

Was it a product of the 21 program?

 

Interesting idea.  I don't definitively know but I doubt it.  The Caterham 21 was essentially a regular Caterham 7 with front and rear de Dion chassis underneath.  The CSR chassis was 10 years later and had inboard front suspension, IRS from  the 1990s era VW Passat and a re-engineered chassis tube design that took it a long way from the original Caterham 7 chassis.  There were likely lessons from the Caterham 21 that fed their way into the CSR but its not a direct lineage I can discern.