Could be the rear steer effect of having radius arms instead of Watts linkages, hitting a bump will cause that wheel to move up and forward slightly, steering a miniscule amount in the direction of the side the bump is on. The Watts linkages can be easily retrofitted if that is the case and definitely were in use on the R400D (although I don't know if it was an option or not). Currently the kit is out of stock, though: https://caterhamparts.co.uk/watts-linkage/4897-watts-linkage-kit-metric-chassis-de-dion.html
The "alloy endplates" may also be different, being available in 0.25, 1.50, 2.00 and 2.50 degrees camber (https://caterhamparts.co.uk/76-alloy-endplates), it is not clear what the toe settings are for each one, but as the toe moves from toe-in to toe-out, the feeling of rear stability will decrease. Different camber settings will also change the feel somewhat.
The rear anti-roll bar thickness and setting could also be different between the two cars, changing the feel.
I'm pretty sure the WL was standard until around 2015, James. My R400D has them and I didn't spec them.
There were a few changes in 2015 when the 270/360/420 were launched, some aimed at reducing cost and standardising things across the range. I wouldn't be surprised if the WL was discontinued then.
I believe the 620 still has it, but stand to be corrected.
Could that be the difference?
Factory spring rates on the 620R are significantly higher at the rear than the 420R. the damper valving is also different.
check the front radius arm mounts ,these are often overlooked and can loosen in use
Assuming neither have had bump steer checked or corrected, also caster these is set basically with thick washer however it often needs thin ones to set it correctly, this will affect the feel mid corner as it applies more dynamic camber.
The 620 also has more mass on its side too
Having owned a 420R and now a 620R my two pence worth ...., no science behind it and obviously just my personal views .... based on only road usage and only on measurements using my bottom, and the caveat that both cars were/are "standard" supplied by Caterham with no geometry/flat flooring etc.
My 620R appears to be much more firmly sprung than my previous 420R too firm I believe IMO, and for me the factory springs don't appear to be particularly well matched to the factory 620 damper response for road use (which on the face of it is no surprise since it's essentially designed as a track car).
Having said that given decent tarmac the 620 feels much more stable/planted when pushing on than my old 420 did, however when the going gets rougher shortfalls in the 620 setup start to appear.
One of my regular routes features a road with a smooth surface but with a series of underlying small humps and dips where the road has been subjected to subsidence in places I'm fortunate enough to also own a Lotus Exige and when driving this road the Exige feels completely planted, you can feel the suspension really doing it's stuff.
Driving the 620 along the same road up to a certain speed everything feels OK, but then trying to push the 620 at the Exige pace you soon become aware that the suspension is not "keeping up" with what's being asked of it, at times to me it feels it almost "floats" along for want of a better phrase, rather than follow the actual undulations, which can lead you into a false sense of security. I don't recall having similar experiences to the same degree with the 420, the ride being more "choppy" but feeling more connected, more of the time.
Sorry if none of the above waffle helps you CtrMint ...... but in pure "bottom" terms for me my ex 420R and 620R feel completely different suspension wise when used on the road, with each having their strengths and weaknesses depending on road state encountered.
I'll still take the 620 at the end of the day though
Thanks Benton, I actually traded an Exige V6 Cup for my 620R. The Cup car was a totally difference experience, completely planted, but less exciting for the same reason. It's interesting you also recognize the difference between the 420R and 620R, to some degree that's good to hear, I'm not imagining it.
Tyre pressures...? They are checked most times the car is driven, due to lack of use and seem fine. I can see how the radius arm might cause it. Though again, I don't feel its something loose, it's kind of like if you twanged a spring, resonance from side to side, just briefly. We're talking a feeling in the arse, rather than significant movement. If something was loose surely you get a knock and sudden drop off. This feels almost like a natural resonance.
Could be the rear steer effect of having radius arms instead of Watts linkages, hitting a bump will cause that wheel to move up and forward slightly, steering a miniscule amount in the direction of the side the bump is on.
Just been for a nose around both cars, and yes the 620R has the Watts linkages, the 420R doesn't. Based on the description from James I have to say it sounds like a promising hypothesis. It is something which I sense over rough tarmac, not weight transitions or acceleration etc.
Yes, the effect will be a bit of a figgity back end as the wheels move separately on bumps due to the radius arm pulling the wheel forward and rotating it inward then pushing it back again by a fraction of a millimetre as the wheel returns to the normal position, but no appreciable direction change. If you hit a ridge or dip in the road with both wheels at the same time you shouldn't get that feeling, although with any deDion car you get a bit of a different feeling of overall rear end grip loss with a transverse dip.
Would it be a reasonable statement to say upgrading the 420R to WL would be an upgrade, adding improved handling etc? If so a worthy upgrade and potential solution to my observation?
I would say it could be, I don't notice the effect on my R400D with Watts linkages. The only thing is to confirm the mounting boss is in place in the chassis for the rear arm position, otherwise it should be a direct replacement bolt-in.
From what I've seen from previous discussions on this, some say it is worthwhile, others say it isn't, it may well be down to personal preference as opposed to any appreciable dynamic improvement. It is a bit of an expensive upgrade at just under £500 for the whole kit, although going by the Caterham prices, the arms, bellcranks and spacers can be bought for about £230, then the rod ends and fasteners can be sourced elsewhere a lot more cheaply.