Feel at the rear, the difference between the 420R and 620R.

[CtrMint]

I thought I'd post to get some input regarding the difference in feel between my two Sevens.   Hope you can follow me with this.

The rear suspension on my 620R feels so much more direct and controlled/stable than the 420R, is this the difference between the chassis or suspension.   

My 420R doesn't feel wrong and handles great, it's just now and again you can feel a sort of oscillation from left to right at the rear.  It's not a movement in the parts, more a lack of sophistication in the rear end, almost like a resonance that causes a sort of low-frequency jiggle during rough road surfaces.   The 420R has been flat-floored and as I say handles great, but it's the comparison between the two that's made me aware of the difference.  Now of course I can't but focus on it.  The 620 is just planted, never does anything similar.

Everything is tight at the back, with the only knocks coming from the LSD as per normal.

Any advice? Am I destined to be ordering a suspension upgrade?

thanks

[ScottR400D]

Whats the difference, mechanically? Different dampers/springs/wheels/tyres? 

[SLR No.77]

What is the difference between the two, is it only shocks and springs?

I'm not familiar with the current Caterham setups through the range, my only contribution that I can make is that my SLR with Nitrons and basic radius arms is completely different to the original Bilstein and Watts link setup, exactly as you describe. 

Stu.

ETA clearly I must type faster!

[CtrMint]

They're both  S3, both on 13" 8" rears.  620 is on ZZR and the 420 is on ZZS.  There is extra bracing in the chassis on the 620.  Both are on the suspension provided with their R packs.  The suspension is different, but I'm sorry I don't know much about the specs.

[aerobod - near CYYC]

Hi Mark, do you have the geometry settings for both cars? Although they are both flat floored to optimize the diagonal balance, there can still be major camber and toe differences that will affect stability a lot. The rear deDion wedges could be different (affecting camber and toe) and the overall adjustment of the front settings could be optimized more for grip than stability, i.e. tow out that would make the car generally less stable but turn in better and high camber that could reduce stability under braking. The amount of wedge will also affect the amount of oversteer.

[ChrisC]

The extra bracing is to support the diff and prevent the diff mounts breaking loose.  The dampers and springs are different, but the trailing arm / dedion tube are the same.  Neither the 420 or the 620 use Watts links. 

Is the jiggle under acceleration, or coasting, i.e. is it a power deliver observation?

Assuming all the geometry is equal, and thats a big assumption. I suspect the tyre to be the biggest difference, ZZS to ZZR is apparently a bit performance jump, especially in the softer compounds now available. Obviously owning both a simple wheel swap could rule that out.  Beyond that spring rates then dampers, although there are those who complain about the spring rates used on the 620. 

The one thing that might be worth checking is your race nylon washers on the A frame, they don't last long, I think 2 years is about maximum, and that depends on use.   

[aerobod - near CYYC]

Neither the 420 or the 620 use Watts links.

 

Has this changed since the R400D and R500D? My 2012 R400D has longitudinal Watts linkages to control the fore-aft deDion position at the wheels, as opposed to basic radius arms that are certainly simpler, but don't control the deDion position as well.

[CtrMint]

My 420R only has the frontward radius arm, would need to check the 620.

Is the jiggle under acceleration, or coasting, i.e. is it a power deliver observation?

I can't say I've noticed it under hard acceleration, its more when you are driving in normally in say 30mph traffic and you catch a rough bit of tarmac, not a pothole, just some rough stuff. 

My 420R has done about 4500 miles, and hasn't been on track. 

do you have the geometry settings for both cars?

I don't, our local specialist Paul Dangerfield did the setup on the 420 when it went on the road. 

To me it feels like the horizontal positioning of the dedion isn't as firm as the 620, and it can result  a brief amount of low frequency resonance shocked in the right direction.  Would the camber etc, cause that? 

Don't forget. I am being hyper sensitive here, I don't consider the behaviour an issue on the 420R, it's just that 620 behaviour feels far superior as it doesn't do it.

[Wrightpayne]

I'd do a nut and bolt check of the rear suspension to check everything is tightened to spec. Also, ensure there are packing washers between the A frame and chassis both sides.

 

[ScottR400D]

As #9 says. If there's lateral movement it has to be a poorly located A frame so check the packing washers, or as Chris said, the nylon washers on the central location, though they're maybe less likely to produce a noticeable movement. 

Can't say it's a sensation I've ever felt so it sounds like somethings not right. 

 

[CtrMint]

I'll do the nut and bolt check, it's worthwhile anyway.  

I'm confident the packing washers are there as I built the 420R.  I wonder if I need more though.

Just sat here considering my reply... I wouldnt call it lateral movement, as I say its a sort of oscillation, its so difficult to describe.  Like I say, only really considered less than ideal having owned the 620 which seems to have a more sophisticated ride.

[aerobod - near CYYC]

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.

[ScottR400D]

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?

[Mark w]

Factory spring rates on the 620R are significantly higher at the rear than the 420R. the damper valving is also different.

[7 wonders of the world]

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.

Tyre pressures...?

Rack..?

The 620 also has more mass on its side too

[Benton]

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 

 

[CtrMint]

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. 

 

[CtrMint]

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.

[aerobod - near CYYC]

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.

[CtrMint]

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?  

[aerobod - near CYYC]

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.

[CtrMint]

I've still only done 900miles in my infamous 620, but even so I can tell I prefer the feeling at the rear on the 620.  I wouldn't say it's about laptimes etc, I just want to make the car the best fit to my preference.  It sounds like I prefer the WL.  I agree the WL isn't cheap for what is some simple parts, though its cheaper than some of the suspension upgrades I've been browsing.

I've rummaged out my build photos and I think I have the mounting at the rear, the boss towards the back in the image.

 

[aerobod - near CYYC]

That is it, this is what it looks like on my R400D. For the front mount you use the higher hole, as opposed to the lower hole the radius arm is currently likely fitted to.

[CtrMint]

Thanks James, and everyone else for your support.  I just need to wait for CC parts to have the kit in stock.

[ChrisC]

News to me 620(s) have WL   Every days a school day.