620 S Build

[Croc]

On 11/15/2025 at 10:37 PM, Croc said:

 

No idea.  But you will not find a 15x8 sold by Caterham for a de Dion chassis.  You will find a 13x8 for a de Dion chassis.

 

I always understood it to be clearance issues somehow.  But I have never checked myself.  I do know it came up in a conversation with Bruce B two weeks ago when I saw a 420R with the widest set of 15 inch rear wheels I ever seen on a de Dion car.  He said it was clearance issues?  

 

 

After some FAFOing in the garage, I can see no reason why a 15x8 wheel would not work.  I think even a 15x9 might work too.  Its all about the offset of the wheel you are using.  Caterham do not offer a 15x8 other than in CSR wheel and the offset is wrong for a de Dion car.  But use a spacer and the CSR wheel would fit.  A custom wheel with the correct offset would fit.  

 

Still does not change your preference to use 13 inch wheels if you can make the rubber tread pattern work.  

[Croc]

18 hours ago, CarYenta said:

My rear toe-in measured via toe plates and tape measures is currently 1/4" total toe in across about 22 inches (toe plate width). With an isosceles trapezoid calculator I get 0.33 degrees each (0° 20' each) or 0.66 degrees total toe in (0° 40' total toe in).

 

This seems kind of high compared to other cars I've had. Do people shim this out to get closer to 1/8-1/16" toe-in?

Or do de dion just like to scrub tires? This is a 98% road car.

 

66.5" across front of rear wheels

66.75" across rear of rear wheels

 

 

Do you have any shim on the rear currently?  Or is that the unshimmed measure?

 

[JohnCh]

6 hours ago, CarYenta said:

Also received some dpr mirror mounts which apparently donates to this forum for each order, sounds like there's a back story somewhere. 

 

There's no back story, because it's not true  I suspect they meant BlatChat.

[CarYenta]

8 hours ago, Croc said:

 

Do you have any shim on the rear currently?  Or is that the unshimmed measure?

 

Welllll waddyaknow, there's a sneaky shim in each side.

 

[CarYenta]

9 hours ago, Croc said:

 

 

After some FAFOing in the garage, I can see no reason why a 15x8 wheel would not work.  I think even a 15x9 might work too.  Its all about the offset of the wheel you are using.  Caterham do not offer a 15x8 other than in CSR wheel and the offset is wrong for a de Dion car.  But use a spacer and the CSR wheel would fit.  A custom wheel with the correct offset would fit.  

 

Still does not change your preference to use 13 inch wheels if you can make the rubber tread pattern work.  

Looking at tire options, it seems like I should probably just run 15". I see several combinations of 205/50-15 and either 225/45 or 225/50 and also three 245/40.

 

Am initially thinking 

15x7 et13 for 205/50

15x9 et25 for 245/40 with continental extreme contacts in mind for summer, then converting my 15x6.5 into the winter set with a wetter tire option.

Edited November 18, 2025 by CarYenta

[Croc]

4 hours ago, CarYenta said:

Welllll waddyaknow, there's a sneaky shim in each side.

 

 

Good - thats what I wanted to see.  Rear end should be shimmed to get to correct rear toe under load.  So your measurements will include that.

[CarYenta]

Changed the front toe to zero, decided to keep the rest of the alignment as-is to 1) learn what the behaviors are first and 2) I imagine the bushings will settle somewhat, changing the alignment anyways.

 

For anyone like me wondering where the heck the flats are on the tie rods after loosening the lock nut, they are hidden under the inner rod corrugated rubber boot, so remove the zip tie, push it back a bit, and there are 11 mm flats.

 

Every 1/4 turn changed my total toe by around 1/16".

[Croc]

Is your 620S a de Dion rear end car or one of the less common watts linkage cars?  Caterham have used both types of rear end on the 620.

[CarYenta]

1 hour ago, Croc said:

Is your 620S a de Dion rear end car or one of the less common watts linkage cars?  Caterham have used both types of rear end on the 620.

I believe it does not have a watts linkage - this is the right hand side looking forward, there would be a second bar coming off the top of the de dion tube going towards the rear if it had a watts, right?

Edited November 19, 2025 by CarYenta

[CarYenta]

Going for gold (anthracite w/ polished lip actually) since the black friday sale is ridiculously cheap compared to anything custom, and have had a good time with Konig on my miatas:

Konig Hyperspec 15x7 et35, cb 73.1, 11.3 lb

Konig Hyperspec 15x9 et35, cb 73.1, 12.7 lb

 

Australian Caold converters, 4x108 w/ cb 63.4 to 4x100 w/ cb 73.1

20 mm thickness front, 1.1 lb = effective et15 front for total of 12.4 lb

15 mm thickness rear, 1.5 lb = effective et20 rear for total of 14.2 lb

 

 

 

 

How it will sit compared to the 15x6.5 et13 on the car currently:

 

front:

 

 

rear:

 

[MV8]

Croc, are you referring to the dedion standard radius arm versus the dedion watts radius arms?

 

Check the adapter lug thread if you plan to use different lug nuts. I've had adapters that look like that and claimed to be a specific stud thread size but were not. They only accepted the included plain nuts. The studs were not to a standard either.

Edited November 19, 2025 by MV8

[CarYenta]

50 minutes ago, MV8 said:

Croc, are you referring to the dedion standard radius arm versus the dedion watts radius arms?

 

Check the adapter lug thread if you plan to use different lug nuts. I've had adapters that look like that and claimed to be a specific stud thread size but were not. They only accepted the included plain nuts. The studs were not to a standard either.

I might need to put in slightly shorter studs too, will find out! Can't determine the wheel rear fascia pocket depth. No idea what the contour shape is of the nuts either on the adapter so may need a few parts. An adventure!

[Croc]

21 hours ago, CarYenta said:

I believe it does not have a watts linkage - this is the right hand side looking forward, there would be a second bar coming off the top of the de dion tube going towards the rear if it had a watts, right?

 

 

Yes - you are thinking of it correctly.

 

There are three types of rear suspension that you can find on a 620 - de Dion, de Dion and watts, and just plain watts.  Not sure why there are variations tbh?   Its only on the 620 too.  Not seen them in person so not sure how they are set up/configured.

 

I think the R500 may also have had a de Dion and watts rear end set up.  

 

 

[KnifeySpoony]

How can you have a watts without dedion? I mean, you can have a watts linkage on a live axle, but 620 is well after the live axle era. 

[CarYenta]

3 hours ago, Croc said:

 

Yes - you are thinking of it correctly.

 

There are three types of rear suspension that you can find on a 620 - de Dion, de Dion and watts, and just plain watts.  Not sure why there are variations tbh?   Its only on the 620 too.  Not seen them in person so not sure how they are set up/configured.

 

I think the R500 may also have had a de Dion and watts rear end set up.  

 

Fascinating.

 

I took the wheel off to measure the hub bore, so it's easier to see the sunpension also:

 

Is it typical to have the coilovers near minimum height yet only have 10 mm distance from de Dion to side rail at full droop? I was going to raise the rear up 10-20 mm but not anymore! Is the 150 mm ride height not applicable to 620?

 

 

Also look how long these springs are, 13 inches! ~50 cm end to end on the shock body.

 

 

And some hub measurements if anyone or future me needs it:

 

[KnifeySpoony]

Oh and yes the SLR models (which includes the r500) were in the dedion era, and all came with factory watts linkage afaik. After changing to the current model lineup, the only street car that came with watts from factory as standard was the 620r i believe. However it could be specced as an option or added later of course. I built my 420r from new with watts. 

[KnifeySpoony]

Yes the tube gets close to frame tubes. Mine actually contacts it centrally (search forum for my thread on the subject). Non issue it turns out. Re: ride height, raising the rear won't make the clearance any less. 

[CarYenta]

24 minutes ago, KnifeySpoony said:

Yes the tube gets close to frame tubes. Mine actually contacts it centrally (search forum for my thread on the subject). Non issue it turns out. Re: ride height, raising the rear won't make the clearance any less. 

Oh really, so this full droop actually has the shock fully extended already then and increasing ride height only increases spring preload?

[KnifeySpoony]

38 minutes ago, CarYenta said:

Oh really, so this full droop actually has the shock fully extended already then and increasing ride height only increases spring preload?

Yes, the suspension will go into full droop no matter where the collar is set on the shock body. ie the length of the shock in full extension is a fixed entity.

 

[CarYenta]

Checked my throttle position sensor voltage - idle was at 0.53 V site 0.0. Site 0.1 starts at 0.54 V. Idle screw completely out (throttle closed) was 0.43 V. Fully open was 3.38 V.

It seems like my idle up/down surge repetition is probably something related to thermal expansion and going from 0.53 to 0.54.

I set it down to 0.52 V to see if that helps.

 

I did notice that with very, very small throttle actuation, the throttle site value goes up to 1/4 of maximum. This is almost certainly related to the difficulty in small throttle modulations. 

---> is there a mod to make the throttle cable actuation more linear at the initial actuation? Like a larger diameter butterfly

 

Build support had me test continuity between green-black and green-black on the fuel gauge dial and fuel gauge connector on the rear left tank side and, it was good. 

 

 

 

Decided to raise the rear up a little since I was going to take the rear wheels off anyhow to bleed the brakes for the seventh, yes, seventh time trying to get more air out. Went up to 165 mm each side in the rear when loaded with my weight and kept 150 mm in the front.

Edited November 22, 2025 by CarYenta