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Should I soften my rear ARB?


Richard_420R

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Hi - hope everyone had a good Christmas.  Quick question regarding rear ARB - from the factory my rear ARB is set to the second rear-most hole (2019 420R).  I found this nice and pointy for the track but on the road it can snap quite quickly.  Would moving to the rear-most, softest setting be a better choice for the road?  Also is this a simple un-bolt and re-bolt or are there some extra steps - thread lock?

Cheers, Rich. 

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I prefer mine disconnected for road, although I am using it on the softest setting. The S (street) 7s come from the factory without a rear ARB. I think the racers buy the extra connectors so they can easily swap between settings to suit the track/wet/dry conditions.

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I have a 2018 420R and running the rear ARB on full soft did not go well for me.

I found that the bottom joint of the N/S drop link would impact the driveshaft under compression. I have an S3, 13" wheels and track suspension.

Yours may be fine but just check you have enough clearance. It was an expensive mistake for me.

Some images in this thread to explain what was happening.

https://www.lotus7.club/forum/techtalk/more-driveshaft-woes

I would also add that with these cars being so light, the ARB assembly is a very flimsy, flexible thing and with the soft adjustment settings being at the extremities of the long, narrow gauge blades, the difference between full soft and one hole stiffer was much less than I had imagined it would be. For track I am happy at one hole off soft but if I was using it for mainly road use, I would just run with out the ARB.

Regards

Andy

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I’ve had issues with the handling balance of my 420R recently. Self inflicted in my case, as I swapped the 15 inch Orcus wheels the car came with for a set of 13 inch wheels (wider at the rear) & I find the car is now wanting to understeer. If I’m very heavy with the throttle, it can still be persuaded to oversteer, but the back slides very abruptly. 

I’ve got it booked in for a flat floor & general set up session in the spring, which will hopefully sort it out, but as an attempt to improve things in the short term, I stiffened the rear ARB, going from the second hole from the end, to the third hole from the end (the second stiffest setting). It definitely made the car feel more pointy & eager to turn in, but the downside was the eventual oversteer felt even more abrupt & ‘snappy’. After one blat I reverted to the original setting! 

Perhaps if you started by removing the drop link from one side, you could try it & see what you think? If it’s a success, you could then remove the ARB completely. They’re not difficult to remove, you just have to remove the drop links & then the alloy clamps, two bolts each side. Good luck anyway! I’ve never tried the softest setting. It will be interesting to hear how you get on. *thumbs_up_thumb*

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In comparison there seems to be a marked difference between 420 & R400 from my experience with an R400D

With the 420 it depends on which suspension pack R or S? The R400 came with the full race suspension of the time which I believe was just fettled for road use straight from the race car. The rear springs were made a bit more progressive & that was about it. I stand to be corrected.

Size of wheel/tyre combination can also come into play? I have 8J x 13" fitted.

The R400 ARB drop link is connected to the de Dion via a horizontal spacer.  But not the 420. Could this slightly alter the geometry?

The ARB rubber is yellow on my R400. I presume the same on the 420?

My drop links are set on the second hole which gives very precise, pointy, predictable handling. Experimentation found that any further forward than the second hole on the ARB blade produced progressively extreme, unpredictable back end handling. The furthest forward hole is definitely to be avoided! The next back is still interesting in a dangerous sort of way.

There is some history on Tech Talk concerning later cars (420 era) showing signs of the drop link fouling the drive shaft if using the first hole on the blade. Is this due to a combination of a floor set up at the lowest extreme, S pack suspension (softer) & no ARB lower spacer? 

 

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I recently fitted a rear ARB kit to my metric chassis S3. The mounting arrangement is quite different to how I remember it being on earlier cars - I think. There is a bracket that attaches to the de-Dion ear rather than a horizontal spacer that bolted straight on. It uses the existing speed sensor bracket on the RHS. 
I have had a close look at mine and trying to work out if there is a possibility of it fouling the drive shaft on the rear most hole but I don't think it is possible to tell how it behaves under load.

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There is some history on Tech Talk concerning later cars (420 era) showing signs of the drop link fouling the drive shaft if using the first hole on the blade. Is this due to a combination of a floor set up at the lowest extreme, S pack suspension (softer) & no ARB lower spacer? 

No, this isn't the case. My car is an R fitted with track suspension. My car has now been set up and corner-weighted by Northampton Motorsport purely for track use and the handling is now sublime. It has sufficient rake, ie. the rear is not set up at the lowest extreme.

I am now running the rear ARB one off soft because the drop link would still impact the driveshaft under full compression over the curbs and rumble strips if it was set at full soft

Andy.

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I have had a close look at mine and trying to work out if there is a possibility of it fouling the drive shaft on the rear most hole but I don't think it is possible to tell how it behaves under load.

In order to try to determine the cause of my problem, I slung my go-pro under the car at Anglesey, because whilst stationary, I could not see how the drop link could cause the impact damage where it was.

On viewing the footage, I was surprised to see how much lateral movement occurred at the drop link under cornering. This was due to both end-float in the ARB itself within the bushes and also because of flexing of the ARB blade. This showed that there was indeed enough movement to cause the damage sustained.

Andy

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Thanks Andy.

Apologies if this is a stupid question but why does moving it one hole forward from the rear most setting stop it happening? Is it because it reduces the travel of the suspension by being stiffer or does it move the parts that collide  out of the way?

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Hi Steve

All of the above really.

It stiffens the whole assembly, therefore reducing the arc of movement at the end of the blade.

It also moves the drop link joint forward, out of the way of the driveshaft

It also further restricts sideways movement of the blade so it can't 'twang' about under load and hit the driveshaft.

Andy

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'interesting in a dangerous sort of way'

Brilliant! Quote of the week. *biglaugh*

I didn't realise there were so many differences between the R400 Duratec & the 420R. I had assumed they were pretty much the same, apart from a few minor changes in terms of what came as standard & what was optional. I have been told the later, Mexico built Duratecs have a slightly lower compression ratio & therefore are a bit down on power in comparison to the earlier ones. 
 

Interesting reading, anyway.

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AG2728 #10 the R suspension must be soft(er) than the original (and best - sorry!) R400D race derived suspension as it is as hard as a rock & I drove for a while with the link in the rear most hole with no incident. That is until experimentation found the second hole in the best.

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Maybe it is softer on the 420, I don't know. I must reiterate though that the contact was only likely to happen in extreme compression situations.

For example, when I hooked up the Go-Pro under the car at Anglesey, even whilst driving in manner to prompt a reaction from my drop links, I could not replicate the impacting issue at all, I could however, by reviewing the footage and looking at the relative movements of the drop link and the driveshaft, tell exactly how it could easily happen bouncing over the curbs at say Curborough.

For road use, or if every sprint was at Blyton Park, then I would never have had an issue. The set up was effectively good for 99% of my usage. Unfortunately the other 1% cost me £400.

Maybe the R400D does have a different set-up though, as I can only describe my 420R experience of ARB adjustment from full soft to one off soft as "barely noticeable" whereas I found even the most minimal height adjustment at the rear would effect a very obvious change in behaviour. I had put this down to a high spring rate vs a weedy ARB. YMMV.

Andy

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I have only done one track day since I fitted mine [on the weakest setting]. It was very wet all day and no signs of contact. I have moved it back to one hole from the weakest to be safe - especially as I have road springs. The whole assembly felt quite loose before and tighter now. I am looking forward to trying it out on road and track in the Spring.

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