Limited Slip Differential

TL;DR Quaife ATB is a Torsen T2-type diff

Very few fundamental changes differentiate the 170S and the 170R. Playing with the car configurator just leaves me with the feeling that you cannot have a spare wheel on the R, and can't have a lightened flywheel or a limited slip differential (LSD) on the S. That's about it.

While I've already expressed that in my eyes a Seven has to have a spare wheel on the back -and I'm lucky mine was configured with that option-, the lightened flywheel is not something I would really be willing to use on a daily basis cruising around, never on a track.

The LSD on the other side is something I've looked into earlier on. At the time I would've had to replace the complete rear axle as it was the only option available from Caterham.

Other companies of various reputation and some eBay sellers offer alternatives, but when the product seems good, shipping costs and import duties wouldn't make sense.

The topic was raised again recently on the Facebook group, and the contact currently busy with a tune-up of my engine found that it was really lacking a LSD to be fully enjoyable - that and he's not a fan of the turbo-equipped sevens in general 😂.

That's how I came back to the subject and discovered that the Caterham shop is now listing the LSD as a standalone unit, the product was announced as both new and out of stock. The pictures show it's a Quaife ATB unit with part number QDF8P.

It's very good news, as the ATB is progressive and does not wear out significantly, with a lifetime warranty to boot.

Reading opinions and reviews, they range from good and effective on the track, to basically useless. The common consensus is that it's great on the road. The YouTube videos from Petrol Ped and Quaife convinced me that's an upgrade I want, and I want it to be an ATB. This comparison of the Quaife versus an awful copy really brought the idea home.

As the QDF8P was not available from Caterham and was not listed on the Quaife website either, I filled the Quaife contact form and got a prompt answer that confirmed that this item can only be bought through Caterham. Excellent customer experience, immediate response first thing on a Monday morning. Good on them 👌

Next target then, a message with the contact form on caterhamparts.co.uk. I somehow missed the answer, that also came in rather quickly. It's on the next day that I decided to create a customer account and look into the follow-up of my tickets/messages. The answer was there : the best would be to order via House of Speed, my official dealer.

Pushing my luck a bit further, I told the customer representative that it would be great to have it available when we come and visit the factory next week, if they have any inventory. And lucky I was. Those messages resulted in them showing two pieces available on the web shop. That's how I was finally able to 'just' place an order, no fuss, as it should always be.

For good measure I did add a new badge for the engine cover as mine is already missing letters, and it was placed upside down anyway.

The plan is now to pick up the order in person at the factory, and have House of Speed fit it at their best convenience with the next service. It's supposed to be plug and play, let's see.

Weight-wise : 4655 grams, compared to 3023 grams for the standard open diff'.

The Cusco DA64V RS.15W, a clutch-type LSD alternative, weighs 4425 grams. Thanks Simon for the check 👍

Update - It took me a while to figure out how or why this arrangement would behave in a different way than an open diff with usual planetary gears. The trick seems to come from the friction of the "plantetary" pinions that are not supported by any axle, they are pushed against the casing instead. The helical cut assures they will tend to push harder when a wheel is slipping. The 31-degree cut seems to be moderate in the grand scheme of things, but totally adequate for such a light car.

I've since 3D-printed a functional model that confirms spinning one wheel does rotate the other one in the other direction, but the small pinions are supported by pegs and aren't able to exhibit any transfer of torque. A later change of the casing model to have it closer to the real deal didn't demonstrate much either.

To conclude, I left the car at House of Speed during the week of February 10th for the 2 years service and have the Quaife fitted. The entire rear axle was taken off the car, the conical bearings could be extracted and reused. A good opportunity to see that a trailing arm bushing was shot on the left-hand side. The anti-roll bar dust covers were replaced under warranty, as they became dry and were already cracked since mid-year.