Stefan, you might be interested in the work I have been doing on my Renault. I had a bit of a problem with the chain coming off at Stretton and with dreadful understeer because of the solid axle driving both wheels.
I don't want to drive/brake just one wheel either and was pondering what can be done when Rhys told me about a simple device he made for his son’s go-kart many years ago. His design used a friction material between two discs, one attached to the free turning wheel hub and the other attached to the driven axle. It worked very well.
I spent ages trying to develop the idea into something that might work on my Renault, without the need for chopping the back-end about to fit additional bearings and all that goes with that.
I came up with a design but, knowing nothing about friction materials I got in touch (via Google) with a guy called John Price who owns a company called 'Industrial Friction Materials Ltd.' in Cardiff. Not quite next door.
He and I spoke on the phone at length and I decided to go and see him.
He has lots of experience and assured me that the design would work well. He is happy to make all the parts required. 
Rhys popped in today and I showed him my developed design. I said I thought many cyclekart owners might like one (if it works of course) but he pointed out that most cyclekarts have 25mm diameter axles and that the pit-bike wheels are very popular too and that they might struggle with a 30mm diameter axle and he's right.  I have a set of pit-bike wheels which are destined for my next cyclekart so I drew up the hub to see how I might be able to adapt the design to suit them on a 25mm diameter axle.
After a lot of head scratching, we came up with a design that will work for this combination but by now it was tea-time and time to go home so a drawing ‘proper’ was not finished.
I have just caught up with this topic and your Bugatti’s problem with understeer.
Since the Bugatti has a 25mm axle and pit bike wheels I thought I might share this with you now rather than just make one for me some months in the future.
The design will allow you to install the ‘Diff’ without changing any part of your Bugatti’s construction. If you are interested, I can get the necessary metal parts laser-cut and the friction material bonded in place. I’ll try and get a drawing done tomorrow and I’ll post it here.
I still have some design work to do on the compression spring (rate/size) but that’s about all.
If it works as it should, I’m sure other members of CycleKarts GB might well be interested too.
Cost is not a lot but the price will drop if we can do a batch of ten.
This is a ‘not for profit’ idea of course and my objective includes doing it for as little as possible.
 

The drawing (I hope) shows the design for the pit bike wheels and 25mm dia axle.
For this application, the mounting lugs are on the outside. The drive comes in from the right.
I'll make a separate topic for this so everyone should see it.

Are we saying this will only work on the pit bike wheels ,  Different construction on Honda type wheels ?

I ask this as i am picking up some Kart parts in a couple weeks  , including wheels and not sure what they are yet.

Thanks Adrian.  Will do .

Chris

How time flies!
I have just realised it's almost 25 years since I used this on the go kart for Sean, and that wasn't the first time I used it!

Hi Stefan, remember when photos were not digital? This was all done long before digital cameras, so, no I have no pics from back then. TBH I would be surprised if I ever took any, didn't see the need to document stuff I did.
When you ask about how "the hub was supported on the axle" I'm unclear on what you are asking. If you mean the wheel, it was a relatively close fit on the axle. If you mean the drive hub, it was keyed to the axle. If I were doing it now with a different style of wheel I would have bushes, not bearings as they are too free, in the bearing cavities as Adrian has drawn.
It has become more sophisticated as there is a need to match to the different wheel designs (Pitbike, Adrian's artillery and the Honda), and my "back of an envelope" designs don't seem to cut it with the 21st century.
My original just had a steel washer against the wheel, but the man Adrian spoke to felt the need to use high carbon steel which introduced another level of sophistication.

At 15mph on a 25ft radius corner the rpm difference between the inside and the outside wheels is 2.33. Now my sums are a bit iffy but thats about what I thought it might be.
Makes you think. Would a friction disc turning against a steel plate at 2.3rpm do any damage at all?
I dont think so but it has to last for years so I'm going for it.
Tomorrow I will order the discs and the friction materials for the 30mm diameter design for my Renault.
Then I should have nothing to concern my little brain with other than how fast can I get round Stretton now?