1972 Hillman Imp "FAF"- Pedal Box

As I mentioned in some other threads, I have been lucky enough to procure some Hayashi Streets, which I had shipped over from Japan.

This presents a problem in terms of mounting them....

Imp PCD is 4x101.6mm (4x4")
The PCD of the wheels is 4x114.3mm

Re-drilling the hubs isn't an option- not enough meat on them
Adapters is not a solution in my mind either- it's a cheat, not a "proper" solution

So I started exploring other options.
>The first was to find suitable hubs from another, donor vehicle. Throughout this build, I am trying to make sure that it is a build for longevity. I.e. if I take parts from other cars, I hope to choose some which will be readily/easily available in years to come.
4x114.3mm is common on a lot of older Japanese cars, and some less old ones- like hyuandai coupes and nissan primeras.

So, on the rear of imp, I have elected to use a Nissan Primera front hub- this gives me a suitable spline for a drive shaft and the right PCD. In order to mount the hub to the arm, a Kia Pride wheel bearing will be used- this is only 1mm greater than the standard imp wheel bearing and the correct inner diametre for the primera hub.

However, on the front- whilst there where available hubs that could have been made to fit the imp stub axle, they were much bigger and heavier. They also would have required the imp stub axle to be turned down and sleeved, in order to get the bearings to fit- which I wasn't sure about as a robust solution.

Thankfully, this forum is full of very skilled, clever individuals, such as a certain Johnny Dennett who it turns out is a bit of a dab hand in a machine shop!

Offering lots of advice on the way, including taking receipt of my new wheel bearings to ensure they would fit correctly, Johnny has sorted me out with some VERY good looking hubs. Almost too good looking to fit to the car, especially with their sexy zinc coating!









Massive thanks Johnny

Looking forward to seeing this come together. Really miss my old Imps. Kept all the manuals and tuning books, one day....

There's plenty of hubs that would work tbh, but the hyundai ones are cheap and easy.... £20 or so brand new.

Although now I think about it they might be Nissan Primera... should probably check the box!

Damn fine piece of engineering!

Some Bavarian brawn eh Frank!?

looking good frank !

No paint left sorry Frank, I think it was used as base coat many moons ago!

So.... Brakes!

With the suspension mostly sorted and front hubs done, I've decided to turn my attention to the brakes, which I've been quite looking forward to, because it gives me the chance to do some maths and I'm a nerd, who loves maths.

I'd already decided that I was going with front and rear discs because.... well.... why not?
Front disc conversions do exist for the imp, typically based on fiesta or Micra parts, however, all you're really paying for with one of those kits is a set of discs re-drilled to suit imp PCD, hoses and a set of brackets to mount the calipers. You then have to go off and find suitable calipers etc.
Having changed the PCD therefore, one of the key parts of the kit (the redrilled discs) is redundant!

I had previously bought a disc conversion, but the subsequently got sold on eBay. Also, I get a bit OCD about stuff and re-drilled discs (same as PCD adapters) just isn't "proper" enough for me and I was a bit underwhelmed by the quality of the caliper brackets.




So.... maths time!
In terms of braking specifications, one of the most important things to get right (IMO) is the balance between front and rear. Yes, you can use a bias bar or a prop valve to correct the balance, but I would rather have that as an option for tuning a system that is close to being right, not depend on it!

First thing I did was to use the drum radius and piston sizes of the standard system to understand what the Rootes engineers thought the balance should be (one factor I left out was the "shoe factor" which shifts the bias slightly towards the front due to the effect of the twin leading shoe at the front).

Then, using estimations for the longitudinal position of Centre of Gravity and the Centre of Gravity height, as well as front/rear static weight distribution (yup, lots of guess work!)... I looked at what the weight distribution is (roughly) likely to be under braking.

Too much rear bias- the rears will lock as weight is transferred to the front axle under braking and you'll go backwards into a wall

Too much front bias- your front tyres will reach the limit of adhesion too quickly, they'll lock up and you'll have LESS braking available due to the under performance of the rear brakes

For the record... I LOVE a spreadsheet.



I did have a brief foray into Wilwoods- there's a massive range of products and plenty of technical specs/details with them, making them easy to work with. But when I bought a caliper, it was MASSIVE and didn't even come close to fitting in the wheel. So I decided to go back to floating single pot calipers from a production vehicle.
Next came LOTS of parts catalogue trawling.

For discs, I find the Apec catalogue to be FANTASTIC:
www.apecbraking.co.uk/catalogue/size

I simply searched for suitably sized discs from vehicles that I know have a suitable PCD (4x114.3mm), such as 90s Nissans and Hyundai coupes.
- The reason being I want to make sure that service parts such as discs will be readily available in years to come, preferably without modification.

And for calipers, the key consideration is the piston diameter, for which the website brakeparts.co.uk is excellent (although you do have to search by vehicle. I just tried lots until I found something):
www.brakeparts.co.uk/#!/shop/ROVER/MGF/95-02/MGF 1.8 MODELS inc VVTI/Front - Brake Calipers and Wheel cylinders

Using my spreadsheet, I arrived a MGF front and rear calipers, which should provide an excellent front/rear split consider the imps weight distribution and rear slung engine.

The MGs feature 240mmx22mm vented discs front and rear.
As it turned out, whilst I was also able to find a 240x22mm vented disc from a Nissan Primera, I also found a 256x22mm disc, which I will use on the front. This shifts the braking slightly towards the front, which is the "safer" option vs the rears locking and can be tuned out with a bias bar if need be.
The numbers show that they are ~1% from the standard imp set up (excl previously mentioned shoe factor).

Good work there!