If you've read the article from our Targa High Country or Challenge Bathurst events from last year you might remember that we suffered a brake failure at the end of Targa. We tracked this down to a split crossover pipe in the front caliper and got it sorted but it got us thinking again about the ongoing brake issues we have had with the car.

Over the life of the car the braking system has evolved from the stock disc/drum combination with the first upgrade being an R31 disc brake setup for the rear and a Toyota 4 spot caliper and vented rotor for the front with a Patrol master cylinder.

This first upgrade had much better fade resistance and was overall an improvement but as the car got quicker the balance proved to be too much to the front and the rears really weren’t pulling their weight (up) so a new solution was in order.

Next major upgrade was the popular Arizona Zed Car conversion kit comprising 310mm vented rotors front and rear with aluminium 6 and 4 piston calipers respectively. This was classified as a "fast road or Autocross" setup and uitilised two piece rotors with aluminium hats and necessitated a new set of larger wheels moving from 15" to 16" diameter. This also opened up the option of Advan A050 tyres which we couldn’t get in 15" so seemed like a good excuse to change.

Despite the popularity of the setup I was somewhat surprised to find that this combination was now over braked at the rear and very prone to locking the rear wheels. Time to break out the calculator and science the hell out of this bad boy.

Before making any further changes we needed to do the maths and work out exactly what was going on in the hydraulic system and what the relative brake pressures were at each end. There are a number of online calculators for doing exactly this but the maths isn't that hard to run yourself if you want to better understand the relationship between leg pressure at the pedal and clamping force at the calipers.

In our AZC setup the wheel/tyre is the same front and rear and the brake rotor is also the same diameter with roughly the same radial location of the centre of the caliper pressure (basically the centre line of the caliper pistons.

This means that to work out the relative brake force we just need to calculate the caliper clamping force at each end which comes down to the hydraulic ratio. With the same sized master cylinder front and rear the balance would come down to the difference in total caliper piston surface area from front to rear.

Armed with a measuring stick and some maths we determined that we had close to 55:45 split front to rear so as soon as you introduce some weight transfer under braking there was way too much rear bias and the rears would lock.

The simple solution might be an in-line pressure reducing valve but some research suggested these would not give the linear change we needed so another plan was hatched. My Elfin runs a dual master cylinder setup with a mechanical bias bar so this seemed to be the best option but there were a few issues. Moving to a dual master would mean deleting the brake booster so the pedal ratio would need to be changed so that superhuman leg presses were not required, and also there wasn't an off the shelf item available so we would have to fabricate one or modify something.

Some Googling unearthed a generic setup from the UK that looked like we could make it fit so an order was placed. The unit was a nice bit of kit but the supplied masters were too long for the space and clashed with the inner guard and strut tower so some "shorty" AP masters were sourced to fit.

The change also required removing all the factory hard lines and components such as the proportioning valve and failure indicator so a new set of hard lines were constructed and the firewall modified slightly to fit the new setup. The brake pedal needed a change to the pivot point to reduce the required leg pressure and once assembled the next track day beckoned.

Unfortunately all the maths has failed us and with the bias bar set all the way forward we still had too much rear bias so another smaller front master was ordered to increase the front line pressure and a proportioning valve also installed in the rear brake line for good measure.

The final setup could be described as adequate – there is enough adjustment to give sufficient front bias (almost 70% of the total brake force) but the trade off is a longer pedal travel than ideal. Still, this setup worked for many years.

But we also started to see increased "pad knock off" where the pads get pushed back into the calipers making the pedal go long and necessitating a "comfort tap" on the pedal to bring it up just before a hard brake. This isn't an ideal situation and takes some brain power that could be better used to optimise the braking segment so we tried a couple of fixes.

First we ruled out overheating causing brake fluid to boil by installed bigger air ducts, changing to an expensive brake fluid and marking the rotors with heat sensitive paint. These measure assured us that we weren’t overheating and getting air in the fluid.

In addition we discovered that the knock off would occur only after cornering and not on a straight run which ruled out poor rotor run-out and started to suggest there was flexing in the stub axle or caliper mounts. Stub spacer / stiffeners were machined and installed and while we had everything apart we also installed light pressure return springs inside the calipers to exert a small residual pressure to help keep the pads in light contact with the rotors. All these things helped and while the pads were new and fat the issue was manageable but as the pads wore thin the issue got worse.

So this is where we are now – the brakes work fine but the pad knock off is still a pain although manageable. But the current problem is availability – the AP racing rotors we have been using are simply no longer available at an acceptable price. Enter MADDAT racing with some ideas!

MADDAT are helping us to switch to a new rotor and hat combination while moving to a full floating rotor design and incorporating a stronger steel caliper mount to reduce the flex and hopefully reduce the knock off.

Parts are ordered and the machines are working – watch this space, install and test coming soon.

Warning - Technical content!