Vitara caliper - smaller than the standard SJ caliper!

[Anton]

Heya, peeps.

I bought some vit calipers on ebay a while ago - 4 of 'em, for about £30ish. Not complete, but I made a full set and kept the rest for parts. New pads, and just ordered new seals etc from brakes international.

The issue is, according to brakes international, the "upgraded" callipers off the Vitara have SMALLER pistons than the Asin calipers I bought them to replace. Here's the info I nicked from brakes international's webbie:

Tokico (Vitara):


Asin (SJ413):


So the SJ calipers have a piston diameter of 51mm (2042.82 square mm), the Tokico's have a piston diameter of 48.1mm (1817.1 sq mm) meaning that the Asin calipers have 12.4% greater surface area, and therefore 12.4% more torque/stopping force.

So if I put the Asin's up front, I should get 12.4% more braking up front, right? That doesn't seem like enough to balance the system to me.

The Grand Vitara calipers I *SHOULD* have bought (Sumitomo) have 57.2mm (2569.6949564sq mm) pistons - 25.8% larger than the SJ's Asin's, 41.4% more than the Vitara's Tokico's.

So my options are: 12.4% front bias with what I have, which seems like *asking* to have the rears lock up before the fronts, 25.8% bias to the front by buying grand vit calipers, which seems more like it (although still seems to be a bit too rear biased - 33% seems more reasonable to me, but I can't tell you why!!!) or 41.4% bias to the front by going Grand Vit front, Vit rear.

Or... Buy an adjustable brake bias valve for £40 or so, use the callipers I have, and adjust the rear brake balance until it's *exactly* right and the fronts lock up *just* before the rears in the wet, and before the rears in the dry.

I'll be running 31x10.5x15 BF Goodrich Long Trail T/A road tyres on the road, which normally go on LWB shoguns (double the weight if not more) so I'll have grip coming out of my ears - but I have no idea how easy it would be to lock them up, even if my contact patch with the asphalt is much bigger than either calliper was designed for. Especially in the wet...

I'm leaning towards the bias valve, although I have conflicting information regarding their legality - some say they're illegal, others say they're only illegal if they're within reach of cabin occupants, and that fitting one under the bonnet is perfectly legal.

Opinions on what I should do here would be most welcome, especially if any of you lovely lot have concrete info on adjustable bias valves legality.

[Anton]

I did some research into this, looking into vehicles that come from the factory with discs on all corners, and found that most are much, much more biased toward the front brakes than I realised.

For example, a Defender 90 with rear discs has the same size pisons front and rear, but it has single piston calipers at the back, double piston calipers at the front - meaning it's double the brakes up front. A BMW X1 has 80% more up front. A Toyota Land Cruiser LJ70 is 100% up front.

I looked mostly at 4x4's, because those seemed most appropriate for the research, but also looked at the Ford Sierra 4x4 Cosworth (because some use their calipers in the conversion - handbrake built in) which was 96% toward front, and the Rover 400 (again, because people use their rear calipers) and found it was 218% more up front.

None of this takes account of any bias valves installed in these vehicles, if they have them. I don't think they do - I *think* they just use the difference in caliper sizes to make the difference. But I can't confirm that for sure.

Anyhow, this led me to do a bit of calculation regarding the options most SJ users use when going for a rear disc conversion - and what biases this would give. This is what I came up with:

Calipers Front Rear Bias (%) Front to rear bias ratio
Ford Sierra RS Cosworth 4x4 60.0 42.8 96.52% 66/34 (factory spec, for reference)
Rover 400 96-00 53.9 30.2 218.54% 76/24 (factory spec, for reference)
Asin front, Tokico rear 51.0 48.1 12.42% 53/47 (what I have)
Grand Vit front, SJ rear 57.2 51.0 25.79% 56/44 (what most people seem to fit)
Grand Vit front, Vit rear 57.2 48.1 41.42% 59/41
Vit Front, Rover Rear 48.1 30.2 153.67% 72/28
Vit Front, Ford Rear 48.1 42.8 26.30% 56/44
SJ front, Rover rear 51.0 30.2 185.18% 74/26
SJ front, Ford rear 51.0 42.8 41.99% 59/41
Grand Vit front, Rover rear 57.2 30.2 258.74% 78/22
Grand Vit front, Ford rear 57.2 42.8 78.61% 64/36

[ScottieJ]

I never checked the size of the vit calipers before slapping them on the front of mine, I just needed some calipers and had them lying around. I did find out they were smaller a whole ago though but still left them on. To be honest I don't think it matters too much on an SJ as long as you havent got a bias to the rear they aren't exactly sports cars and for offroading the bias won't really make much of a difference unless you are racing and doing lots of heavy braking, SJs suffer from so much brake dive in emergency stops the back goes light and locks up anyway even with the stock brakes.

[Anton]

Well, I decided to shove what I have on my SJ, drive to a road near me that leads nowhere, has no houses on it, and therefore tends to be very, VERY quiet, and see what happens when I jam my foot on the brakes.

I've ordered an adjustable brake bias valve too, but to use that requires I rework the brake lines into a Samurai configuration - one master cylinder output to both rears, one to both fronts, instead of one to front left brake and rear right brake, the other to front right and rear left.

We'll see how it goes.