Harrison brake bias - solved!

sambo · #1 (**permalink**) 08-27-2012, 08:03 PM

We've had a few discussions on the Aussie forum about Harrison brakes and various ways to maintain correct brake bias when pairing VT-VZ Commodore front brakes (original spec being the smaller VR-VS series) with R33 Skyline rear brakes.

My setup is as above (all new or reco'd), together with XF Falcon ute master cylinder which provides 50/50 pressure to front/rear. The car pulls up well in most situations, but my engineer was not satisfied with the rear brakes under very hard braking. ie, they don't lock up. The car tends to pitch forward a lot even under moderate braking, which suggests the rears aren't quite up to it.

With some handy work from Kenmer Engineering, I now have Holden Camira front calipers on the rear and the improvement is incredible. These are "only" single pot but the much larger brake cylinder and pad surface area have done the trick. The conversion involves machining the rear uprights and sandwiching an adapter plate between the wheel bearing and the hub. This allows the larger calliper to be mounted square on the rotor. I also fitted new braided lines (different ends) and a $21 set of pads. TRW make a race pad for $80.

Previously the car pitched forward as a result of exaggerated weight transfer to the front. Now the whole car hunkers down during hard braking, the fronts lock and then the rears. It's gone from "stops well" to "my eyes hurt" and I'm rapt.

The beauty of this approach is that a taller adapter plate can be machine up to accommodate larger diameter rear discs and/or calipers.

I have a spare set of R33 uprights so if anyone is interested in doing a changeover, shoot me a PM and we can work something out.

And a few photos...

Merv and Sharon · #2 (**permalink**) 08-27-2012, 09:35 PM

Well done Paul! I have stayed with the R33s on the rear but with the pads I have I can certainly lock them up.

So what is your next step towards registration?

sambo · #3 (**permalink**) 08-27-2012, 09:43 PM

Thanks Merv. All round brake performance benefits greatly from this upgrade. I'm actually glad the lock-up test failed or I wouldn't have been forced down this path - believe it or not.

Next step is to redo the brake test, pass the noise test and off the paperwork goes to VicRoads. So I'm nearly there.

Gav · #4 (**permalink**) 08-27-2012, 11:19 PM

Is your engineer happy with a fabricated mount though Sambo?
I asked about having upgraded calipers when I went through engineering, and even with the AP calipers that are approved, a fabricated mount isn't and would've been required to be tested as a separate item.
I left the Jag stuff on as I couldn't be bothered messing around, hopefully your engineer sees sense and isnt concerned.

BTW good work!

sambo · #5 (**permalink**) 08-27-2012, 11:35 PM

Gav, yes he was consulted along the way. It's 8mm plate with an additional mounting bolt and everything is high tensile.

Aussie Mike · #6 (**permalink**) 08-27-2012, 11:43 PM

That's some tidy workmanship there Sambo. Looks good.

Cheers

Tenrocca · #7 (**permalink**) 08-27-2012, 11:59 PM

Thats some history right there Sambo. Quite possibly the only remaining trace of Camira left on Australian roads....

sambo · #8 (**permalink**) 08-28-2012, 12:00 AM

Hey Mike. Full credit to Marty Govett for the workmanship. There's a fair bit of relief work on the underside - a full day's effort to get it right, including CAD and plasma cut brackets. I was on pull-apart-put-back-together-again duties.

Andrew you could be right about that! Apart from this one...

Modena · #9 (**permalink**) 08-28-2012, 08:17 AM

Well done Paul, great to hear she is stopping well now!

Is this with the OE spec Skyline disc?

Why did you choose Camira calipers? Is that some sort of common caliper or something where parts and replacements are plentiful?

Make sure you replace those $21 pads with something decent before track work or you will come in to the pits smoking!

sambo · #10 (**permalink**) 08-28-2012, 04:16 PM

Thanks Ben. Yes, OEM discs and I have a race spec TRW pad for track work.

There's a bit of history behind these calipers. Race Brakes fitted them to the rear of another Cobra several years ago (not a Harrison) and they worked well with VT-VZ front brakes. That owner has since done a big brake upgrade so we thought we'd stick with a known formula.

Howard at Race Brakes said he'd never pair VZ/R33 brakes on a Cobra - way underpowered at the rear and also very different weight distribution to a Commodore. That sealed it for me.

gjkrv8 · #11 (**permalink**) 08-28-2012, 05:04 PM

Paul, You know I reckon I have that problem with my stock VE rears that are paired with stock VZ fronts.

Up at Winton the last two times my rears got quite hot and the fronts were only mildly warm, and that was with my brake bias bar set about 75% to front. Have to put a brake upgrade on "the list" of things to do. Heading down to Yarra Valley Exhaust to get my exhaust upgraded next week and then Errol at EFI in lilydale the week after - in prep for Nationals.

Cheers Gregg

sambo · #12 (**permalink**) 08-28-2012, 05:43 PM

Sounds like you have the opposite problem Gregg - requiring a "downgrade" to your back brakes or even an upgrade to the front ones?

gjkrv8 · #13 (**permalink**) 08-28-2012, 09:03 PM

Paul, you might be right. I'd like to get some nice 4 spots for the front one day.

I think I will wind the bias a bit further towards the front and test up at the Nationals to see if I can get the fronts to work harder.

cheers Gregg

Philm · #14 (**permalink**) 08-28-2012, 11:37 PM

Gregg,
Have you tried swapping mastercylinders?

Phil

gjkrv8 · #15 (**permalink**) 08-29-2012, 12:31 AM

Quote:

Originally Posted by Philm

Gregg,
Have you tried swapping mastercylinders?

Phil

They are the same size. Was it the size you were thinking or the Cylinder might be faulty??

Aussie Mike · #16 (**permalink**) 08-29-2012, 12:44 AM

Quote:

Originally Posted by gjkrv8

They are the same size. Was it the size you were thinking or the Cylinder might be faulty??

Our older cars ran 3/4" front and 5/8" rear master cylinders. This can vary depending on what size calipers you have.

The theory is to work out the ratio of piston area between the master cylinder piston and the caliper pistons. The rear calipers are single piston and the front are twin or 4 piston so have more surface area. Oposed pistons calculate as a single piston in area so for a 4 piston calipers you take the total piston area and halve it they are opposed pistons.

When you know those ratios you can then work out the difference in ratios brtween front and rear and adjust the size of the cylinder to get you in the ball park of where they should be for the weight of the vehicle etc.

The balance bar is really just for fine tuning the front rear balance.

Cheers

Philm · #17 (**permalink**) 08-29-2012, 02:21 AM

As mike has said, i thought you had 2 different size master cylinders.

That is the good thing about the cr system, you can tune the brakes for a perfect bias.

Phil

Modena · #18 (**permalink**) 08-29-2012, 04:04 AM

the CR round-tube "manual' says 3/4" for front and rear.

gjkrv8 · #19 (**permalink**) 08-29-2012, 04:36 AM

Hmmm, yes 3/4 front and rear is what I have. Maybe too much braking force is going to the rears, anyway, I'll do some "tuning" soon. thanks for the tips. Sambo sorry to steal your thread. cheers Gregg

sambo · #20 (**permalink**) 09-08-2012, 12:21 AM

Good news, the Cobra passed the brake test yesterday with flying colours. We fitted a proportioning valve on the rear lines and it needed a couple of turns to get the bias absolutely spot on.

While the car was on the hoist I also upgraded to a HICAS lock bar that uses solid bushes instead of ball joints. Plus it's much easier to make toe adjustments and it looks nicer too - not that you'll see it.