Big front brakes

Adam · Site Admin Joined: 04 Oct 2001 Posts: 682

Been looking at a brake upgrade to skyline R32 GT-R (non V-spec)/R33 GTS-T rotors, DBA part number 926, these are 296x32mm which makes them pretty much the thickest rotor for the given diameter, they have the same centre hole size as a gemini and are available undrilled from DBA, the problem is the 'height' of the rotor is 7mm greater than a gemini, meaning the inside of the inner friction surface sits 7mm closer to the hub.

Fortunately due to their larger inside diameter most of the hub basically sits in the 'bell' section of the rotor, *except* for the arm of the stub axle which goes forward for the tie-rod end to attach to.

Now this arm runs parallel to the rotor for a fair way and then bends away about an inch before the end, if it bent inwards an inch earlier, it wouldn't be a problem, but as it stands it looks like it will hit the rotors (they are on special order to get them undrilled).

I've drawn a line on my stock rotors showing the inside diameter of the skyline rotor.
http://www.hotgemini.com/images/workshop/56_brake%20inside%20diameter.jpg

Currently I've got three thoughts, there is some room to machine some metal off the inner side of the hubs, but that material also constitutes the backing to the countersunk holes which attach the rotor to the hub so I don't want to remove too much there.
http://www.hotgemini.com/images/workshop/56_brake%20machining.jpg

The next option is to machine some material from the arm of the stub axle, but I'm probably looking for around 4mm additional clearance and I couldn't safely remove that much metal.

The final option is to slightly bend that arm of the stub axle, which is entirely practical and possible, but I wouldn't want to bend it too much, as this will alter the ackerman angle.

So my current thought is to go for a combination of the three, machine a millimetre or two from the hub, slightly bend the stub axle, then gently mill a millimetre or two off the arm if necessary.

With the exceptions of posts of horror re: bending the stub axles, does anyone have anything constructive to add?

GasGem · backyard mechanic Joined: 05 Nov 2001 Posts: 755

Assuming you would be mounting these disks to Gemini hubs,

Machine a little of the hub as discussed
Machine a little of the Rotor ( Mounting face)as well,

then look at bending and maching the arm,

From memory if you could get some Piazza Hubs they would give you a little more room,

Cheers
Eggy

Adam · Site Admin Joined: 04 Oct 2001 Posts: 682

Hi eggy,

Do you have any details of the difference of the piazza hubs or a set to measure, I'd be very interested, any machining which can be avoided is worth twice its weight in hassles saved.

GasGem · backyard mechanic Joined: 05 Nov 2001 Posts: 755

You are so so lucky I just took the front end apart yesterday, so I can replace all the bushes and ball joints,

I'll get some pic's and dimm's for you,

cheers
Eggy

Adam · Site Admin Joined: 04 Oct 2001 Posts: 682

Much obliged, if the mounting surface for the brake rotor is further outboard than a gemini hub this could save a lot of hassles. It isn't particularly important, but do the piazza rotors/hubs share the same 4x88mm PCD of the gemini hubs and rotors?

GasGem · backyard mechanic Joined: 05 Nov 2001 Posts: 755

will have to look into that, had been advised that they don't, but will check all tonight,

cheers
Eggy

GasGem · backyard mechanic Joined: 05 Nov 2001 Posts: 755

Photo's emailed to you,
Hub bolt pattern appears the same,
about 10-12mm room between steering arm and rotor,

Cheers
Eggy

Ben Wight · backyard mechanic Joined: 04 Oct 2001 Posts: 946

Hey Adam,

Assuming the piazza hubs are no help, I would be inclined to try and machine a bit off the hub, and even less off the arm, unless there is a lot there to be taken.

I would think that not only would you disrupt your ackerman angle, which may play havock whilst circuit racing, which is what I assume you are eventually planning, but I just dont like the sound of bending the arm.

Incidently though, I was told a lot of race cars dont abive by the ackerman principle, and go with a completely straight steering geometry, if you get what i mean. I still think though that the ackerman principle can only help cornering.

Any, sounds like piazza might be best bet. Wonder how hard a set of hubs will be to get.

Ben Wight.

Loz · backyard mechanic Joined: 05 Oct 2001 Posts: 943

Just curious as to why you are looking at the skyline rotors?
If you havent already had a look then I would consider using DBA 807's (can be bought undrilled). I have used them before with no dramas concerning clearance.

What are you planning on useing for calipers? Would you stick with the brembos from the skyline?

At the moment we have a setup in the works. Mainly for track use. I'll keep you updated.

Loz

Adam · Site Admin Joined: 04 Oct 2001 Posts: 682

To be 100% entirely honest, I was attracted to the 926's over what I considered to be the other three options, 807's, 241's and 924's, primarily for looks, it is no secret that the 926's are more front brake than a gemini is ever going to need.

In short, it is a wank... but I'm much happier being a wanker because of my obsession with overdoing it, rather than because I glue fibreglass all over an otherwise stock gemini.

Adam · Site Admin Joined: 04 Oct 2001 Posts: 682

Got 'em, will have a close look at them around lunchtime today and get back to you then.

Adam · Site Admin Joined: 04 Oct 2001 Posts: 682

Had a good look at the photos and it doesn't look positive, I couldn't say for certain without sitting down with verniers but I'm prepared to say that the gemini hubs and piazza hubs are dimensionally the same in terms of the location of the mounting face for the brake rotor with 95% certainty.

The difference in the amount of room available at the rear of the rotor is because the piazza rotors are 15mm less deep than the gemini rotors.

GasGem · backyard mechanic Joined: 05 Nov 2001 Posts: 755

Dammm.

So what is Plan C?

Interesting thought tho is you could just bolt Piazza rotors straight on then? DBA do stock them,

just need to find a caliper,

Cheers
Eggy

Adam · Site Admin Joined: 04 Oct 2001 Posts: 682

Any part numbers on that caliper?

Mark · tinkerer Joined: 04 Oct 2001 Posts: 153

Hey Adam, youve seen my conversion, it works very well. Ive shown some of the other guys on this site how to do it and they seem to agree. My car now runs a Big turbo and nos, (rolls 327hp at rear wheels) and the brakes are more than adequate to pull it up time and time again. You may go for bigger rotors for more wank factor, but that generally requires bigger calipers, which takes more fluid to move the bigger or larger number of pistons, which then requires a larger master cylinder, and then you have to find a different bias valve as the rear gets "taily" etc etc. Sounds like a lot of work when you already know how to do a setup thats been tried and tested and costs bugger all to do........