Brake Bias Speculation

[Mechazawa]

So we know the 818 will have roughly opposite wight distribution from the donor car, and overly front weighted brake biasing is a common problem on mid engine cars.

What do we think FFR is going to do about it, or will we be on our own to figure it out.

Multiple master cylinder solution, custom stock style prop valve, aftermarket adjustable prop valve, just eliminate the factory prop valve, use the best wrx rear brakes with the most basic Impreza fronts. Lots of options here.

Anyone else care to speculate?

[PhyrraM]

It's hard to even begin to think about brakes when it's not known which spindles/knuckles FFR will use and at which end of the car they will use them at.

I wouldn't be surprised to see the same front assembly that is used on the Roadster and Type '65 with a fabricated adapter to use Subaru front brakes. The rear is still wide open also.

[Mechazawa]

I am kind of (and perhaps wrongly) confident that the stock Subaru uprights will be used, and that the fronts will be in front and the rears in the rear. I am guessing that the Subaru front axles will plug right into the Subaru rear hubs (maybe one of the Subaru guys can confirm or deny this) it would make sense from both cost and engineering perspectives. Not sure if the transmission is wider than the rear differential though so we may have to build hybrid front/rear axles anyway.

In any case using different uprights would add a lot of cost either for FFR or for the builder. I think the kit will come with four bent/welded steel pieces that bolt to the stock uprights just the way a strut would, but have a ball joint in the proper position. This is the type of fabrication FFR does the most of, very economically, and to a high standard.

I think the fronts will stay in front because they would have the lower ball joint and steering arm to allow for it. Of course I am only speculating.

[thebeerbaron]

I agree that the fronts will likely stay up front, the rears in the rears, and some hybrid axles will be used, just as you speculated. Knowing the differences in mounting points and rotors between the fronts and rears, I'm pretty sure the brakes will stay in the same place as on the donor. Following this long trail of assumptions, that'll put any bias-solving solutions in the hydraulic system.

It's pointless to speculate, but for fun I'll throw my hat in on Jim using aftermarket master cylinder(s). If not dual masters, then with some kind of prop valve. If you look at the brake booster assembly on the donor, it's a massive thing that doesn't seem to fit with the ethos of a lightweight, minimalist car. Guessing it gets binned. Not sure if you can use the donor master without that big booster.

[PhyrraM]

All good guesses.

I don't have quite enough stuff in my possesion to see if the axleshafts will plug right into rear uprights. But I know enough folks to borrow components if I can find time.

IIRC, FFR has already got a source for 'cheap' SLA uprights and could possibly implement them cheaper than an adapter. The common sense approach would to be using the Subaru front assembly at the rear and avoid any CV shaft fitment issues, but then you introduce brake fitment issues.

The best solution is likely custom CV shafts to fit the transmission output to the Subaru rear uprights, but at what cost?$?$

The one thing I am pretty sure of is that every component needed will be on a 2002-2007 Impreza or in the kit. I don't think FFR wants the eventual buyer to source anything outside of those two basic components. So, in addition to the frame and body, every bracket and adapter must fall into the $10K goal. It's gonna be tough enough as it is - much less appeasing the 'roofers' and 'windowers'. ;)

[Mechazawa]

I don't even think it will take custom axles, with luck the fronts will simply work with the rear hub, if not that, than we might just have to build shafts with front iners and rear outers, would just need boots grease and bands.

Looks at the parts pages on the website the factory five uprights are way more money than the ball joint adapters.

I don't know about ditching the booster even this car might be a little heavy for that. I once drove an MGB with depowered brakes and thought it was awful to have to work that hard to stop the car. My bet is that if the booster goes the whole master cylinder goes with it.

[mn_vette]

My guess would be the front axles will plug into the rear hubs, with the rear hubs in the back.

As for the brake biasing, I would have to guess FFR will throw in an adjustable proportioning valve. Since the car needs to work with any kind of brake setup that we decide to put on there, some adjustability would be needed. I believe this is what they did for the GTM, but please correct me if I'm wrong.

Anyone using the WRX ABS module will not have to put in a proportioning valve since the ABS module does that, but you may not have the ideal split for the car and may need to adjust the braking power by upgrading or downgrading certain components.

[Gollum]

Let's talk about weight distribution though. The 818 will have a fairly long wheel base by most of our standards, and the WRX drivetrain won't be taking up a whole lot of space or weight.

To the point: I could see this vehicle coming close to a 45/55 weight split, which isn't far off from many FR vehicles. And personally I prefer a little bit of rear weight bias, when it comes to brakes especially! :-D

[StatGSR]

simply put, a $40 proportioning simplifies and solves this problem with ease regardless of what brakes you get from your donor car.

also, would anybody actually like to make the 818 with ABS? (keep in mind the subaru ABS pretty horrible even on the cars it was designed for)

[BrandonDrums]

I agree that the fronts will likely stay up front, the rears in the rears, and some hybrid axles will be used, just as you speculated. Knowing the differences in mounting points and rotors between the fronts and rears, I'm pretty sure the brakes will stay in the same place as on the donor. Following this long trail of assumptions, that'll put any bias-solving solutions in the hydraulic system.

It's pointless to speculate, but for fun I'll throw my hat in on Jim using aftermarket master cylinder(s). If not dual masters, then with some kind of prop valve. If you look at the brake booster assembly on the donor, it's a massive thing that doesn't seem to fit with the ethos of a lightweight, minimalist car. Guessing it gets binned. Not sure if you can use the donor master without that big booster.

The OEM 2 stage booster is why Subaru brakes feel like oatmeal, they are notoriously over-boosted so a smaller unit would be very easy to implement. A simple after-market unit with more rear brake bias would make sense on many levels.

[PhyrraM]

Cetain older Imprezas (maybe others) with the 1.8 liter motor (maybe others) came with a single stage booster that would be a bolt in for any application designed for the 2 stage booster.

[Scubynubie]

I'd say that the simplest path would be to use a modified hub that allows you to keep your front axles attached to the transmission, but to go through the rear hubs so you could still use the WRX rear brakes in the back, and then use a modified front hub and knuckle that will let you keep the WRX front calipers. That would be ideal because replacement parts are readily available, and you have a lot of options.

Up front, the WRX rotors have been the same from 2002 - 2011, so you can bolt on the Subaru 4-Pot calipers (V6 STI/2006 - 2007 WRX) and use the OEM sized rotors. The Brembo calipers from the STI also are direct boltons to the WRX hubs with 04 STI rotors, though I would guess that would be overkill for a 1800 lb car.

In the rear, there is a cheap H6 up-grade that uses the same rear caliper, with a larger diameter rotor from an H6 Legacy, or you can adapt the 2-Pot rears, or even the STI Brembo rear calipers.

So with those options, even with the stock master cylinder there should be a combination that will give good stopping.

Throw in an adjustable proportioning valve, and you should be all right.

Also, FWIW, even if you lose the power assist, you can get performance back within reason by going with a more aggressive pad compound. That would be more for a track only car, but it has worked in Rally prepped WRX's. Though I will say driving a WRX with no brake boost is pretty spooky the first couple of stops...