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 Post subject: consideration for brakes
PostPosted: December 10, 2020, 9:20 pm 
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Joined: November 22, 2020, 10:03 pm
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In the early stages of dreaming.

What consideration do you make for rear brakes when using a fwd platform in the rear? Is it enough to run the rear disk and calipers in the rear on the front spinals with the added weight in the back? I know most of the stopping should still be on the front, but curious if you need to think about larger brakes in the rear, there's going to be less weight transfer to the front when braking, so it seems like bias might need to shift to the rear slightly... thoughts


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PostPosted: December 10, 2020, 9:50 pm 
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Joined: November 22, 2020, 10:03 pm
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or larger front rotors and calipers all around with a brake bias adjuster? and some sort of hydraulic line lock for parking brake? but later might not pass a vehicle inspection for ubuilts.


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PostPosted: December 10, 2020, 10:05 pm 
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I don't have a definitive answer for you. Consider that these cars are ~1/2 the weight of the donor's. Brake performance is hardly a limiting factor no matter what size you chose. Brake balance, on the other hand is important. I seldom see anyone using a brake booster. Somewhere on this site is a good spreadsheet addressing brakes and sizing IIRC. Maybe someone can link to it. I never used it myself.

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PostPosted: December 10, 2020, 10:23 pm 
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Link to the brake spreadsheet: http://www.locostusa.com/forums/viewtopic.php?p=40140#p40140

You can probably avoid re-sizing the donor fwd brakes on the rear via proper selection of master cylinders and a prop (limiting) valve.


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PostPosted: December 11, 2020, 12:10 pm 
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SeattleTom also made a few corrections and updates to the brake spreadsheet, as found here: http://locostusa.com/forums/viewtopic.php?f=5&t=14424

I think you'll find it rather difficult to get the rear brakes from the FWD donor to mate to the front spindles. So I believe most just keep the 'front' brakes when the move everything rearward. I haven't been paying close enough attention to any in recent memory to recall what is typically done for the parking brake.

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PostPosted: January 5, 2021, 10:00 pm 
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Joined: January 5, 2010, 12:13 pm
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Location: Little Rock, AR
I recently completed a mid-engine project (replica of a McLaren M8B CanAm car). I used the exact same master cylinder size/rotor/caliper combination for all four corners. This was based on estimated CG, and that effect on braking. It was petty close in that the brake bias adjustment is just slightly (Just over a single turn) off center. This was a track setting where the weight transfer is greatest. On the street is was almost perfect balance without the adjuster. Weight distribution was 39/61.

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PostPosted: January 6, 2021, 12:53 am 
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The answer to the OP's question is: Yes, though it depends almost entirely upon the application. If you do car show cruises, it doesn't matter. If you drive to lunch, it doesn't matter. If you do autocross only, it doesn't matter, or if it's got 140 hp. But the generated heat goes up with the speed squared, so if you're running high power, and using it, then it does matter. Than again, if you're only going fast in a straight line, it again doesn't matter. So yeah, "it depends."

On my Midlana (mid-engine "Seven" with turbo Honda K24 drivetrain), at up to about nearly 1G deceleration, the rear brakes are doing more work than the fronts. I initially ran with stock Miata discs for a while, then later went to the "big brake kit" offered by the V8 Miata site. Expensive—and you can certainly piece it together yourself cheaper—but very, very effective. Equal size discs all around, effortless braking, and never any fade, even at California Speedway (a NASCAR track) running up to 150 mph and 450 hp.

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PostPosted: January 6, 2021, 11:38 am 
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blueovalz wrote:
brake bias adjustment is just slightly (Just over a single turn) off center.

Adjusted for more rear or front bias?


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PostPosted: January 6, 2021, 1:06 pm 
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Yeah, good question. It is currently (for the track) biased toward the rear brakes, which is confirmed by my observed temperature measurement of the rotors after each run. Didn't record the actual temps, but believe the rears were about 75-100 degrees hotter at time of measurement. Comments made earlier are so true in that there are a LOT of inputs/variables that impact this, so it's not a one-size-fits-all. With the 39-61 distribution, an unknown at this time CG, can't recall my longitudinal roll center (dive/squat), and tire sizes (245/335), and compound, this is just a reference, and was surprisingly close to the calc values I made when I was building the design 12 years ago.

One thing I failed to mention was I'm running a different compound front to rear on pads, using the DTC60 and DTC70 pads, but cannot recall which end has which pads. Thinking now I need to look at that and experiment with swapping ends. Just a thought after responding to this question.

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PostPosted: January 6, 2021, 1:13 pm 
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Another consideration is you would not want to downgrade F brakes used in the rear because you need enough brake power to counter the motor power.

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PostPosted: January 6, 2021, 4:39 pm 
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Bent Wrench wrote:
Another consideration is you would not want to downgrade F brakes used in the rear because you need enough brake power to counter the motor power.

Unless I'm missing something, why would you ever press on the accelerator and the brake pedal at the same time?

I'm in the planning phase of building a mid-engine Locost type car using a FWD drivetrain moved to the back, and have thought about how to address the rear brakes. In order to have a parking brake, my plan is to mount the rear calipers from the donor car onto the front hubs, using a custom adapter plate. I would also have to use the rear discs from the donor car on the front hubs, as the vented front discs are quite a bit thicker then the non-vented rear discs, and would likely be too thick to fit in the rear calipers.

Option 2 is to keep the front discs and calipers as is, and just add a mechanical "spot caliper" for the parking brake. This is similar to what I have seen on the Tesla Model S. Here is a link to the page on my website discussing this:

https://midixsportscar.wordpress.com/re ... nd-brakes/

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