1.75" 4-pot on vac boosted 23.8mm master cylinder ?

[jupilerman]

Hello!

I recently purchased 4 pot calipers with pistons 1,75" (44,45mm) in diameter. To be used on front wheels

OEM caliper: 1 piston, floating type, 48mm (1.89")diameter

Master cylinder : 23.8mm (0.95")

Vacuum booster ratio: 4.6:1

brake pedal ratio: unknown

From what I calculated I will need to displace 70% more hydraulic fluid. I don´t know what that will do to my pedal travel. But I believe it will have to be floored. So I believe I bought unsuitable calipers (pistons)

Can anyone confirm or better denie?

Thanks in advance,
david

 

[patprimmer]

I guess you did, unless you think you can cut a hole in the floor to allow for 70% more travel.

Regards

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[jupilerman]

Well I thought is wasn´t that straightforward!
BTW, Measured the brake pedal ratio which is 3,3 : 1

 

[patprimmer]

Seriously

The booster ratio, and pedal ratio have little to do with it, except for their effect on pedal travel.

The limiting factor except for floor board impact will be the master cylinder bottoming out.

A larger bore master cylinder might fix the front brake problem, but will not correct the change in the bias between front and back brakes.

Bottom line, you need smaller bore calipers, or larger master cylinder and larger rear brake calipers.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[jupilerman]

>>The booster ratio, and pedal ratio have little to do with >>it, except for their effect on pedal travel.

From reading this sentence, I´d say they do have effect on pedal travel!

Point taken re the bias though.

Don´t know the stroke of the MC, but I believe it will not have 70% more volume on board than it has now.

kind regards
david.

BTW: I´m not an automotive eng. but Env./civ. How do you change the caption on the name. It doesn´t show in the "change profile" part

 

[CoryPad]

You should contact the site administrators using the "Contact Us" link in the upper right corner.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[patprimmer]

Booster ratio has nothing to do with travel, just load.

Pedal ratio will have very little effect on it as the MC will bottom out first if you change the pedal ratio.

You will only fix the problem by reestablishing the appropriate ratios of surface area of all the pistons involved in the hydraulic system.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[jupilerman]

Agreed. Booster decreases the load to be applied to the pedal to get the same pedal/MC travel as w/o booster. Should have known this!

Off course it is merely the pedal lever that determines the travel distance of the pedal relative to the plunjer. If the plunjer as you say doesn´t bottom out first.

Now, I have searched the internet for some papers or guidance to do these calculations. I have found some (

http://www.stoptech.com/tech_info/tech_white_papers.shtml).

Perhaps you know other interesting information.

Kindly appreciated.
david

 

[jpr2004]

David,

How did you make out with the 4 piston calipers?

I was interested in your calculations, if you had treated the single piston caliper as two when comparing it to a pair of opposed piston. It appears that you did.

As already mentioned, you can swap to a larger bore mc (master cylinder). I did a similar swap on my daily driver by installing the factory racing calipers & large rotors. Also installed were Goodrich stainless steel braided hoses.

After the upgrade, there may have been a bit more travel, but when the brake pedal got stiff, it was not moving anymore, period. No problem with it hitting the floor. So that concern was not an issue.

Now onto the real issue. If you want to keep the balance correct, you would need to use an after market distribution block with a balance adjuster. Since my car had four wheel anti-locks, i decided against these changes to the distribution/balance and let the anti-locks work over time. If the driver stood on the brakes, the front anti-locks would kick in.

I did upgrade the rear brakes to larger vented rotors instead of the solid rotors and larger calipers and got the balance back.

 

[jpr2004]

One last note, regarding liability. When I sold the car, the entire brake system was returned to factory parts. Even if the car stops better, in the event of an accident with someone else behind the wheel, you could be first on the list of defendants.

 

[jupilerman]

Hi JPR!

I did some more research and came up with a spread sheet that calculates the brake torque requirement, hence clamping force dependent upon the rotor diameter.

Using 1.75" pistons in the front, lead to massive shift of required line pressure. The rears would need way less pressure, hence they´d lock up all the time.
So certainly not a wise option.

Additionally an increase of 70% in piston surface does not equate a linear increase in pedal travel as well, but probably half of this linear increase. Didn´t actually calculate pedal travel this (travel to actuate pad ect is µm figures, so error margin needs to be small, hence IMO useless to calculate). But Since you have a bigger surface, required clamping force is reached sooner (F*surface)and with application of a smaller pedal force (I calculated this to be merely 8kg)

So the calipers are for sale and I am looking at a 1,25" to suite 300mm discs or 1.36" to suit 11" (280mm) discs. Upgrading the rears to 280mm would balance hydraulic pressures again. Meaning I could keep the existing pressure valve installed in the rear hydraulic lines.

In doing these calculations I found it strange that with the numbers I use (deceleration, hight of center of gravity, pad/rotor µ, etc) rear line pressure is slightly higher than the front pressure (64/52psi). I don´t know what variable causes this. Calculated Brake torque with 0.8G deceleration seems to be correct (front 45/ rear 55)for a mid engined car.

A balance bar is not an option, as my car was meant for RHD,
and the MC still is in the RHD postition, to convert to LHD, they introduced a rod leverage thing that actuates the MC from the LH side of the car. Kinda diffucult to explain.

HTH,
david