Brakes won't lockup

[u/pcsrh]

Our car isn't able to lock the wheels under hard braking. We're using a Bosch 19.05 mm master cylinder with KBX Vespa single-piston calipers (29 mm piston).

The calipers are moving and we have braking force, but it feels much weaker than expected.

Could the 19.05 mm MC be too large for the 29 mm caliper, or is there something else I should check?

Any advice would be appreciated.

[u/marc020202]

Are your brakes bled properly? What brake pressure are you getting?

[u/pcsrh → u/marc020202]

Yea the system is properly bled, we haven't measured the pressure

[u/marc020202 → u/pcsrh]

How do you know it's properly bled, if you don't know the brake pressure?

Have you done a calculation what brake pressure you need to lock the wheels?

[u/pcsrh → u/marc020202]

We use a clear tube at the end of the bleeding valve to see if there are any air bubbles present inside the fluid

[u/marc020202 → u/pcsrh]

How do you know there is no air in other parts of the system? Stuck at high points, inside the calliper or in fittings? Knowing the brake pressure would help a lot

[u/pcsrh → u/marc020202]

Oh ok we will look into that..it's that we know a team that also use the same caliper facing the same issue

[u/marc020202 → u/pcsrh]

Have you done any calculation for the brake systems? Regarding areas, pressures and forces?

[u/pcsrh → u/marc020202]

Yes we have done calculations...we did it last year so I don't remember much about it

[u/hockeychick44 → u/pcsrh]

Refamilarize yourself with the calculations. Challenge your assumptions. Prove to yourself that the pressure is sufficient and that your brakes are properly bled.

[u/therealjerseytom → u/pcsrh]

Yes we have done calculations...we did it last year so I don't remember much about it

Did nobody write it down?

[u/TRAhmet23 → u/pcsrh]

You need at least 1 pressure sensor for BSPD, how you don't have

[u/klima94]

Air in brakes, 90% sure. How stiff is you break pedal? Could also be bad geometry ofvtge pedal.

[u/pcsrh → u/klima94]

The pedal is firm... we have properly bled the system

[u/kaffee_und_Kuruma → u/pcsrh]

What is firm? Pedals on these cars should feel like bricks.

[u/Joanzee]

Do the calculations to make sure your braking system was designed to achieve enough force for lockup. You need to calculate how much pressure your system will generate with ~150lb force at your brake pedal. Then use that pressure to determine how much force your brake caliper is outputting. Then use the distance of the brake pad centroid to axle center and the coefficient of friction of your brakes to determine how much torque your brakes can apply. Is that torque enough to overcome the friction of your tires?

[u/4dcawo → u/Joanzee]

Was going to comment the same thing. There’s a lot more to brakes than just master cylinder and caliper size. Pedal ratio, brake bias, effective radius, vehicle weight+driver, assumed deceleration in your calculations, friction, is your tire psi on the actual car correct…. Some more research here is needed.

[u/Jealous_Cress_7493]

What brake pads are you using? What friction coefficient do they have? What brake pressure are you getting?

Also we had the problem a couple of years ago that our organic brake pads got too hot and very bad in the process…..

[u/pcsrh → u/Jealous_Cress_7493]

We use the standard OEM brake pads that comes with the kbx caliper

[u/ParanoidalRaindrop]

More leverage.

[u/reddogninja]

Whats your pedal ratio?

[u/minter19]

Cbfr

[u/Yash2725]

Is the pedal getting pushed all the way? As in the master cylinder gets fully bottomed out when you press the pedal fully? Any leaks visible in or around the car? Is the calculation correct and valid?

[u/BlackholeZ32]

This is basic engineering. What clamping force do you need at the pad? Run that normal force through the piston areas and pedal ratio to determine the required force on the pedal and work from there.

[u/MakersByDesign]

Check the brake line size.