Fiat 125 rear brake calipers on 124 ?

[tiaf124]

Trying to solve the poor handbrake performance in my 124 i was thinking of the Fiat 125 rear calipers instead of the 124 ones.They look identical with the only difference of a bigger diameter piston in the 125 which is 38mm instead of the 34 mm in the 124.
Now there are 3 issues
1.Is it a straightforward install or further modification is needed to install the 125 calipers ?
2.Do Fiat 125s had better handbrake performance than the 124 ?
3.Do i have to adjust the rear brake regulator since the 125 pistons are bigger and if so how ?

If it isn't possible what else can i do to improve the handbrake performance ?
Thanks in advance.

[kmead]

The issue is not the size of the piston, it is the amount of force you can apply using the hand brake. The Fiat arrangement doesn't allow enough force to be applied to the pads to properly grab the disk.

Something one might look at is increasing the ratio of the hand brake pull to piston movement by which I mean more pull on the handle creates less movement of the piston. I am not sure how this would be accomplished, one needs to change either the length of the pull rod on the caliper or the hand brake lever end.

I have suffered this problem as well having watched my X1/9 roll down my driveway and roll across the street.

My VWs have very effective disk type emergency brakes unlike my Fiat versions.

[tiaf124]

So most rear disc braked Fiats ( 124s ,125s, x1/9s) suffer from poor handbrake as most of them share the same rear brake caliper type!

by which I mean more pull on the handle creates less movement of the piston. I am not sure how this would be accomplished, one needs to change either the length of the pull rod on the caliper or the hand brake lever end.

More pull of the handbrake means less movement of the piston i thought the opposite!! the more you lift the brake lever the more the rear brakes are enganged or am i wrong?

[kmead]

Sorry I wasn't clear. Yes that is correct but once the pads are in contact with the disk the caliper piston isn't moving much farther. Hand brakes or emergency brakes are mechanical, not hydraulic so they work by moving a lever which moves the piston outward to compress the pads against the disk. It is due to this reason that the diameter of the piston doesn't matter in regards to hand brake performance and why you don't need to look at a larger rear caliper. The hand brake doesn't work well right now because you don't have enough mechanical advantage with the amount the hand brake handle moves relative to the amount of movement of the piston against the pads.

This is a function of the ratio of movement of the cable at the end of the handbrake lever to the amount of movement of the piston. By reducing the relative amount of movement of the piston with the same amount of motion at the hand brake lever, you would be increasing the amount of force you can apply to the pads to grip the disk. Effectively "gearing down" the hand brake motion to increase the force you can apply.

To do this either the attachment of the cable to the hand brake needs to change or the length of the lever at the caliper needs to change. The hand brake attachment would be the easier and safer of the two to modify.

Alternatively one could go with grippier pads to increase the friction at the disk but that will effect wear and I am unsure of much availability of softer, grippier rear brake pads.

[tiaf124]

Thanks for the clarification

To do this either the attachment of the cable to the hand brake needs to change or the length of the lever at the caliper needs to change. The hand brake attachment would be the easier and safer of the two to modify.

According to my 124 service manual there is an adjusting bar (or cable rod as other manuals name it)just in front the return spring and the cable equalizer in the middle of the floorpan and there is also the hand brake rod which connects the hand brake lever to the control arm ( where the lift of the hand brake lever transforms to pull or push wire movement ) .Which of these parts needs to be modified... shortened or do i just need to shorten the length of the wire it self

Alternatively one could go with grippier pads to increase the friction at the disk but that will effect wear and I am unsure of much availability of softer, grippier rear brake pads.

Somebody once suggested a pack of a worn pad along with a new stock pad altogether as a solution for a thicker stuff for the rear calipers

[kmead]

Clearly the first thing to do is to ensure it is properly adjusted. I presumed you had already adjusted it.

I would recommend jacking the rear axle up to ensure the cable is equalized in the cable holder and to know that you have them both working. Of course using jackstands under each spring perch of the rear axle to support it while working.

On my VWs I set it up for two clicks before the brakes lockup on the third click. On my Fiats I tend to set it up for 4 clicks to try and get more engagement/force on the pads.

I wouldn't make changes to the mechanism at this point.

If your were to make a change to the mechanism I would make changes to the bell crank the pull rod attaches to. To achieve this one would lengthen the long arm of the bellcrank to increase the amount of force you can apply to the cable without deforming the hand brake lever. Alternatively the short arm could be made shorter. (drill a new hole closer to the pivot of the bellcrank.

Karl