Running four front calipers?

carl

carl

True Classic
Has anyone used front brake calipers at all four corners? The race X came that way although with a manual brake bias adjustor. While I wait for new e-brake cables to show up I thought I would run my current X with those four same calipers (without a bias adjustor). Wouldn't this make sense as the X has a rear weight bias, certainly as compared to a 124 spider which runs the same rotors and calipers as an X.

This turkey isn't ready for the road but hopefully in the next month or so I should have it ready for tags and road use.
 
In my opinion...

carl said:
Wouldn't this make sense as the X has a rear weight bias, certainly as compared to a 124 spider which runs the same rotors and calipers as an X.
Click to expand...

The static (just sitting there) weight bias goes out the window when you stomp on the brakes. Weight shifts dynamically to the front under hard braking, and the few hundred lbs of static weight up front in an X vs a 124 won't make much difference.

With bigger pistons in the rear and no other changes to front pistons or master cylinder, I would expect that your braking distance would increase somewhat and be less stable. My line of thinking is that you have the same amount of fluid leaving the line and entering the larger piston area, resulting in diminished pad pressure/friction in the rear. This leaves the front to do proportionally more braking. But I'm not certain on my thinking.

Bottom line is, Fiat engineered the brake system the way they did for a reason, choosing the best compromise in a variety of braking situations for maximum safety. If you decide to run the car this way, I would take it to a big parking lot and try to test it safely under as many conditions as I could to get a feel for its braking characteristics, especially at impending lockup.

:2c:

Pete
 
Your right Carl

The X19 is biased heavily to the front. That way you hit the wall with the front of the car, it really is safer on the street. By putting the larger front calipers on the rear and a proportioning valve on the rear brakes you can dial them in perfectly and it really decrease the braking distance. The other way is to just put a proportioning valve on the front, that works just as well but it increases the pedal pressure. The only disadvantage to the front calipers on the rear is the loss of the parking brakes. Another solution is to put the Scorpion rear calipers on the rear, they are larger than the x19 calipers and you will still need to put the proportioning valve on the front brakes as the front will still lock up first but this will help the pedal effort problem somewhat. For the street stock is the safest but for racing the change is required. Just remember either way you go some proportioning valves will work on the rear only, like Tilton, and some can be used at either end.

Good luck
Charlie
 
Thanks guys. I am aware of the physics of weight shift on braking but was just curious on input from forum members. When MWB gets my e brake cables in I'll go to a proper stock caliper layout and this will hopefully happen before the car is properly roadworthy.
 
Not true, the exxe has it's weight biased in the rear. About 55-60% rear, 40-45% front. Not a front engine chassis or layout.

As for using front brake calipers in the rear, it works fine. We did this on the LeMons racer and a host of other exxe racer folks have done this. It is similar to installing 38mm rear brake callers in place of the 34mm rear brake calipers in the rear for a street-road exxe.

This shifts the brake bias towards the rear where it is needed to achieve better front to rear brake balance, significantly prevents cooking the front brakes and helps turn-in under braking as it reduces front tire loading.

If possible leave the current dual front brake caliper with limit valve set up. Try it and see how that goes. Going back to the stock 34mm rear brake caliper returns all those original brake bias problem.


Bernice


Old Fiat Driver said:
The X19 is biased heavily to the front. That way you hit the wall with the front of the car, it really is safer on the street.
Charlie
Click to expand...
 
Bernice, can you explain this to me?

Rupunzell said:
This shifts the brake bias towards the rear where it is needed to achieve better front to rear brake balance, significantly prevents cooking the front brakes and helps turn-in under braking as it reduces front tire loading.
Click to expand...

I wasn't sure I was correct in thinking about the physics of the hydraulics, and your post would indicate that I was incorrect.

My thinking is that if the same amount of fluid fills a larger area, some of the resulting braking force is lost because the piston does not move as far, thus providing less clamping force on the rotor.

Where did I go wrong in my thinking?

Thanks,
Pete
 
Master cylinder

Pete from memory and mine is bad and hopefully others will correct me, the master cylinder is made up so the front brake section of the master cylinder can push fluid in to the rear section of the master cylinder so once the fronts have pressured up anything extra goes to the rear.

I do know this happens as I lost a rear brake hose when I hit a fox and very soon afterwards found I had lost all brakes which was a bit unnerving, when I checked when I got home I had to bleed air out of the fronts.

Dave
 
My experience

I once converted a street X to an EMRA time trial car, and put front calipers on the rear with a bias adjuster. My experience was that the best brake balance was with the adjuster set for full rear pressure (same as if it had not been installed). You will have much more trail braking capability than the car in stock form. The only time I got surprised with rear lock-up was with cornering while braking in the wet.

Paul
 
Hi Paul,

That's what I did on my GT car (four aluminum front calipers). For one, the front calipers are less complex and probably a tiny amount lighter than the OE rears - but the real big benefit was that I could use four front pads all the way around the car and the same hardware for Earl's rebuildable braided brake lines.

I used a brake bias on the rear and set it up nicely and never had to adjust it ever again (and hated the rain anyway).
 
Rupunzell said:
Not true, the exxe has it's weight biased in the rear. About 55-60% rear, 40-45% front. Not a front engine chassis or layout.

As for using front brake calipers in the rear, it works fine. We did this on the LeMons racer and a host of other exxe racer folks have done this. It is similar to installing 38mm rear brake callers in place of the 34mm rear brake calipers in the rear for a street-road exxe.

This shifts the brake bias towards the rear where it is needed to achieve better front to rear brake balance, significantly prevents cooking the front brakes and helps turn-in under braking as it reduces front tire loading.

If possible leave the current dual front brake caliper with limit valve set up. Try it and see how that goes. Going back to the stock 34mm rear brake caliper returns all those original brake bias problem.


Bernice
Click to expand...

After I first installed the k20 in the X, I couldn't install the hand brake cables due to configuration issue (since resolved),so I tried the stock front calipers on the rear. This was in combo with wilwood 4x in the fronts. The car felt VERY stable while braking.
 
Pete Whitstone said:
I wasn't sure I was correct in thinking about the physics of the hydraulics, and your post would indicate that I was incorrect.

My thinking is that if the same amount of fluid fills a larger area, some of the resulting braking force is lost because the piston does not move as far, thus providing less clamping force on the rotor.

Where did I go wrong in my thinking?

Thanks,
Pete
Click to expand...

Pete, don't think about flow or volume think about pressure when the brakes are applied both front and rear pistons move out to the rotor and stop, the amount off travel is volume and a larger piston will require more volume to move the piston out meaning more pedal travel. Once the piston/pad has hit the rotor, in a simplistic model, the pedal will stop moving and the fluid pressure will increase with the increased leg pressure. Now the larger piston will have a larger surface area for that fluid pressure to work against meaning it will have an overall larger force applied to the brake pad than the smaller piston.

The master cylinder is just the pump, leg force times mechanical advantage will stay the same but a smaller piston will increase the pressure because the pedal force is applied to fewer square inches than with a larger piston, but will also require more pedal travel to achieve the same volume of fluid.

Think about hydraulics the larger cylinder will lift more than a smaller cylinder at the same pressure but it will take a greater volume to do it
 
Thanks Brian

bpimm said:
Think about hydraulics the larger cylinder will lift more than a smaller cylinder at the same pressure but it will take a greater volume to do it
Click to expand...

I think I am starting to grasp it... the larger the area of the back of the piston, the more force the piston exerts, because pressure is basically a function of square area. It makes sense when you think about it in terms of PSI - more SI, more P.

Pete
 
Exactly.

Pete Whitstone said:
I think I am starting to grasp it... the larger the area of the back of the piston, the more force the piston exerts, because pressure is basically a function of square area. It makes sense when you think about it in terms of PSI - more SI, more P.

Pete
Click to expand...
 
Hi Repunzell

The X19 is weight biased to the rear as you say. What I was talking about is the brake Bias towards the front as brakes are what the conversation was about. As you know locking up the front brakes first means you hit the wall with the nose of the car and locking up the rears first means you hit the wall with the rear end. I also use the 38 MM calipers on the rear with a proportioning valve in the front.

Charlie
 
This thread now has me thinking about what to do with the rear brakes - I have the Vicks larger vented brakes up front, but the rears are not up to the task with the larger wheels & tires I have now..... yet another project :rolleyes:
 
From previous discussion about 38mm piston diameter rear calipers.

http://xwebforums.com/forum/index.php?threads/14256/

Read here.. Unit one & Pascal's Law:
https://www.grc.nasa.gov/www/k-12/WindTunnel/Activities/Pascals_principle.html

Previous links are dead.


:)
Bernice


Pete Whitstone said:
I wasn't sure I was correct in thinking about the physics of the hydraulics, and your post would indicate that I was incorrect.

My thinking is that if the same amount of fluid fills a larger area, some of the resulting braking force is lost because the piston does not move as far, thus providing less clamping force on the rotor.

Where did I go wrong in my thinking?

Thanks,
Pete
Click to expand...
 
Last edited by a moderator:
I run all 4 corners with fronts

but my car however is atypical because I have wide rear tires vs the front as well as non stock spring rates. I have a bias valve next to the handbrake lever and use it when I am on the track. I never need to use it on the streets. I have it full open at the start of the track laps and as the temp comes up I dial out the rear. If I don't the rear will start to walk and come around under hard braking.

With that said on the street it is a bit of of a hassle not having a parking brake when parked. I carry a small 2x4 block I use as a wheel chock and turn in or out the front wheels so it rolls into the curb if it ever decides to let go or pop out of gear.
 
Prop valve

Hi Charlie,

how do you plumb in a valve in the front circuit with it having two separate lines to the front wheels?
Do you go 2-into-1, then the valve and then 1-into-2 to the wheels?
 
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