RX7 Single Pot Brakes on Trials Truck Rear Discs

[SirGerald]

Right time for some thoughts.

When we built the truck we were missing the rear callipers from the prado disc brakes.

We had some rear single pot RX7 callipers lying around so figured sweet save some coin and put them on.

Problem we have is that we need to re-kit them (cost over $100) just for the kit. Can do the work myself so cost to rebuild lets say $100.

Question is we are unsure if they will be strong enough to allow the fiddles to lock the wheel up???

Don’t want to spend $100 to rekit and then find out that they won’t work.

We have a set of hilux 4 pot callipers that we hope will fit, although haven’t gone to the step of trying to fit them, or even seeing what shape they are in. Can probably assume they will need rekitting, and they way things work won’t be an easy fit.

Therefore in short do we spend the $100 on a gamble and hope they work, or save the $100 and put towards costs for the hilux???

Does anyone know what sort of setup other trials trucks use on rear prado discs??

[kaney]

hilux rebuild kits are $75 from BNT @ trade price (kit does two front calipers) will be more than that if you need new pistons/pots as well

...might be helpful

[Jafa]

http://www.offroadexpress.co.nz/Forums/ ... 64#p179364

[De-Ranged]

I would spend the little bit more and sort the hilux calipers, it boils down to piston surface area this is the strength of your braking ( yea there is a bit more to it but thats the main bit)
Don't know about the MR2 Calipers that jafa is recomending, I've never seen em how big is the piston in them ??
From memory the RX7 calipers were a single large piston which don't have half the push of a hilux caliper
Here's some info from some stuff I did awhile back to prove a piont to a local trails guy
Alloy v6 commy calipers big single piston (54mm dia)
Gives a piston surface area of 22.9 cm2
Standard hilux or surf caliper (2 pistons 29mm dia, & 2 at 37mm)
Gives a surface ara of 69.5cm2
(Surface area of a circle is "Pi x radius sqaured")
Roughly 3 times the force
Now if you are like most people you'll be using imp master cyclinders with no booster that makes for stronger brakes on a softer lever

cheers Reece

[SirGerald]

Cheers for that.

Think will put the coin towards the hilux ones and do it properly.

Can't be trying to do skids and the brakes not work.

On the other hand my co-driver does like the brown bubbly stuff (says he is just a drunk not an alcoholic as alcoholics go to meetings) and might get a bit confused with all the leavers in front of him, so perhaps unhook them all together for my safety!!

[cool__bananas]

single pot calipers are no where near big enough, most of the time the four pot ones struggle in the dry at a trial

[callum007]

hence why mine dont work unless they are clean and dry on wet grass... only got single pot galant calipers..

[Heath]

I guess the only problem will be if they require a huge volume of fluid to get them to move.

[BlackCruiser]

hilux rebuild kits are $75 from BNT @ trade price (kit does two front calipers) will be more than that if you need new pistons/pots as well

...might be helpful

$17.00+gst Autostop, Kit per caliper, Pads $16.00 per axle + gst,BNT must be Ripping you

[SirGerald]

$17.00+gst Autostop, Kit per caliper, Pads $16.00 per axle + gst,BNT must be Ripping you

Gee that is cheap. Think when I did the front the kit was about $80 from repco at trade price.

[kaney]

$17.00+gst Autostop, Kit per caliper, Pads $16.00 per axle + gst,BNT must be Ripping you

I been done

[suzolla]

Alloy v6 commy calipers big single piston (54mm dia)
Gives a piston surface area of 22.9 cm2
Standard hilux or surf caliper (2 pistons 29mm dia, & 2 at 37mm)
Gives a surface ara of 69.5cm2
(Surface area of a circle is "Pi x radius sqaured")

Hi,
Agree that the single pot is 22.9 cm2.
Don't quite agree with the calcs for the 4 pot. Having pistons boths sides dosen't double the braking force it just means that the caliper does not have to move plus more likely for pads on both sides to retract from the disc and hence less wear and drag.
So for the hilux caliper.
29mm dia = 6.6 cm2 and 37mm dia = 10.8 cm2 giving a total hydraulic area of 17.4 cm2. Not sure how they got 69.5 cm2.
The size of the brake pads will also have some effect as this will affect the contact stress, too big and the pads will tend to float over the top of mud and water, too small and the pads will wear quickly and not have enough friction area.
Perhaps the commodore pads are a bit big for the piston size if they aren't as good as the hilux brakes.

Tim

[De-Ranged]

Yea just had alook a the calcs in the spreed sheet yep for the 4 pot it didn't add them up right,, its 34.75cm2 mind you it still proves my piont thats 1.5 times the force of the commy caliper

So you are saying the pots on the far side only retract the pads... or are you saying the preasure in the lines would is halved because of the extra pistons ??? how ???

it's hydrolics, the surface area of a piston times the preasure in the lines = force applied.... if you want I can see if I can find a diagram I'm not the best at explaining things
Or are you talking about the size of the master cyclinder piston and stroke to as this is the max displacement or volume you can move, being disks the distance the caliper pistons mover is minimal, for most 4wd masters there should be ample to supply a hilux caliper, and on the fiddles this is actually a bonus as you will get more movement of the lever making the lever lighter for the same braking

Cheers Reece

[suzolla]

Hi Reece,

So you are saying the pots on the far side only retract the pads

No, I guess what I was trying to say was that with the piston only on one side the piston can retract a bit due to the seals where as on the other side the whole caliper assembly has to move for the pad to move away from the disc.

or are you saying the preasure in the lines would is halved because of the extra pistons ???

No, you are quite correct the the pressure is the same on both sides and is the same as the supply pressure.

And yes the force is proportional to the pressure and the area that the pressure is applied to.

No wasn't talking about the master cylinder, but yes the caliper force will be proportional to the master cylinder area, for a given pedal force the smaller the master cylinder area the higher the braking force will be but the pedal travel will be greater.
And with fiddle brakes this also applies but is also related to the leavage ratio of the lever.

Right, back to the caliper with pistons on both sides, either one or two doesn't matter the principle is the same. Having a piston on each side does not double the clamping force, it just means that the caliper can be rigidly mounted. The clamping force of a caliper with a piston one side is the same as for a caliper with pistons on both sides that are of the same area.
Perhaps one way to visulise it is by taking a caliper with a piston on each side and each piston has it own hydraulic supply and there is a disc inbetween, assume that the caliper can float, first pressurise one side leaving the other side at 0 pressure, the caliper will float to the pressurised side until the 0 pressure side piston bottoms out, now isolate the high pressure side so that it maintains its pressure. How start pressurising the 0 pressure side, as the pressure increases nothing will move and the clamping force will not change, this will be the case until the low pressure side pressure is increased so that it equals the high pressure side, at this point the clamping force will be the same, if the pressure is increased any more the pressure on the other side will also increase to match it, this will give an increased clamping force, if the supply lines are now connected together the caliper will now be able to float from side to side without the clamping force changing, the only resistance being any seal stickion.

Hope that makes sense

Thanks
Tim

[bb402c]

commy brakes are good nice and light the slides are a problem in our sport as they sieze up .
i had them on my old truck to stop a 35 and did the job fine only down fall was had to clean and re grease the slides after every round
hilux 4pots dont have this trouble but are heavy and weight is a major part in our sport they are dear to rebuild if you need pistons dear to replace a whole one but then again most pepole run them and carry a spare just rember to get the rights ones as there is to options one for vented disc and ones for sliods
your other option is nissan skyline 4pots alloy light and not to baddly priced

[De-Ranged]

I finally got it.... I think we have crossed wires... had to reread your's and my posts , I think it was my dodgy math thats messed us up

The braking force is based on the surface area of the pistons, where the pistons are and how many of them dosn't change this... single, double, opposite dosn't change this

eg a hilux caliper would calculate out to

piston dia 2.9cm = (1.45 x 1.45) x 3.145 = 6.6cm2 since there are two pistons at this dia that is a total of 13.2cm2 of surface area from them
then you add the second set
piston dia 3.7cm = (1.85 x 1.85) x 3.145 = 10.8cm2 and since there are two you get a total of 21.6cm2
Add these together for a total surface area of 34.8cm2, what my math should have shown all along
The fact they are in oposition does nothing other than add the ability to fit more piston surface area in

Oh and thanks for the heads up on the nissan calipers I'll keep my eyes out for them, just had a look on trademe no differance in price

Cheers Reece

[JimmyT]

whats the difference in Brake PAD surface area of the Big single Pots VS 4 pots like ?

I just fitted some pads to the old mans lux and was quite impressed with the pad area on the hilux 4pot.

If the supply pressure is the same then surely you want to go with the calipers that have more pad touching the rotor ?

I know its more a of a car thing but with a slotted rotor it will clean the pads and give them more bite and keep them working better .But i have only ever seen this on cars.