Rear disc conversion.

/dev/ram

/dev/ram

Ralph Graw
Joined
May 19, 2005
Location
Earth
So, from a towing perspective, school me on rear disc conversion. Pros/cons, special circumstances that make disc over drum the best thing since sliced bread, quirky behavior under some circumstances, etc.

I'm letting my Xmas win go to waste so what would this do for me on the Dodge 2500 from a towing POV? Say at the limits of its capacity?

Perhaps factor in a gooseneck or fifth wheel component. I may need to haul my place of residence with me!

TIA


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Well I might be able to help some, lol!

The standard JB6 calipers are good for most trucks and even towing a normal car. For you I'd recommend the heavy duty JB7 calipers. They do very good. I have sold 100's of kits and always got great feedback on the brake performance. When I first started it was for off road only, but now most of them are for stock trucks.
 
Discs are better, period. They shed heat better, they're easier to modulate at the margins, they provide more stopping power, etc. The only drum brakes in our fleet are on the trailers, and whenever I can stomach the upgrade cost, they might go to disc as well.

When I have had problems overheating the brakes while towing, it's always been the rear brakes on the truck. That axle has the most weight on it, so it kinda makes sense when you think about it.
 
GubNi said:
Well I might be able to help some, lol!

The standard JB6 calipers are good for most trucks and even towing a normal car. For you I'd recommend the heavy duty JB7 calipers. They do very good. I have sold 100's of kits and always got great feedback on the brake performance. When I first started it was for off road only, but now most of them are for stock trucks.
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Shawn, does your e-brake kit work with the JB7's?
 
OK, one last question. Holding power of e-brake drums vs discs, parked on an incline. I rarely use the e-brake, but when I do it's cuz I need it to hold a load. Even if that's just long enough to chock the tires. Seems like the surface area might be a larger on the drums, but the grip of the discs may overcome that - particularly since Shawn's is manually activated. Am I on the right track with my thinking? Been doing a lot of reading while saving up in case I need the e-brake but it's clear as mud... everybody's coming from some other perspective.
 
Well I have heard that emergency brake calipers don't hold well also, but I have sold many hundreds with no complaints. There are a few things I do to resolve common problems.

1. My brackets are made from 1/8th steel not thin formed steel so mine don't flex.

2. I use the bigger 76-78 caliper and not the small 80+ calipers

3. I include and instructions guide that says to apply the ebrake once a month. By doing this you keep it adjusts.


It's my belief that with this you won't have any problems.
 
shawn said:
When I have had problems overheating the brakes while towing, it's always been the rear brakes on the truck. That axle has the most weight on it, so it kinda makes sense when you think about it.
Click to expand...

Axle load doesn't have much to do with it. You're putting more energy into the entire brake system than normal because of the trailer load (you know this already), and the rears have lower thermal mass, lower heat rejection, etc., than the fronts and will usually overheat first because of that. The rears can't have as much torque as the fronts or you'd be very unstable under braking when not towing (and maybe when you are towing, depending on rear axle load). So, the rears are downsized for cost reduction, because that's as big as they need to be.

Ideally you want the same amount of rear brake torque as you already have (for proper bias when not towing), but with higher thermal mass and hear rejection (better cooling) for when you are towing. Some different rotors (vented and slotted) can help with that, and some better pads all around.

You could also put larger brakes in the rear, and use a proportioning valve to reduce the rear bias when not towing. Your stability is very dependant on that rear axle load because it can dictate which axle loses traction first.
 
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Fabrik8 said:
Axle load doesn't have much to do with it. You're putting more energy into the entire brake system than normal because of the trailer load (you know this already), and the rears have lower thermal mass, lower heat rejection, etc., than the fronts and will usually overheat first because of that. The rears can't have as much torque as the fronts or you'd be very unstable under braking when not towing (and maybe when you are towing, depending on rear axle load)
Ideally you want the same amount of rear brake torque as you already have (for proper bias when not towing), but with higher thermal mass and hear rejection (better cooling) for when you are towing. Some different rotors (vented and slotted) can help with that, and some better pads.
Click to expand...

Great reply! I totally agree if the rear brakes are properly working with like new drums.

I think what he may have been referring to is that if the rear brakes are not working properly because of a malfunction like a seized adjuster then the fronts overheat.

I have seen this many times too. If the fronts are overheating most of the time I have found the rears were not working properly and therefore the fronts were working too hard. This is very common and one of the many reasons I have sold 1000s of rear disc kits.
 
GubNi said:
Great reply! I totally agree if the rear brakes are properly working with like new drums.

I think what he may have been referring to is that if the rear brakes are not working properly because of a malfunction like a seized adjuster then the fronts overheat.

I have seen this many times too. If the fronts are overheating most of the time I have found the rears were not working properly and therefore the fronts were working too hard. This is very common and one of the many reasons I have sold 1000s of rear disc kits.
Click to expand...

I think Shawn was talking about rear discs overheating.
If the rears are garbage, like drum brakes, the fronts will often overheat first because the rears aren't doing much of anything to contribute (like you said). If the rears are discs, they'll usually overheat first because they're smaller and aren't able to dissipate heat because they're sized for forward weight transfer under braking so the rear tires don't lockup first when not towing. The fronts are designed for more energy dissipation, so they last longer before overheating.
 
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GubNi said:
I just reread his reply and I see what you mean. On a disc disc system all brakes should be equal temperature regardless of payload or towing.
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Yes, if they are the same.

But when the front is 14" rotors and rears are 12", the rear isnt designed to deal with the thermal energy like the front is, so it overheats first under heavy breaking.
 
paradisePWoffrd said:
Yes, if they are the same.

But when the front is 14" rotors and rears are 12", the rear isnt designed to deal with the thermal energy like the front is, so it overheats first under heavy breaking.
Click to expand...

Well since a normal porportioning valves sends 60% of the power to the front and 40% to the rear wouldn't that equal out if the front rotors were about 20% bigger?
 
GubNi said:
Well since a normal porportioning valves sends 60% of the power to the front and 40% to the rear wouldn't that equal out if the front rotors were about 20% bigger?
Click to expand...

It doesn't quite work like that, because that assumes only the effective rotor diameter is responsible for brake torque.
There's also the caliper piston diameter (caliper clamp force), pad compound friction coeff (which varies with temperature), etc. The proportioning valve isn't a 60/40 pressure split, it's a 60/40 power split, so the pressure split is actually something different.

This goes back to what I was saying about upgrading the rotor and pads to be able to do more work before fading and overheating, while not actually changing anything else about the brake system. Basically just a drop-in replacement to make the best of what you have as an easy upgrade. Can't always fit a wider rotor that's vented without changing the caliper though. Or, do one step better and add brake ducts as well.
 
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why would the rear be smaller? from factory? on a "tow rated" vehicle?
 
ckruzer said:
why would the rear be smaller? from factory? on a "tow rated" vehicle?
Click to expand...

They may not be physically smaller (in terms of rotor diameter) but will usually have a smaller caliper, narrower disc, etc., to save money and keep the clamping force in the proper range for the pad size and compound temperature. Unless there is some way to adjust front/rear brake bias, the bias will always be toward the front so the truck doesn't swap ends under braking because of too much rear brake torque (when not towing).
 
Fabrik8 said:
They may not be physically smaller (in terms of rotor diameter) but will usually have a smaller caliper, narrower disc, etc., to save money and keep the clamping force in the proper range for the pad size and compound temperature. Unless there is some way to adjust front/rear brake bias, the bias will always be toward the front so the truck doesn't swap ends under braking because of too much rear brake torque (when not towing).
Click to expand...


when the yj was driving on stock axles i redid front to rear new everything. drum rear 35 fyi. one time i had to emergency fast and hard. the rear end started to fishtail back and forth. later dicovered had blleed issues in the rear so wasnt reallybreaking there.
(never did confirm exactly why this happened 100%...)

riding bikes. if you clamp down on the front brake too hard you can fishtail. if you do just the rear too hard you can break loose.

so im trying to peice all this in my mind (sans coffee lol)

if the factory gyped the customer and made 4 wheel disc brake truck and put bigger calipers on the front (more clamping power) and smaller in the rear... has to be for some reason? or just the a-typical fubar?

when towing, you want the trailer axle to become the "rear axle" right? or no?
 
ckruzer said:
if the factory gyped the customer and made 4 wheel disc brake truck and put bigger calipers on the front (more clamping power) and smaller in the rear... has to be for some reason? or just the a-typical fubar?
Click to expand...

I don't understand what you're saying. That's the way all vehicles are made, and I've already explained the reason a few times...?
 
ckruzer said:
when the yj was driving on stock axles i redid front to rear new everything. drum rear 35 fyi. one time i had to emergency fast and hard. the rear end started to fishtail back and forth. later dicovered had blleed issues in the rear so wasnt reallybreaking there.
(never did confirm exactly why this happened 100%...)

riding bikes. if you clamp down on the front brake too hard you can fishtail. if you do just the rear too hard you can break loose.
Click to expand...

If your YJ is lifted (so has a high CG), and has soft springs (so that CG moves around a lot), you could likely get enough weight transfer under braking to unload the rears and make it fishtail (little to no rear weight equals no traction). Same with a bike. If all your braking force is in the front, the rear is just along for the ride and isn't doing anything to help the rear of the vehicle track straight because the rear isn't providing any force under braking. You're sort of pivoting about the front wheels at that point, and your CG is behind the front wheels, so that CG will try to move ahead of that "pivot". Plus, you're under-utilizing the rear tires, so your stopping distance increases because you're now limited by traction on the front tires and they'll be overloaded.

That's why on a road bike, mountain bike, etc., it's ideal to drop behind and below the saddle for panic stops. Get the CG of your body low and rearward, so the forward weight transfer is less and the rear brake and tire can be used more effectively. ....And you don't go over the bars...
 
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Fabrik8 said:
I don't understand what you're saying. That's the way all vehicles are made, and I've already explained the reason a few times...?
Click to expand...

sorry. i read your post then made mine then posed the question. sorry need coffee lol. to save money is the reason you stated. to my understanding, and it seems youre saying the same, you equal breaking power front and rear, and when its more to the front youre only setting yourself up for the front being the "pivot point". right? (thats where i was going with the bike example :D )

so i guess what i was trying to understand (and if you said it already sorry! lol) is setting politics asside, why would the engineers have thought it was a good idea to put samller stuff on the rear? like what is the "on the books" reason for it?

which leads to this gooberish idea. if i had a truck and they gyped the rear. what if just flipped the prop valve (assuming not 50/50)? would it help or just transfer the a problem to the rear?

do i understand the conclusion right (even if not the full physcis of it all lol) that we want 50/50 prop and same size front rear disc/calipers?


Fabrik8 said:
If your YJ is lifted (so has a high CG), and has soft springs (so that CG moves around a lot), you could likely get enough weight transfer under braking to unload the rears and make it fishtail (little to no rear weight equals no traction). Same with a bike. If all your braking force is in the front, the rear is just along for the ride and isn't doing anything to help the rear of the vehicle track straight because the rear isn't providing any force under braking. You're sort of pivoting about the front wheels at that point, and your CG is behind the front wheels, so that CG will try to move ahead of that "pivot". Plus, you're under-utilizing the rear tires, so your stopping distance increases because you're now limited by traction on the front tires and they'll be overloaded.

That's why on a road bike, mountain bike, etc., it's ideal to drop behind and below the saddle for panic stops. Get the CG of your body low and rearward, so the forward weight transfer is less and the rear brake and tire can be used more effectively. ....And you don't go over the bars...
Click to expand...


it was RC springs. lmao. tires never broke loose but it was the first time i had ever felt the rear end sway so much in a hard brake. (in a vehicle)

makes much more sense now in relation to the lift and suspension

the moutain bike closed the circuit!!! lol thanks
 
ckruzer said:
so i guess what i was trying to understand (and if you said it already sorry! lol) is setting politics asside, why would the engineers have thought it was a good idea to put samller stuff on the rear? like what is the "on the books" reason for it?

which leads to this gooberish idea. if i had a truck and they gyped the rear. what if just flipped the prop valve (assuming not 50/50)? would it help or just transfer the a problem to the rear?

do i understand the conclusion right (even if not the full physcis of it all lol) that we want 50/50 prop and same size front rear disc/calipers?
Click to expand...

The money saving is because there's generally no want/need for the same amount of braking in the rear than in the front. Under normal conditions on the street (not towing or bed-loaded), weight transfer under braking means that there is more weight on the front axle. That means you have more weight (vertical force) on the front tires than the rear, so you have more traction in the front than the rear. That means that you can apply more brake torque to the front than the rear before the tires break traction. For the same reason of weight transfer, if you had the same braking setup on all 4 corners you'd have the same amount of brake torque on all 4 corners, and would break the rear tires free very quickly because there is less weight on them then the fronts. Then you spin.

So because of weight transfer, you can't use the same amount of brake force in the front and the rear, and therefore the rear brakes can be made smaller and less expensive. That's the "on the books" reason for it.

When towing, you have more rear axle weight, and you can actually take advantage of more braking force in the rear. But if you optimize the rear brakes for towing, when you're not towing you'll have too much braking force in the rear and will break the rear tires free and spin. So brakes are generally still designed for not towing, because stability under all conditions is far more important than taking advantage of a little extra rear braking ability when towing.


ckruzer said:
do i understand the conclusion right (even if not the full physcis of it all lol) that we want 50/50 prop and same size front rear disc/calipers?
Click to expand...

That would only work for towing or with a loaded bed, where the rear axle had at least as much load under braking than the front axle. As soon as you unhook the trailer or unload the bed, that situation completely changes and now you have far too much rear braking force. That's why it's not done.
 
Didn't toyota make a "load sensing" prop valve on some of their trucks? Basically a prop valve that would change rear braking dependent on rear ride height. Not sure how well they worked, but just throwing it out there. I honestly didn't even read the entire thread, just the last couple posts, so if this has no relevance then oh well...
 
Rear brakes optimized for towing is why its scetchy as hell driving a bobtail semi truck, especially in the rain. They have huge rear brakes on the tractor since its the front axle of the trailer. Most semis didnt even have brakes on the front axle at all until the 1970s

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NC-V said:
Didn't toyota make a "load sensing" prop valve on some of their trucks? Basically a prop valve that would change rear braking dependent on rear ride height. Not sure how well they worked, but just throwing it out there. I honestly didn't even read the entire thread, just the last couple posts, so if this has no relevance then oh well...
Click to expand...

That sounds pretty cool, I've never heard of that before. Need to go research that.
 
NC-V said:
Didn't toyota make a "load sensing" prop valve on some of their trucks? Basically a prop valve that would change rear braking dependent on rear ride height. Not sure how well they worked, but just throwing it out there. I honestly didn't even read the entire thread, just the last couple posts, so if this has no relevance then oh well...
Click to expand...
I think most of the import trucks had them. It had an arm that went to the axlehousing. Valve was usually on the frame. It would adjust the brake proportioning off ride height until it rusted up or broke.

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