I have a rather unique application in mind for the RABS system and need to know if it will work and hopefully how.

I'm building a trailer based on the back half of a 88 F250HD. One plan option is to keep the 10.25 complete and use a surge brake tongue with the stock drum brakes. With this plan option is to use the RABS system in the trailer which I think would be awesome, this plan has it's downsides though, most of all drag in that the differential will be turning to provide a signal to the VSS. One thing that might make it absolutely worth it would be if I can use the RABS to electronically solve the reversing issue. In case your not aware surge brakes have a backing issue, cause it pushes back on the tongue it activates the brakes which make you push harder which activates the brakes more, etc. This is often solved with one of many forms of lockouts on the brakes, but these can be a pain and/or unreliable.

One idea I had but don't know if it will work would be to energize the RABS valve with the backup light signal to release the brakes when put in reverse. The ability to do this would absolutely make putting in the RABS worth it as it solves my reverse issue in a convenient and hopefully reliable way.

Thoughts?

 

Kelsey-Hayes RWAL Antilock Brakes


Well I've been doing some reading and I think I have it mostly worked out. All I really need to do is open the dump valve. In doing so all the brake pressure is absorbed by the accumulator and the brakes don't work while in reverse.

This assumes one fact I've yet to confirm, that the accumulator has enough volume to absorb the entire volume of the master cylinder. Even though I haven't yet found the volume of the accumulator, nor have I pined down the volume of my master this seems likely. There's cases of the dump valve sticking open when it shouldn't and the entire volume of the stock master being absorbed by the accumulator. Course if not all I need is another RABS valve to double the volume, I'm sure I got a couple laying around.

I don't know yet for sure if these are 12V solenoids, they may be 5V but most likely 12V. Nor do I know for sure if I'll need to completely isolate the controller while doing this to keep it from throwing a code. Even so a couple double position relays should do the trick.

I don't yet know the purpose of the valve reset?

Thoughts?






x

 

Why not just energize the isolation valve when backing up?

 

Just put a line lock in the line from the tongue to the trailer brakes. Have it activate with the reverse lamps.

 

I considered these but they both have the same problem as they effectively accomplish the same thing. They put all the pressure pushing the trailer onto the master cylinder, this in turn of course pushes on the piston and the fluid. I'm sure this is clear to you but that can be A LOT of pressure on the master and fluid, I'd rather avoid that.

They way I figure opening only the dump valve the master can then move it's full travel freely without building pressure and it's piston and the accumulators piston will just go back and forth swapping fluid. This would probably be pretty good for the system anyway having a flushing effect. I can install a permanent bottoming bump-stop for the master cylinder's travel if there isn't one already so the pressure pushing the tongue back ultimately is on it.

 

This would be the easier choice

 

I would think bottoming out the mc would be bad on it even going passed normal travel could have detrimental effects on the internal seals

 

The travel of the coupler is limited to prevent the MC bottoming, this is true for reverse as well as normal braking operations.


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Even if I decide a line lock type function is a better way to go I'd use the isolation valve of the RABS over a line lock unit they accomplish the same thing. Even if I didn't want to install RABS in the trailer I already have RABS control valves from the donor truck, a parts truck, and my driver(not 100% sure they work) I'd have to spend money on a line lock.

Now that I understand it better I see that the control valve gives me two ways to go about this, isolation/line lock or pressure dump. I'm thinking pressure dump is a better way to go about it.

Have to admit I'm pleased with what I think I have figured out, looking forward to making this work.

 

I'm thinking the purpose of the valve reset is to since flow going back to the master to tell the controller to open the dump valve so it can empty?

 

The pressure on the master cylinder wont be any different if the fluid flow is blocked. Also
backing up wont put near as much pressure on the system as braking at highway speeds.
The only problem I can think of with energizing the isolation valve is if you stopped on a steep downhill and tried to back up you would have some pressure from the weight of the trailer trying to roll down the hill. If you were to energize the dump and isolation valves with the reverse lights it would bleed off the pressure and block off the flow from the master. I think that would work best.

 

How do you figure?

During normal breaking operations the trailer largely stops itself, it does not place it's entire stopping force on the tongue. That is the point of trailer brakes, done right only about 10% of the stopping force is applied to the truck and in turn the MC. But in reverse the truck pushes back on the trailer with potentially about half the total weight of the trailer, 5 times what could be expected under normal braking.

Picture going down a 100% grade and stopping at a stop sign at the bottom of the hill. During braking for the stop sign the trailer brakes are doing up to 90% of the braking leaving only 10% on the truck and MC. Now back up that hill from the stop sign, done right the trailer wheels are coasting, the truck must push the entire weight of the trailer up the hill. At a 100% grade that's 50% of the trailers weight now applied to the truck and if the fluid is blocked the fluid and in turn the MC. For 8000lbs of trailer that's 4000lbs pushing on the MC rod and if a 1" MC piston 3200lbs of pressure in the line and on the MC piston seals.

Yes no?




I completely agree, good point I hadn't thought of that issue recently. If I energize only the isolation valve or only a line lock it will have the function of a mico lock. I'd have to also energize the dump valve to release the pressure.


x

 

I didnt account for the trailer brakes reducing the force on the master during normal braking. I still dont think the pressure would hurt anything when backing up though.

 

Your probably right 99.9% of the time, but I'm concerned about the 0.1% time when I have to push it up a steep hill, out of some mud, over a log, etc. This trailer has 37" tires to match the rig for a reason. On top of that is the accumulated wear of that kind of pressure. Why risk it when simply dumping the pressure into the accumulator could work just as well. This is my line of thinking, sound about right?


I once bent a nice class IV hitch trying to back a car trailer with a F250 on it up a steep driveway. At the bottom of the driveway the road was level so the truck and trailer were like this _/ it did not go well. Point being sometimes we need to push things past anything anyone really thought about when designing it.

 

I understand what you are trying to do, should be interesting. Let us know how it works out.

 

<p>i have one of these: https://www.etrailer.com/Trailer-Brakes/CTS/HBA-10.html</p><p>on my car hauler with falcon 5 lug drum brakes on the wheels.</p><p>got the master used from a trailer store for $100, and the drum brakes off of a parts car for free.</p>