I am trying to figure out what would be the best way to get ABS brakes on a 6 wheeled vehicle.
I'm planning out the conversion of a vehicle from 4 to 6 wheels. The original vehicle already has ABS brakes and want to bring that over to the extra 2 wheels that would be added on the back. For reference,I have experience restoring and working on cars but have never”experimented” with ABS systems.
First off, I haven't found any info on how existing 6 wheeled vehicles like vans and small trucks do this, that is, if they even bother having ABS on the extra wheels??
My first thinking was to add a second ABS pump(the exact same model as originally on the vehicle) to the vehicle. This while leaving the original ABS pump/braking system alone(mostly). For reference, the existing ABS pump is an earlier version that doesn’t have can bus so should be relatively easy to work with when modifying.
My thinking is to connect the rear(L+R) outputs and sensors of the additional ABS pump to the new rear wheels. I would block off the front outputs to the brakes(not connect them to anything). Then connect the front(L+R) sensors either to the existing front ABS sensors or somehow add new sensors to the existing ABS ring on the hubs.
My thinking here is that the additional ABS pump would now have something to reference off for wheel speed difference(the front wheels). Would something like this work? Would not using/blanking off the front outputs work or create a problem? Any suggestions on how to make this work or completely different ideas for the same result are very welcome.
Diagram for reference.
I’m going to move this to the main forum. I think you’ll get more eyeballs there.
Good luck with the project.
Great idea, let me know when we get a solution. 
A simpler solution might be to change the rear ABS from working on a per-wheel basis to a per-axle basis - so the 2nd axle would get a ring on one side and the output from the matching corner split to both sides, the 3rd axle would get a ring on the other side and the output from the other corner split. The downside is that this would be kind of like old-school 3-channel ABS on both rear axles, but production cars with ABS tend to have a ton of front bias anyway and basically never lock up the rears so I think it could work quite well. If similar weight is now spread across both rear axles that could change and you may need to alter the bias to suit.
I was going to suggest something similar to Gameboy.
Assuming a 4 channel ABS, I'd have one circuit for each of your wheels on one side in the front, and have one channel operate both those wheels.
The traction on those two wheels is going to be close enough that the ABS should do a reasonable job operating those wheels in the same manner.
One thing to be careful with is that some ABS units do "electronic brake differential". That is, instead of having a mechanical proportioning valve in the system to lower the rear pressure, they use the electronics in the ABS. Wild speculation here (I'm a software guy, not a proper engineer) but it seems like that might work OK with a dual-rear-drive-axle thing (like a semi tractor) but perhaps not so well if it's got 3 equally spaced axles down the length (like a 6x6 APC)
