Splinter bar by itself (traditional) has roller bolts, no singletrees. You need to have collars on the horses to use roller bolts, since there is no movement in the splinter bar itself. Modern vehicles often have put singletrees under the splinter bar so they can use breast collar harness. You need to drive your horses so BOTH are working, using this system. You won’t get a see-saw effect in starting like an evener can do, if they don’t step off evenly, which is good for training new animals. Singletrees are removable if not being used. Splinter bars, roller bolts, require traces with a loop to go around the roller bolt. You can order special traces with loops or make up slot traces into loops with a Monkey Puzzle sqare.
An evener is nice, to keep both animals working together, pulling equally. Again, Driver needs to look down, make sure animals are pulling equally, because one will ALWAYS be a bit lazy in a Pair. You can tie evener bar down with a new animal, get the same effect as singletrees pulling from under the splinter bar. Lets new one get used to moving off smoothly without being pulled on, moving with his partner on command. Then you can remove the tie, let the evener work as designed.
A vehicle like you pictured, wouldn’t have a yoke in most cases, for use with a Four. You could probably order a pole with a yoke for a Pair, 2nd pole with a pole head (using pole straps) and a crab, for use with the Four. The yoke for Pair with a shorter modern pole, would be the bolted on the metal pole end. Yokes used with modern vehicles, have snap shackles for the breast collar chest rings. Yokes come in various widths, from chest center to chest center of both horses, to shorter widths that snap on offset chest rings of breast collars. Animals don’t need the longer pole for control, using yokes fastened to the breast collars.
Yoke is not usually seen on the Wheelers of a Four, they have pole straps instead, run from chest of animal to the arms of the pole head, with the Leader Bars hung on the Crab out ahead of their chests. This pole strap use is with breast collars or full neck collars. A traditional pole is LONG, ending at the relaxed nose of Wheelers, so they can control the vehicle. Modern vehicle with sprung poles can offer various Wheeler fastening options, but too short a pole with a pole head, does add difficulty in vehicle control. Can make the Wheelers go crooked by pulling them inward and not giving them good leverage.
Guess you need to ask what other options are available for the pole, to hitch Wheelers chests to the pole on a Four. I see a number of home designed features, interesting concepts, but I won’t recommend something you might not be able to get from the seller.
I am not familiar with the “spreader” term, so can’t comment. I know about spreader rings for running reins, is that it? Carriage driving doesn’t use spreader rings. Also not sure of your use of “chain”, though on Coaches, they might use pole chains run from chest ring of full neck collar, to pole head, on the Wheelers. Chains used this way are not correct on certain kinds of vehicles, and are always used with a full neck collar harness only, in Carriage Driving. Not up on my Draft harness details for showing or daily use.
I can see a wagonette, modern made, with both the splinter bar and singletrees or the evener with singletrees. Both could look nice, be appropriate and easy on the animals.
