Rear steering to the rail?

[Nate]

What if you could set the DFW to neutral steer-in with a very low COM so it approaches zero effect on the steering, and have the right rear alligned to cause the DFW to veer onto the rail? It seems this strategy would break past the standing problems with a low COM, provided your allignment was spot on.

Just curious to see if anyone has done this before I spend effort reinventing a wheel that doesn't work.

[Stan Pope]

I think that if you tried doing that, you would introduce more losses that caused by DFW toe-in. The rear axles are more heavily loaded, so additional frictions due to them not tracking together are magnified. The DFW, on the other hand, is lightly loaded, so the frictions introduced by toe-in have less effect.

One way I considered (and discarded) was to make the rear wheel behind the DFW with slightly more friction. This drag would pull the DFW toward the rail. Positive camber on DFW would keep it rolling (not sliding) on the track and the rail. Steering by differential friction is hard to predict and, I suspect, loses more energy than DFW toe-in.

So, talk to us about what rear alignment changes you might make to steer the car and maybe we find a way to keep those losses low enough to make your idea work.

[Nate]

My thought was to use your cantilevered weight allignement method. Allign the DFW to track straight with weight on the nose. Take a couple allignment passes to ensure the rear wheels are also tracking straight. Then allign one rear wheel (Either, but I expect the right, or DFW side, or the side that happens to bear the most weight at the end of build if that's the case) to steer the DFW into the rail.

Alternatively, you could have two contacting, straight tracking front wheels with a dominant rear wheel and a raised rear wheel. Result would be no frictional issues, but the major challenge here would be concentrating the weight so the car doesn't want to tip in the rear.

The goal here is, admittedly, to lower the COM without the risk of chatter. I'm working on the assumptions that
- the rear wheels naturally have more steering effect due to the extra weight
- decreased COM, all else equal, will be more of an asset than additional friction, even with the traditional DFW and two rear wheels differently alligned
- a front wheel, lightly weighted and steered by a rear wheel will chatter/jump less than a DFW bearing more weight and traditionally steering.

[Stan Pope]

"Rear wheel steering" can happen at least two ways:
1. By direction
2. By differential friction

When steering by direction, the dominant rear wheel (the rear wheel most heavily loaded, usually) tries to assume a direction straight down the track. If it has slight toe-out, then the rear moves in the direction of the toe. If the front end does not follow suit, then the result is that the DFW gets a bit of toe-in as a result of the rear movement. The net effect is (1) there is DFW toe-in, which we were trying to avoid, and (2) the rear wheels are not tracking together, so they produce losses.

For differential friction, see my prior comments.

I'm not understanding how your ideas change the actions above.

[Laserman]

Wow! What a fascinating idea!

It seems very complex, and in the end, at best would probably only achieve speeds similar to a normal set up, but I can't help but like it.

My gut tells me that it is a total goose chase, but I like the way you think.

[Stan Pope]

... If it has slight toe-out, then the rear moves in the direction of the toe. If the front end does not follow suit, then the result is that the DFW gets a bit of toe-in as a result of the rear movement.

I think I said this backward for the usual case, i.e. opposite side rear most heavily loaded.

I should have said ... toe out on the opposite side rear moves that rear side out and causes the DFW to decrease toe-in/ increase toe-out. And toe out on the near side rear moves the rear side out and causes the DFW to increase to-in/decrease toe-out.

In other words, steering the direction of the rears alters the direction of the DFW because the rears will move left or right to get lined up straight down the track (if the rail doesn't get in the way.)

[ngyoung]

Your rear wheels are carrying most of the weight, having them ride the rail will scrub off more of you kinetic energy. one of the reasons front wheel steering works is that the wheel is carrying minimal weight. It has just enough force to keep the nose of the car in a straight line which helps keep the rear wheels from touching the rail.

[Vitamin K]

I don't think the point was ever to have the rears /ride/ the rail...just that the steer to push the DFW into the rail would come from the rears.

Your rear wheels are carrying most of the weight, having them ride the rail will scrub off more of you kinetic energy. one of the reasons front wheel steering works is that the wheel is carrying minimal weight. It has just enough force to keep the nose of the car in a straight line which helps keep the rear wheels from touching the rail.

[ngyoung]

What is stopping them from continuing into the rail themselves though when they're toed to steer the front into the rail? You want the wheels that carry the most energy pointing straight down the lane as much as possible.

I don't think the point was ever to have the rears /ride/ the rail...just that the steer to push the DFW into the rail would come from the rears.

Your rear wheels are carrying most of the weight, having them ride the rail will scrub off more of you kinetic energy. one of the reasons front wheel steering works is that the wheel is carrying minimal weight. It has just enough force to keep the nose of the car in a straight line which helps keep the rear wheels from touching the rail.

[Stan Pope]

What is stopping them from continuing into the rail themselves though when they're toed to steer the front into the rail? You want the wheels that carry the most energy pointing straight down the lane as much as possible.

I don't think the point was ever to have the rears /ride/ the rail...just that the steer to push the DFW into the rail would come from the rears.

A way that the rear wheels might influence the DFW to move to / stay on the rail is to toe the more lightly loaded rear wheel slightly off parallel and use it's increased drag to "pull" the DFW to the rail. That would require that the more lightly loaded rear wheel be on the opposite corner from the DFW, which is not the normal case. So, the ballast would need to be bunched on the DFW side of the car.

This condition would not drag the rears into the rail, but, I suspect that the combined losses from the DFW directed into the rail + the drag of the offside rear wheel would more than offset the gain from not using DFW toe-in to steer the DFW to the rail.

[davet]

I have no idea about the physics of such a design. However, I do like that you are actually trying to create, in effect, a 2 wheeler instead of the standard 3 or 4 wheeler. Just seems to me that it would be pretty squirrely going down the track. I'd love to someone try it though.

[Stan Pope]

2 wheelers are an entirely different animal. The usual scheme is to use a DFW and one rear wheel with the CM below the bottom of the axles and centered on the strip between the two wheel treads. Otherwise, the ideas are similar to a rail guided three wheeler. For stability you may have to give up camber on at least one of the wheels. Problem 2 is keeping it under control in the breaking section!

Go for it!