Vague steering at speed

[martin67]

Hello from Oz. My 38 Plymouth special has a wandering problem. I have the toe in set at 2mm and the castor as far as I can tell is at 6 degrees. The steering is VERY light and it turns into the corners ok but when you are going as fast as possible down a straight it will not keep a straight line. My thinking is that the castor should be around 4/5 degrees however I am not all that experienced with the car as I have not long owned it.

Would greatly appreciate any input.

Kind regards Peter.

[greg g]

Is the rest of the steering stock? What are you running for wheels and tires? If you are running bias ply (non radials) they have a tendancy to follow ruts and imperfections in the pavement. Does it have a straight axle, if so aren't tha settings other than toe in preset by the axles location? Are the springs and shackles up to snuff with minimal wear? Are all the connections ie tie rods ends etc in good shape? Is the steering box well connected to the frame?

Does your steering wheel have excessive free play while centered?

Some more pictures of you special would be appreciated. What type of driving do you do with it??

[desoto1939]

According to my motors book here are the spec for the 38 ply:

Preferred caster degrees: +2

Prefeered Camber degrees: + 3/8

Toe In inches: 1/16

King pin inclination in degrees: 5

Rich Hartung

Desoto1939@aol.com

[martin67]

I use the car for mostly hillclimbs and circuit racing. The steering is in all round good condition, it is as you could imagine much lighter than the original car, original 38 Plymouth front axle running radial tyres. The only thing that I can think og is the king pin inclination (castor angle) being a bit much at 6 degrees although the steering arm from the box is quite long and maybe this is setting up a multiplying affect?

Peter.

Edited October 29, 2012 by martin67

[greg g]

maybe with the lightened weight you are getting whats known as bump steer and need a steering stabilizer. These can be had from many 4wd off road shops or you can fab one with a shock absorber between your crosslink and the front fross member.

[martin67]

Sounds good but we are not allowed those modern additions, the car must be as it would have been if it was made up into a race special in 1938.

[greg g]

Usually more caster as in more Positive inclination yields a more stable dynamic, more tendancy to remain straight and take higher effort to turn into a curve, Less yields a less stable easier to turn. Typically solid axle sets ups do not exhibit much bump steering, the tendency of the wheel being lifted by a bump to steer outward as it raises, but it can happen depending on the complience of the suspension, ie: the softer the suspension, the more deflection possible. You might try to crank in a bit more toe in and see if that has any effect on the condition. Usually more positive toe adds to straight line stability, at the cost of slightly more sluggish turn in on curves.

As far as the steering stabilizer, you could make one up in the form of a friction shock and be period correct.

[martin67]

Thanks for that. I tried more toe in which was fine until I slowed down then it got the shakes up with each wheel taking it in turns to fight for forward direction. A bit hairey when you are approaching a corner at speed!

[greg g]

Just did a search on the HAMB forum, lots of straight axle experience there. Most answers revolve around faulty tires, bad belts, bad balance, out of balance brake drums. Might be wrth a closer look

.

[martin67]

I will try pulling the drums off and getting them ballanced, also tempted to try a shortened steering arm from the box.

[Robert Horne]

http://www.ebay.com/itm/1936-1937-1938-Plymouth-car-front-straight-axel-hubs-springs-stablizers-/271091553007?pt=Vintage_Car_Truck_Parts_Accessories&hash=item3f1e50d2ef&vxp=mtr

Check out this listing on ebay of a 38 Plymouth front suspension.

It has a stablizer unit I have never seen before. It may be more for

anti-sway.

[Dan Hiebert]

Usually more caster as in more Positive inclination yields a more stable dynamic, more tendancy to remain straight and take higher effort to turn into a curve, Less yields a less stable easier to turn. Typically solid axle sets ups do not exhibit much bump steering, the tendency of the wheel being lifted by a bump to steer outward as it raises, but it can happen depending on the complience of the suspension, ie: the softer the suspension, the more deflection possible. You might try to crank in a bit more toe in and see if that has any effect on the condition. Usually more positive toe adds to straight line stability, at the cost of slightly more sluggish turn in on curves.

As far as the steering stabilizer, you could make one up in the form of a friction shock and be period correct.

Our lead mechanic here at the office is an old stock car racer, a pretty successful one back in the day. The home built kind. Assuming positive caster is where the geometry of the king pin is leaning back, rather than forward (I don't recall off-hand), we had this discussion about how it makes the car track and handle. You'd have to find the sweet spot on your modified car, but that caster is what gets the car to settle down, track straight, and return to center. Too much and its think and sluggish, too little at its all over the place. The difference can be as little as 1 or 2 degrees. Everything else has to be "square" first.

[greg g]

Positive caster is the difference between plumb verticle, and the contact patch of the tire on relation fo the bottom of the king pin Leaning the top of the kking pin back increases the difference to the front of that contact Patch. For an easy illustration take a look at bicycles. A road bike or cruising bike will have the front wheel axle in front of the line discribed throught he steering head at the front of the frame. This gives a stable ride, which requires some effort to change direction. It makes for a vehicle that is easy to ride slowly and is stable enough at moderate speed to ride "no hands". Now look at a typical Mountain bike. Its front axle is mounted nearly directly under the steering head. This makes for a fast turning, but unstable platfor that is responsive to change of direction but nearly imposible to ride no hands, and pretty wonky at moderate to fast speeds. when you extend this to motorcycles, compare the raked and extended fron wheel mount of a typical chopper, to the nearly vertical stance of an off road bike, same principle applies, the chopper likes to go straight, takes corners like a pig wollowing on mud, and provides about as much feed back to the rider as two strows stuck in a dish of pudding, and bear to turn around in the garage. Now the off road bike is easier to turn quicker to responds and takes a small area to meneuver it.

Going back to the original post I wonder if vague is the correct term we should be chasing. are we talking about slow response to inputs, or wobbly while trying to go straight. So is it vague as in not precise or indifinate or vague as in unsettled or unclear??