This has been bugging me for years.

All of my ford trucks with the twin i-beams seemed to have given me the feeling that the outer wheel wobbles when I'm turning tight.

I think it's normal due to the steering geometry but maybe someone out there can explain it to me. I always avoid doing this, worried something might break

 

Never notice that before on my trucks. The TTB does not have that good of a road feel, but it has never made me feel uneasy unless something was worn out. I am assuming you always kept up with the components underneath. Have you ever replaced the radius arm bushings? You should have had to do that at least once if you owned one of these trucks for any length of time.

 

I mean, from a standing start and cranking the wheel all the way over, it seems like the outer wheel does'nt track smoothly, feels like the way it would feel if it where oval.

I've just had this latest truck (86 f150) safetied, paying particular attention to the front end, everything is very tight.

My previous truck (84 f150) did the same after an alignment.

Apreciate the reply btw

 

Ford pickups have better clearance then Dodge and Chevy because of the longer arms but that puts a little more stress on the pivot points. The geometry points are very different as the center of gravity of the truck is distributed differently.

You can change the bushings to a harder material than factory but some of the road vibrations will be greater felt, that is why the factory uses a little softer bushings.

Do some research on the bushings as hardness and working life of the bushings are far from the same.

 

I have noticed this too. There is engineering reason as to why - Unless the front end geometry is perfect, this will happen, not just on Fords, but ,more noticble on a twin beam / twin traction beam front end = so as to what might need improvment or be made perfect here goes -

Remember that not only does that geometry have to keep the caster and camber in spec, but the inner tire needs to progessivly turn in a tighter arc relative to the angle of the turn. Ford, it seems inherited a extreme case of this by basicly taking a straight axle and cutting it in half. Stong and durable as heck, but more prone to what you are asking about.

This is something that is hard to do in a production truck, made to be built well, but as inexpensivly and fast as possible. Straight axles have a advantage in this one point in that the arc is simpler to maintain as both front tires are affected by a bump or hole on either side. Of course, on the road, then this is also the biggest disavantage, as hitting a bump say with the right front with a straight axle will transfer some of that affect to the front left as the castor / camber changes with the axle plain relative to the body plain. Please note that I am not addressing off road issues here such as articulation, ground clearance, etc, just more to the means of on road behaviors.

Then, if this is not enough, on leaf spring equpiped trucks, you also have a slight fore / aft movement of the axle(s) as it moves up and down in response to the bumps and holes. Thiis is caused by a fixed piviot point on one end of the leaf, and a movable pivot (shackle) on the other end. Coil spings have a definate advantage here, although at a debateble durability disadvantage.

Plus one there is the steering linkage itself, it also has to move with the axle, (again the straight axle has a easier to define movement here, with greatly simplified linkage) but the piviot points of the steering linkage have to have the exact same arc as the axle halves. This is directly related to the point in the second paraghraph.

There is a bit more to getting your truck down the road straight, such as the before mentioned bushings, and that old bugabo rear axle steering afforded by leaf springs, but these are some engineering reasons as to why the outer tire tends to scrub on a turn.

David

 

Thankyou, Moxy123, dmanlyr


Had to read that a couple of times,heh,heh. I always suspected it had something to do with the design, that extreme looking camber they all have.

If you guys happen to return to this thread and have any links on the subject, it would be greatly apreciated. I've been googling and have'nt found all that much.

 

I would like to add to what was mentioned;

The inner tire turning more than the outer is called the "Ackerman" principle. It is achieved by drawing a line from the left balljoint to the center of the rear pumpkin, and another line from the right balljoint to the rear pumpkin. The outer tie rod placement on both sides needs to be placed along these lines. Any vehicle(including our trucks) that has the tie rods in front, usually does not use the Ackerman principle, since the tie rod would have to be placed way to the outside where the tire is located. That's part of the reason why so many vehicles "squeal" during a tight turn in a parking lot with lots of tar on the surface. The engineers have thrown out the ackerman principle on some vehicles, I guess for space reasons. Do a web search on "Ackerman" and they will explain it better than I can.

As far as mantaining caster and camber, the engineers purposely change the camber when turning, and when the suspension compresses. So when you go around a turn, the top of the tire kicks in, giving better handling. Just go out and turn your tires sharply in one direction, and you can see right away that the tire is kicked in at the top. This is one reason I believe the twin I beam system was used. Many cars use the "short arm, long arm" a-arm system to do the same thing.

Our trucks with coil springs also have fore and aft movement. The radius arm is what causes this.

 

One more thing that tends to affect this issue. The I-Beams and steering linkage are the same, regardless of the trucks wheelbase. As the center to center measurement differs, the turning differences in the front *should* also differ. It wasn't that Ford was cheaping out, no other manufacturer bothered adjusting the geometry according to wheelbase either.

 

Another interesting read, Franklin2, I will certainly look up this "Ackerman" deal.


Good point Rogue_wulff, I hope the long bed was the benchmark.

I have found a great front end guy in my neighborhood, older fellow who's used to working on these trucks, I will be passing some of this information on to him as I would really like to tweak this steering as much as we can.

Thanks again guys