Hot Rods straight axle drag link angle

Slowly getting the axle/ steering set-up mocked in and need some insight from someone who's done it before, this is my first attempt. Everything fits great with the steering box here when the drag link is at this much of an angle from being inline with tie rod. It is horizontally parallel with tie rod but to clear oil pan with drag link it fits in this way perfect....motor height, firewall fitment, angle. etc. Just worried about possible bump steer or other problems.

 

If the link is level, when the full weight car is on the suspension, then you'll have minimal bump induced steer, as the driven (opp. side) spindle moves up or down.

 

will the tie rod hit the pitman arm when the springs are fully compressed? (like when you hit a big bump going too fast, or land from a wheelie)

 

Shouldnt the steering box be moved back so that the tierod and the drag link are parallel?

 

Run it back and forth and see if it binds anywhere, otherwise it should be fine.

 

This is the common "urban myth".
If the drag link moves up away from horizontal [eg 1"] or if the drag link was mounted 1" below and moves up 1" to be horizontal, their is the exactly same amount of bump induced steer [just in opposite directions]
This is basic triangulation [maths]

By mounting the drag link 1" higher at the pitman end, If there is 2" of travel the draglink arcs through this minimal bump steer range and "cancels" itself back to zero.[at 2'' travel]

 

Kerry, your approach is favoring upward movement...a 'level' tie-rod, at rest, burdened by full weight, would offer minimum bump in either up or down movements, no? A slight down-side position as you state, passes thru center, a sound approach. When however, you crank a left or right turn, my best sense, is to start centered ie: level.

 

Actually a lot of oem cars have the outer tie rods just a little lower than the rack and most tie rod linkage is the same, my guess and it's just educated guess is so that the wheels toe out slightly thru a dip to help keep the car stable thru the initial dip. Toe out (within reason) makes a car very stable.

 

Comparing a straight axle , fixed camber car with cross steering to a rack and pinion with camber thats changes because of the A arms when hitting a bump ?
Toe in for rear wheel drive and toe out for front wheel drive.

 

The car would turn sharper to the left than the right the way it is now set up.

 

A straight frame rail...gasser style suspension...and a V8....but you can't find room beside the engine to move the box back 6"?
What the hell are you running for an engine??? LOL

Seriously though.
You say everything "fits" where it is now.
Well, perhaps things that fit perfectly now need to be misaligned/modified/changed a little and you can add a couple of U joints to the steering shaft, bend a brake pedal arm etc...because steering box location trumps convenience.
I don't believe in placing important stuff simply for convenience.
It tends to compromise how well an important part can do its work.
Certain things need to go in particular places and that placement is ruled by geometry and common sense.
If you don't follow rules (and continously think several steps ahead as you do,) you often find yourself up against a brick wall later in the build.
Do it the way it needs doing and make slight changes to the less important bits to get you there.

 

I set it up with single leaf and compressed it as squirrel suggested and the draglink/tierod clears oilpan good at max travel,still have to mount bump stop. Setting the tierod and drag link horizontally pararell is no problem, just want to know if where the drag link is in pic will be ok being a couple of inches in front of tie rod. If I move it back straight above the tierod I'd have to move the engine back about 4 inches and cut out the firewall, would like to not do that if possible. Nothing seems to bind as is. Thanks for all the info from you guys

 

The drag link on my car angles even more than that, but it's all behind the tie rod, rather than in front of it, because the steering box is in it's original location, while the axle is 4" ahead of where it is supposed to be (altered wheelbase car).

The front to back offset doesn't really affect the steering geometry noticeably, because the drag link is so long. And it doesn't induce bump steer, since it's horizontal, not angled up or down.

 

Make it a front steer setup? That looks like a lot of work for a rear steer to work.

 

Thanks again squirrel, your pic of your suspension and explaination of the geometry kinda settles my question.Couldnt see why it wouldnt work but gives me a lot more confidence seeing it done on yours. Have a good one... now back to the shop, after I shovel a trail back to it

 

You are only going straight

 

You feel the affects of mechanical bump steer at tyre footprint.
During jounce there is more load on the tyre than during droop, so a tyre dropping into a pothole would barely be noticeable until it climbs out of the pothole.

Early Ford Falcon/Mustang suspension is a good example!
They get excessive toe-in at droop. This can be seen here during bodyroll in this Road Race 'Stang Turning into a downward hairpin corner [attached]
All the weight is transferred to the outside tyre so the affects of the inside wheel toe-in is none existent

With suspension it is nearly always a compromise. With cross steer it would be better to have a panhard bar that swings on an identical arc to the drag link.[Especially a wheelstander]

 

Toe out in a straight line is unstable [ a little bit is used to counteract camber thrust with Neg camber]
With OEM McPherson struts there is always dynamic neg camber during jounce [basic trigonometry] so a slight amount of bump toe-out helps.

In normal straight line driving [with excessive toe-out] if one wheel gets more load, eg: hitting a bump , the wheel wants to "dart-off" in the direction of the toe-out. This then transfers weight to the opposite side causing that wheel to have more load. Starting an oscillation back an forth.

Excessive toe-out feels like standing up in a dinghy

 

I didn't put 20,000 miles on the car in less than 4 years by only going straight.

 

That's why I said within reason. Toe a vehicle out 1/2" and most people will feel no difference in drivability toe it in a 1/2" and most people will notice a vehicle that wanders. When newer (early 70`s) cars were designed they were designed to go negative camber thru compression to put the load into the center of the vehicle, toe went out again to maintain more positive road feel. Most older vehicles of this era can change toe out up to 3/8".

When aligning race cars we always take suspension height into account at half track to counteract positive camber and toe in as the suspension is dropping not compressing, just the opposite of compression.

 

sorry forgot it wasn't trailered