Setup for sweeper and hairpin turns

[hardingfv32-1]

This is a new thread based on a subject that emanated from the Mono Shock thread.

We are discussing the reason the same setup does not work well for both a sweeper and hairpin type turns. This is not really an Ackermann issue. It is really all about the operating angles of each tire relative to the direction of travel of the car. I am focusing on slip angles and disregarding all the other factors that effect a tire's cornering performance.

The rear tires are never at optimum slip angle when going through turns. They are basically in a fixed position if you disregarding bump-steer. The divergence from an ideal slip angle varies depending on the radius of the turn. The front tires are variable but the angle they assume is dependent on the radius of the turn IF you disregard all the other factors. Let us ASSUME a tire's cornering performance is only effected by the slip angle it is running at. Given these constraints, we are probably only going to be able to balance the car for one radius turn.

This is my first go at this explanation, see what you think. Remember to keep it simple. As an example, I have purposely disregarded the very important and complex issue of weight transfer.

Brian

[jpetillo]

Brian,

What are you looking to find out? Are you looking for the slip angle of all the tires based on the turn radius, the direction of travel, the steering angle and instantaneous toe? What are you defining as balancing the car in the scenario that you describe - obtaining a certain average slip angle? John

[brian]

My reason for varying ackermann dealt with reducing the scrubbing off of speed by the increased angle of the inside tire. We know the ackermann is a function of how tight the turn is but it will still scrub speed on sweeping turns. Nascar guys use less akermann for qualifying then for the race where tire wear becomes more critical.

The most effective scrub angle for a tire depends more on the construction of the tire. Radial vs bias ply work completely differently.

[lbaconll]

Brian M. I would like to understand better the effect of ackerman / non ackerman on the Vee. Am I wrong in assuming that zero ackerman is better for high speed stability/speed, and some ackerman is good for turn in- transition in slow speed stufrf? Larry Bacon

[brian]

Ackermann does nothing for stability directly. Besides the tire wear improvement, ackermann, which is really toe out in turns, gives the car it's initial response to steering input. I often refer to it as "lead", an old sprint car term. Unlike setting up a front end with a bit of toe out, ackermann will give you the same results without the dartiness or wandering, under hard braking. Laying out the inside tire with a tighter radius will aid in changing the direction of the car like using more steering input. If done properly, the outside tire will not turn as much which will reduce initial scrub. Ackermann will not help as much in the middle and exit of the turn but having the car respond so much better on turn in will aid the rest of the turn.

Drag link steering systems, which incorporate a single solid tie rod from side to side, cannot produce ackermann nor do they have bump steer. That's why on some cars, like the Adams Aero, it calls for a bit of toe out on the front end. Like this design, any car with parallel steering, or no ackermann, will tend to be sluggish on turn it.

Hope this helps Larry.

[Mystique Racing]

Drag link steering systems, which incorporate a single solid tie rod from side to side, cannot produce ackermann nor do they have bump steer.

I don't necessarily agree. Drag link geometry will have zero ackermann only if the tie rod pick up point is directly behind the king pin center of rotation on each side, which in a Vee requires a special tie rod mounting block attached to the spindle. Additionally, a drag link could be beneficial under heavy braking when the front end dives, however, when one wheel goes up and the other goes down, like in a corner, there will be a change in toe between the two front wheels. The March issue of Circle Track has a great article on ackermann and why it is not favorable.

[problemchild]

I agree with Scott's statements. Regarding the bump steer with a single drag link, not eliminated, but certainly minimalized in relation to most 2 link configurations used on FVs.

[jpetillo]

I agree with Scott's statements as well - both about ackermann and bump steer with a single drag link. Ackerman should only be zero in the case that Scott described. Then you can adjust ackerman by changing the length of the drag link.

About bump steer, there should still be none when the front tires move up and down in unison with respect to the chassis. I assume we're talking about a single spindle-to-spindle link like Brian described. Scott, do you agree with that?

Greg, why do you say that bump steer is not eliminated? Are you also talking about when the tires are not moving in unison, or even when they move together? John

[problemchild]

Both front corners will never move in complete unison. Using the single long link with droop limiters, travel is so limited that, that I expect any bump steer changes would be immeasurable without a dial guage. Most FVs can be measured with a ruler. I did not even need to have my glasses on to measure it on my FST car

[Mystique Racing]

John,

I guess my thought would be if we enter a left hand cornering situation, the right spindle compresses, or goes up, and the left spindle extends, or goes down, I think that will cause a toe change with a drag link.

Maybe we should call it roll toe? because I think that it is different then roll steer. I would think that this would happen on most FV's because of our steering geometry.

[jpetillo]

Scott,

Before you said that I was thinking roll steer since it is a standard term, but you're right it's not specific enough. Even when we say bump steer, we're usually talking about bump toe, using the new separate terminology.

Getting back to what you said, I agree, you'd certainly get roll toe. Any roll steer (separate from toe) would be based on how it got connected to the steering box.

Getting back to bump as opposed to roll, if we separate steer from toe, then I agree with Greg that you could still get bump steer, but not bump toe if you had a single drag bar. John

[lbaconll]

Thnaks Brian, a simple explanation for a complex scenario! It makes sense, maybe We can talk at length in the future re set up? Regards
Larry bacon

[hardingfv32-1]

Does the Citation rocker system come the closest to eliminating these toe issues in a FV?

Brian

[Mystique Racing]

Brian,

I had the same thought. Don't know the answer.

[jpetillo]

Brian,

That's an interesting thought. The Citation linkage does nothing special to eliminate bump steer. It's close, but the rockers are not quite the right length to result in no bump steer. Other cars with conventional Vee steering are probably close as well - maybe not as close as the Citation.

If the Citation steering were modified to give no bump steer, then I believe you could also have no roll steer or toe. That's the case for the car going dead straight. Once you steer the car, then I believe you'd get bump and roll steer and toe. We're not talking about a single drag bar system for this.

However, with the Citation steering it seems that you could adjust your ackermann by changing arm lengths. John

[brian]

First, can someone explain to me how the dimension between two tires can change when locked together by a single tie rod? I can see roll or bump steer in one direction, but the overall toe setting cannot change if the distance between steering arms cannot change. With two tie rods, the difference in radius of the two tires will result in the two tie rods acting independently. I'd like to read the article in Circle Track. While oval specific, the roundy round guys do some weird stuff, I'm always ready for new info. Scott, can you tell me the issue?

Regarding bump steer in vees, if you imagine that the radius of the arc that occurs on the end of the turkey leg, at the point of the steering arm, what you will see is a cone. The center or pointy end of the cone is on the centerline of the h-beam. The citation uses the bell cranks to transfer the ideal tie rod angle out to where it can be connected to a steering box. If you eye ball the Citation outer tie rod, and extend the length beyond the bell crank, you'll see that it intersects the centerline of the midpoint of the h beam.

I accomplished close to the same thing by putting my steering box in front of the h-beam and have the pitman arm terminate as close to the centerline of the h-beam as possible. My car has virtually no measureable toe change with ride height. I say virtually since if I were to use something more precise than a tape measure, like a dial indicator, I'd probably see some change.

[problemchild]

First, can someone explain to me how the dimension between two tires can change when locked together by a single tie rod? I can see roll or bump steer in one direction, but the overall toe setting cannot change if the distance between steering arms cannot change. .

The single tie rod length will not change, nore the length of the spindle arms .... but the length of the 4th side of the apparent parallelogram will vary as the wheels travel.

[jpetillo]

Regarding bump steer in vees, if you imagine that the radius of the arc that occurs on the end of the turkey leg, at the point of the steering arm, what you will see is a cone. The center or pointy end of the cone is on the centerline of the h-beam. The citation uses the bell cranks to transfer the ideal tie rod angle out to where it can be connected to a steering box. If you eye ball the Citation outer tie rod, and extend the length beyond the bell crank, you'll see that it intersects the centerline of the midpoint of the h beam.

I accomplished close to the same thing by putting my steering box in front of the h-beam and have the pitman arm terminate as close to the centerline of the h-beam as possible. My car has virtually no measureable toe change with ride height. I say virtually since if I were to use something more precise than a tape measure, like a dial indicator, I'd probably see some change.

Brian, what you are describing is sufficient to have no bump or roll steer. My Citation rockers seem to be off using rough measuremenrts by about 1/2" to do the job. That's why I commented as such before. In my opinion, what you're doing is right, though. John

[jpetillo]

The single tie rod length will not change, nore the length of the spindle arms .... but the length of the 4th side of the apparent parallelogram will vary as the wheels travel.

Greg, since the pipes in the H-beam are parallel, and the torsion arms swing inside those pipes, I don't see how what you're saying can happen with just bump. Can you explain further? John

[problemchild]

John,
Your question is different than Brian's question.
"Just bump" is a non-existant condition. No car will ever hit a bump equally with both front wheels. Because each side is operating on travel curves that are parallel,as soon as one side travels up or down, the linear difference between the same points on each kingpin will change. You only have a parallegram in 2-D when viewed from above. Once the wheels start moving up or down, the theoretrical parallelogram disappears. This may be "no bump steer" by FV standards, but there will be some steering with any suspension movement.

[jpetillo]

Greg, I understand what you mean now and I agree with your statement. Brian mentioned that he didn't expect the single drag link to prevent toe changes in roll, so I believe that he was talking about no toe change for a bump when both tires move in unison, even though that exact situation happening may be unlikely. John

[hardingfv32-1]

With the normal ultra soft FV front suspension, wouldn't tow changes due to weight transfer under acceleration or braking be of some interest? Is this the one time when both wheels travel in unison?

Brian

[brian]

Braking has always been my concern. Under braking while absolutely not perfect, both wheels are traveling a basically the same rate and distance. Im still thinking about the parallelogram

[jpetillo]

Great, we all seem to agree on that point! I'd like us to focus back on the original thread.

Brian, I think this is the article that Scott was referring to. I need to read it first before commenting further.
http://www.circletrack.com/chassistech/ ... index.html

John

[Mystique Racing]

Toe change under heavy braking is of little importance to me. I am more concerned about where the toe is when you turn the wheel such as transitioning from braking to turn in and throttle application. My Vee will dive under hard braking about 1" Once the throttle is applied the front end will lift about .600" What was even more surprising is from the center of the turn to the track out point the car will lift another .500" This information is from a left hand corner at the end of a long straight (Laguna Seca turn 2) with the potentiometer on the right front shock. Additionally, you will need to multiply my measurements by the motion ratio to get actual dimensions. The data is very interesting and is quite different depending on the type of corner. What this all means to me is that there is a critical range of motion where the toe needs to be stable and linear.