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PostPosted: May 8, 2008, 12:05 am 
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The voice of reason
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Jack, just thought I'd mention friction will cause a lack of centering and also make a car or motorcycle wander. You will feel things like wander before you notice the friction itself. I think it might come up when people rebuild racks if they make them tight, but maybe that's unlikely...

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PostPosted: May 8, 2008, 9:04 am 
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Randy, the spindle was positioned in 7 degrees of caster when I took the measurements of the balljoint locations from it. You can see that not only do things move in the x axis as you change caster, but in the y axis also. Just a few degrees of caster change will move the pivot at the end of the steering arm a considerable distance.


You should have your balljoints 1" apart in the x axis. That's about where they were when I took the measurments.

You don't need to worry about the lateral position of your instant center. That field should say "swingarm length." The lateral movement you are trying to minimize is that of the roll center when you introduce roll to the chassis. How about a screen shot in 3 degrees of roll?

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PostPosted: May 8, 2008, 10:02 am 
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You can also hit ALT and ENTER to get a full size MSDOS window. Now hit Prnt Sc or Fn+ Prt Sc and you can paste into a Word file. (You could also take a photo of this full screen and upload.

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PostPosted: May 8, 2008, 10:33 am 
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RandyBMC wrote:
Image

As you can see, there is no caster. Do you set the caster by changing the X axis for the control arm mounts? Not sure how to do that. ...

Thanks!
Randy


Move the upper ball joint toward the rear of the car to get more caster.

-dave

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PostPosted: May 8, 2008, 5:28 pm 
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OK, here is the car shot with caster, first shot with zero roll, second shot with 3 degrees of roll:

Image
ZERO ROLL

Image
3 degrees of roll

Here are the questions...

Do the ball joint locations look appropriate? Without the donor pieces in place yet, I am basing all of this on what I have seen on other posts for the Miata measurements.

Is the camber gain realistic? Do I need to introduce steer with roll to get it right?

Thanks for the help guys!

Randy


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PostPosted: May 8, 2008, 6:18 pm 
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The balljoint locations (assuming they are the ones I posted above in this thread) are sound in terms of their releationship with each other. Of course without having the exact measurements of thier location mounted with your chosen tire and wheel at your chosen static alignment there is some speculation involved.

Since you are using 23" tall tires I added 1.5" to my balljoint heights to arrive at the ones I posted for you. Since my design is based around a 20 tall tire. (racing slicks)

I built my car using those numbers and it seems to have worked out fine.

The camber gain looks about like what you would expect with that swingarm length. Steering angle shouldn't effect camber. What are you looking for? Based on what I've seen and read, a loosely sprung car with lots of suspension travel won't need as much camber gain per inch of bump as would a tightly sprung ride with very little suspension travel.



If anyone else has some good measurements of the Miata spindle please share them and compare them.

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Last edited by chetcpo on May 8, 2008, 7:07 pm, edited 1 time in total.

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PostPosted: May 8, 2008, 6:40 pm 
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I plan on having about 4" of travel in bump at this stage of planning. I would like to have the car soft enough to deal with tracks that aren't that great surface wise (like La Junta here in Colorado), but not so much lean that the car is a cruiser. I plan on using swaybars as well as the springs for roll control, so I think I may get away with softer rates than folks without them.

With that in mind, I am basically after having the camber gain equate somewhat to the roll, as to keep the outside (loaded) tire vertical throughout the travel. I would have to say you are right in that if you have less travel, the wheel arc is reduced, so you will remain closer to your static camber setting. If you are going to limit body roll to 2 or so degrees, than the static camber setting of 2 should work.

How did you figure optimum camber gain per inch of bump? I've been looking for a formula for it and can't seem to find it.

Thanks again for your help! I'm stuck in SFO waiting for my delayed airplane to get here...

Randy


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PostPosted: May 8, 2008, 7:07 pm 
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dhempy wrote:
Move the upper ball joint toward the rear of the car to get more caster.


Not to hijack the thread Dave, but this brings up an interesting point. In Bill Mitchell's manual that accompanies his WinGeo3 software, he states the following:

"SAE Caster Angle - The angle in the side elevation between the steering axis and the vertical. It is considered positive when the steering axis is inclined rearward (in the upward direction) and negative when the steering axis is inclined forward."

Nothing revolutionary, but this is opposite to what wishbone provides. The interesting part is that he stated just before this section that:

"Usually the line slants toward the front of the vehicle"

So this would indicate that he would expect a negative caster value and that the UBJ would lead the lower. This is contrary to what you have explained.

So which is right? When you think about it from a self-centering persepctive, Bill Mitchell's explanation seems to make sense. But I can also see how a leading LBJ might be beneficial in counteracting torque moments from potholes and the like.

Any ideas?


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PostPosted: May 8, 2008, 7:13 pm 
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The lower ball joint should always lead the upper. I'm no expert, but I can't think of any exceptions to that rule.

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PostPosted: May 8, 2008, 7:14 pm 
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Image

"When you turn the steering wheel, the front wheels respond by turning on a pivot attached to the suspension system. Caster is the angle of this steering pivot, measured in degrees, when viewed from the side of the vehicle. If the top of the pivot is leaning toward the rear of the car, then the caster is positive, if it is leaning toward the front, it is negative. If the caster is out of adjustment, it can cause problems in straight line tracking. If the caster is different from side to side, the vehicle will pull to the side with the less positive caster. If the caster is equal but too negative, the steering will be light and the vehicle will wander and be difficult to keep in a straight line. If the caster is equal but too positive, the steering will be heavy and the steering wheel may kick when you hit a bump."

I copied this from a google search, but it explains it pretty well. You are moving the upper ball joint rearward, which gives a more positive caster. The rearward is made by using a negative number, but it is in fact giving the proper caster.

Hope that makes sense!
Randy


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PostPosted: May 8, 2008, 7:35 pm 
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That's what I've always been told too, until I got this software. Now Bill Mitchell is no slouch when it comes to race car design so I'm trying to understand his logic rather than dismiss it right away.

Allow me to play devil's advocate here. Although not formally mentioned in what I've read so far, I can guess that by using negative caster I can build in some self centering into the steering, what is the reasoning behind using positive caster as opposed to negative?


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PostPosted: May 8, 2008, 7:41 pm 
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Ah ha! Got it..Milliken & Milliken...

"With positve caster angle the outside wheel will camber in a negative direction ... while the inside wheel cambers in a positve direction..."

Thanks for allowing the diversion.


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PostPosted: May 8, 2008, 8:15 pm 
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Randy, 4" of bump is a lot of travel. To use that you will need a lot of ground clearance, giving you a high CG. You want this for good reasons, but the high CG is really going to hurt on the track. More weight transfer, more roll, more trouble trying to get the geometry right, more clumsy in transients ( swerving and even just turn entry ), more likely to roll.

I forget what priority track use you are going for. Slicks?

If your surface is rough, try to keep your unsprung weight down. These cars are lighter then your mini so will run much softer tires, that might help with the roughness. Mount your battery and seat low for CG, less weight transfer and roll will help with bump travel.

Ask Chet about 20" slicks...

Modernbeat runs 2" ground clearance or less on his rack car. So the compromise for bump travel seems towards limiting for ground clearance.

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PostPosted: May 8, 2008, 10:33 pm 
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Is it direct though - bump vs. ride height? I'm asking this because I just had a 15 hour work day, and my brain is toast right now. If I want 3.5" of ground clearance, do I also want 3.5" of travel, or is there a difference in the trig because of the angle of the shock. If I am thinking about it right, I may even need less than 3.5" of travel for 3.5" ground clearance.

I plan on using the car on the street with some R-compounds but not slicks. Going with a 225/45-15 on the new Toyo 888, about 22.8".

So let's say 3" travel instead of 4".

Same questions from above though!

Thanks again!
Randy


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PostPosted: May 12, 2008, 8:31 pm 
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Jack,

Can you just use a straight to mount the heim, then angle the bars coming from that to keep from having to use the slanted mounting points? If so, the arm length would just be measured differently - taking the distance from the heim mount to the ball joint.

Does that sound right to anyone? I want to use Jack's mounts and not have to modify my LA and LB after all the work to make it!

Thanks,
Randy


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