yet another vert baja

DesertGuy · Post by **DesertGuy** » Mon Jul 28, 2008 12:24 pm

Scabbyfab wrote: I've questions about the different styles of a-arm setups too.
It seems the ones with the shorter upper arms have better
"scruff ratio" but they look kinda goofie to me. I don' think
scruff will matter that much in the sand. what do you guys think?
Do you think 3/4 heims are stout enough for the front spindles?
Come on throw a guy a bone.

Unequal length A-Arms help dial in the KPI in relation of the spindle to the contact patch of the tire. This helps with steering and you can fine tune the camber change of the wheel/tire through the suspension cycle. Although it may be less in the sand to a point, srcubbing with effect handling.

If for a sand only play car you could most likely get by with 1/2" chromoly heims. For a dual sport/desert/or stoutness, a 3/4" chromoly heim should do well. I run 3/4" heims... alot has to the design of the front suspension and how you plan on loading the heims and in what orientation.

An excellent read is Herb Adams "Chassis Engineering" book for basic fundamentals. It is geared towards pavement but the principles are similar... if not the same.

...It all sounds "technical" but in reality it makes sense once you start piecing it together.

Scabbyfab · Post by **Scabbyfab** » Mon Jul 28, 2008 4:35 pm

What is KPI?

bajaherbie · Post by **bajaherbie** » Mon Jul 28, 2008 4:43 pm

king pin inclination....

how much of an angle it leans in and/or back.....

DesertGuy · Post by **DesertGuy** » Mon Jul 28, 2008 4:54 pm

Although this article addresses RC car handling it is actually a pretty decent primer.

http://www.rctek.com/handling/index.html

zancat · Post by **zancat** » Mon Jul 28, 2008 7:30 pm

Scabbyfab wrote: I've questions about the different styles of a-arm setups too.
It seems the ones with the shorter upper arms have better
"scruff ratio" but they look kinda goofie to me. I don' think
scruff will matter that much in the sand. what do you guys think?
Do you think 3/4 heims are stout enough for the front spindles?
Come on throw a guy a bone.

Like DG said, the inner upper pivot location is what defines how the spindle acts, the outer draws a line through the lower forming the inclination angle. The intersection of this line and the tire patch is where the tire pivots for steering. If you want the tire to pivot about the center of the tire patch, then that line needs to run through the upper and lower outer pivots and intersect the ground at the center of the tire patch.

The location of the inner upper pivot defines camber gain / loss, etc.

Decide how you want the suspension to behave, then build it to do that.

Good luck and nice job on both cars!

Zancat

Scabbyfab · Post by **Scabbyfab** » Tue Jul 29, 2008 6:37 pm

A friend drew this up in solid works for me. It has about 1.5" scrub
and 20 inches of travel using a 12 inch shock. the spindle is from
kustom 1. The lower arm length is about 27"
[img][img]http://i304.photobucket.com/albums/nn17 ... assem1.jpg[/img][/img]

So correct me if I'm wrong, the inboard/outboard motion of the tire
from the arc of the radius of the a-arms is compensated by the
negative camber produced by the upper arm being shorter than
the lower arm, so that the tire patch has minamal scrub. right?
It (the tire) just does a lot of tilting? The shock angle is produced
by having the lower mount 2/3 down the length of the arm
and the upper mount placed so that at full bump the shock is
90 deg to the arm. Does this sound right? How does the negative
camber effect bump steer? Is bump steer more important than tire
scrub? Can you have alot of tire scrub and still have little or no bump steer?
It seems many of the sand car builders are using close to equal length arms,
and leaning the shock way over. Is that just for looks?
Or is there a purpose? Isn't the Kpi dictated by the Kustom 1 spindle?

zancat · Post by **zancat** » Tue Jul 29, 2008 7:56 pm

So correct me if I'm wrong, the inboard/outboard motion of the tire
from the arc of the radius of the a-arms is compensated by the
negative camber produced by the upper arm being shorter than
the lower arm, so that the tire patch has minamal scrub. right?

Yes. Sorta. It not so much the length, but the location of the the inner pivot that dictates how much camber gain the spindle has (which ends up being a certain length).

It (the tire) just does a lot of tilting? The shock angle is produced
by having the lower mount 2/3 down the length of the arm
and the upper mount placed so that at full bump the shock is
90 deg to the arm.

This is called motion ratio. How much travel opposed to how much shock travel. 90* to the arm will be the most progressive spring rate, but some claim +/- 10* is negligable.

Does this sound right? How does the negative
camber effect bump steer? Is bump steer more important than tire
scrub? Can you have alot of tire scrub and still have little or no bump steer?

They are independant for the most part.

It seems many of the sand car builders are using close to equal length arms,
and leaning the shock way over. Is that just for looks?

It will be a softer ride. More leverage against the spring. Like laying down the shocks on an old motorcycle, but that's probably before your time

Could be for packaging reasons too.

Isn't the Kpi dictated by the Kustom 1 spindle?

Yes.

Having fun yet? It's an excercise in spacial skills isn't it? Don't forget about caster... LOL

Zancat

Scabbyfab · Post by **Scabbyfab** » Tue Jul 29, 2008 8:10 pm

Thanks for all the useful info from everyone, special thanks to Zancat
Well it's time to make the jigs for front and rear arms I'll keep ya posted

Oh and one last question for now. Won't the 9 deg angle of the front
a-arm rails give me my caster? Oh and lay down shocks are not before
my time

Projectile Motion · Post by **Projectile Motion** » Tue Jul 29, 2008 8:15 pm

Well it's time to make the jigs for front and rear arms I'll keep ya posted

Mock ups first! This is coming from some one who's first attempt at A-arm design ending up with 6" of travel and 10° camber change.

It looked good on paper I swear!

zancat · Post by **zancat** » Tue Jul 29, 2008 8:18 pm

Scabbyfab wrote:Thanks for all the useful info from everyone, special thanks to Zancat
Well it's time to make the jigs for front and rear arms I'll keep ya posted

Oh and one last question for now. Won't the 9 deg angle of the front
a-arm rails give me my caster? Oh and lay down shocks are not before
my time

It should. I have never done it that way. 9* is a lot of caster... I hope you are using power steering.

Here I gave you a chance and you went and dated yourself.

Good luck and please do keep us posted.

Zancat

DesertGuy · Post by **DesertGuy** » Tue Jul 29, 2008 8:21 pm

Scabbyfab wrote:Oh and one last question for now. Won't the 9 deg angle of the front a-arm rails give me my caster?

That is a fairly common practice to jig the components at 0 and build the castor into the frame. However, remember that suspension is a trade-off of positive and not-so-positive. What you want to gain on one end... you need to lose (to a point) on the other... hence mock-ups are typically a better idea, unless you have money to blow.

Scabbyfab · Post by **Scabbyfab** » Tue Jul 29, 2008 8:32 pm

Thanks guys for that warning! We (as in I'm watching him over his shoulder) are using the solidworks program with all its features to do the mockups. It allows you to cycle the travel and analyze all the motion and angles, pretty neat really but I'm with you guys, who's got the money to blow. surely not I.

zancat · Post by **zancat** » Wed Jul 30, 2008 5:57 am

Scabbyfab wrote:Thanks guys for that warning! We (as in I'm watching him over his shoulder) are using the solidworks program with all its features to do the mockups. It allows you to cycle the travel and analyze the all the motion and angles, pretty neat really but I'm with you guys, who's got the money to blow. surely not I.

That's excellent! Much better to screw up virtually. LOL

Nice chassis Desertguy!!

Zancat

Scabbyfab · Post by **Scabbyfab** » Wed Jul 30, 2008 7:01 am

Indeed Desert Guy, Very nice chassis, I really like the way you carried the upper a-arm rails further back into the car, is there any issue with foot room there? What prompted you to go with sqaure tubing for the lower frame rails? Was it for ease of fastening to the body? Is square tubing stonger than round for that application? how much caster did you build into your frame?

DesertGuy · Post by **DesertGuy** » Wed Jul 30, 2008 7:42 am

The pic above is not of a frame I have done, but used it to show what I was talkin' 'bout. In the pic the square tubing is for "plumbing" purposes only.

Most chassis that I am involved with are similar to the spine and bulkhead like the picture. I will see if I can find those pics so you can see what I mean.