front frame geometry design help

[350skabarat]

Hey guys im busy designing a new front frame for a banshee, I just need abit of help. Im designing the front end towards the 250r geometry in terms or how the setup acts during travel not in terms of exact measurment. Ok here are my questions:

 

 

1. During the travel of the suspension ,due to the fact that the radius of the top a-arms is smaller than the radius of the bottom a-arms, you have a camber adjustment which how i understand normally increases negatively during travel. Now what i would like to know is what a good value for camber adjustment is? like for example on compression of the suspension you want your camber adjustment to increase by 3 or 4 degrees negatively (you want a camber of negative 3 degrees to go to negative 6 degrees) hope im explaining correctly.

 

2. During the travel of the suspension you also have caster adjustment. This happens because the top a-arm mount rail is not parralell to the bottom a-arm rail from a side view of the frame. So what id like to know is would you want your caster adjustment to increase positively on compression or should it decrease positively. For example say your caster on full extension of travel is positive 5 degrees, now when the suspension compresses should it increase to positive 8 degrees or decrease to positive 2 degrees? Also how much should it increase or decrease (would like it to be like a 250r)?

 

my guess on the caster is that it should decrease on compression so that its easier to turn on decceleration? Im not sure so thats why im asking. Thanx for any help guys

[350skabarat]

Is no one ganna help me:(

[DirtDiggler69]

Go talk to sicivicdude on www.blasterforum.com , he'll set you up on the right track .

[Larry's Shee]

http://bansheehq.com/forums/index.php?showtopic=75316 Have you checked this thread? Maybe PM him for ideas

[sicivicdude]

@Blastard:

 

Or Sicivicdude could come talk to him

 

I got your PM over on BF but figured I'd go ahead and answer your question where anyone might be able to benefit from it. Knowledge is only powerful if everyone knows it, otherwise it's only dangerous!

 

 

Hey guys im busy designing a new front frame for a banshee, I just need abit of help. Im designing the front end towards the 250r geometry in terms or how the setup acts during travel not in terms of exact measurment. Ok here are my questions:

 

 

1. During the travel of the suspension ,due to the fact that the radius of the top a-arms is smaller than the radius of the bottom a-arms, you have a camber adjustment which how i understand normally increases negatively during travel. Now what i would like to know is what a good value for camber adjustment is? like for example on compression of the suspension you want your camber adjustment to increase by 3 or 4 degrees negatively (you want a camber of negative 3 degrees to go to negative 6 degrees) hope im explaining correctly.

 

2. During the travel of the suspension you also have caster adjustment. This happens because the top a-arm mount rail is not parralell to the bottom a-arm rail from a side view of the frame. So what id like to know is would you want your caster adjustment to increase positively on compression or should it decrease positively. For example say your caster on full extension of travel is positive 5 degrees, now when the suspension compresses should it increase to positive 8 degrees or decrease to positive 2 degrees? Also how much should it increase or decrease (would like it to be like a 250r)?

 

my guess on the caster is that it should decrease on compression so that its easier to turn on decceleration? Im not sure so thats why im asking. Thanx for any help guys

 

 

The radii of the a-arms is set. With any unequal a-arm setup where the upper is shorter and the wheel is at 0 camber at sag point (with rider weight static) the camber will always be negative as it moves past that point (into the compression region). The upper arm will ALWAYS keep a shorter arc than the lower arm if they're are unequal length with the upper shorter.

 

What this means is two things. If you run "flat bottom" tires like MX tires, you're going to be chewing up the inside lip of the tire when the entire front suspension travels (like coming off a big jump or hitting a large whoop). This is the disaavantage of the unequal length suspension, there is however, an advantage and the advantage outweighs the disadvantage. While running on the "edge" of the tire isn't a real detriment on dirt suspension isn't only design to travel in 2D (wheels straight ahead while both a-arms experience the same travel and forces) it's designed to operate in 3D. The unequal length a-arm setup helps during frame roll to keep the contact patch larger on the outside front tire during a turn. This means that the weight tranfer goes onto the "solid" tire and you grip easier.

 

To the question of what's the "proper" camber setting actually is determined by frame rigidity, rear tire ply rating, and compression control setting. A heavy side wall rear tire on a super stiff frame with strong shocks requires little to no negative camber at sag. Frame roll will be limited by the rear tires and frame and any bouncing (that would disturb the contact patch) will be limited by the strongly valved shocks. If you have a stock ungussetted frame with balloon rear tires, you may want 0-3° of static camber so that as the frame rolls in a turn, your outside tire will but more, not less.

 

99% of the quads out there do not have variable caster (the upper and lower a-arm pivot planes aren't parallel). In fact, I can't think of a single one that does off the top of my head. Most designers design in "static" caster (including the venerable TRX250R) in order to simplify the operation of the suspension. When you're talking about caster (versus camber) a little bit IS A LOT. Most automobiles have ~2° caster built in for "centered" steering while most atv's have less than 4° (more demanding turning but also more predicatable) The notalble exception being the TRX250R, which has a bit more.

 

There are two ways to adjust caster, you can either "tilt" the a-arms so that their travel moves backwards as it moves upwards or you can stagger the outer pivot points so that the lower is "in front" of the upper. One means that the spindle moves in relation to the steering stem (which affects your ackerman angles) and the other means that the a-arms don't do any correcting themselves, only the spindles do. Most suspension designers go for both actually. The blaster (and banshee) both have a tiny bit of "cant" to the frame which points the pivot plane upwards towards the sky. (the TRX250R is an extreme example of this) while the upper ball joint is ALSO located about 1/2" behind the lower ball joint (to roll the pivot plane of the spindle "back") to increase caster.

 

You COULD design a suspension with a "violent" swing in caster by changing the angle between the upper pivot plane and lower pivot plane. I'm not sure what you'd want however. Seems to me that you'd want approximately the same amount of caster (between 2-5°) throughout the entire travel but if you were going to make it change, you'd want more caster towards the upper end of the travel to make the suspension want to self center during large jumps with full suspension bottom.