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    Re: How to design and build A-Arm suspension
    #21
    Senior Member deaner's Avatar
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    I've got another question: If the inner-lower a-arm mounts are higher (at ride height) than the outer-lower mount at the spindle (like most are), then when the spindle moves up during compression the spindle moves out. To keep scrub at zero the wheel would need to go into positive camber, right when you want more negative camber. Neg. camber would make for more scrub. What am I missing?
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    Re: How to design and build A-Arm suspension
    #22
    Bend-Tech Dragon Master
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    Scrub is a constant full determined by the spindle kpi / hub / offset / tire. While camber does change your horizontal measurements imo its a non-issue. You need neg. camber to compensate for body roll more than worrying about inc. scrub.
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    Re: How to design and build A-Arm suspension
    #23
    Senior Member deaner's Avatar
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    Quote Originally Posted by JGSturbo View Post
    Scrub is a constant full determined by the spindle kpi / hub / offset / tire. While camber does change your horizontal measurements imo its a non-issue. You need neg. camber to compensate for body roll more than worrying about inc. scrub.
    I think you mean scrub radius not [tire] scrub.

    After studying it and running some scenarios I've come to the same conclusion. Concentrating on scrub and disregarding roll center can make for some awful setups. Not that scrub isn't important, but as the body starts to roll into a turn it needs neg. camber right away, not when it's near full bump.
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    Re: How to design and build A-Arm suspension
    #24
    J.M. Action Figure flyerrider's Avatar
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    Most desert cars run their lower A-arms very close to level at ride height however this is simply due to the 60/40 thought on ride height and wheel travel. This makes the negative camber gain increase in both compression and droop from ride height. For desert cars, roll center is not nearly as important as chassis stability in a high speed straight line (where we place the majority of emphasis). You don't want a lot of TIRE scrub at high speeds simply because it reduces chassis stability.

    If we focused on roll center we could virtually eliminate swaybars however I have yet to see a competitive off-road race car without one. The issue with roll center is that it changes as the suspension cycles. Compression travel lowers roll center and droop travel raises roll center. Roll center is very easy to optimize for road cars given their very low wheel travel and the fact that they are on a SMOOTH road so it makes for a focus point on suspension design. For long travel off-road stuff it is kind of a wash (at least for me).

    As far as camber gain for body roll, with how much travel we pull with these suspensions, the camber gain from minimizing scrub will typically more than cover the required camber gain for body roll. Once again, varying locations in the suspension cycle will change the roll center so the actual axis of rotation for body roll is highly variable. This makes it very difficult to effectively factor into the design.

    Another factor is castor. Castor effectively increases the negative camber of wheel and tire as it turns inward and reduces camber as the wheel and tire are turned outward. If we based our suspension design and camber gain on roll center, how would you effectively factor this in? Generally speaking, to induce body roll you have to be turning. Once again, this is "easy" to factor into a road car simply because weight transfer, body roll, camber, castor induced camber gain, anti-squat, and anti-dive can be optimized to work together through the various chassis modes. This is NOT the case for a long travel suspension. Watch a Formula 1 car under acceleration and braking. There are fractions of an inch of chassis movement. Why? Anti-squat and Anti-Dive. If you had this much anti-squat and anti-dive in an off-road car it would possibly be the worst handling off-road car of all time. It would probably last 2 minutes before it was bucked ass over tea kettle by the anti-squat.

    Another huge variable is how will the car actually turn? In pretty much all aspects of road racing, if you're going around a corner sideways you are doing it wrong. This is completely different in the dirt. A good 75% of turning in off road is done with large amounts of counter steer. With this much being said, do you increase your camber gain to counteract the fact that the castor induced camber gain is inducing positive camber to the outside wheel?

    Do you see where I'm going with this? There are simply too many highly variable aspects to effectively factor in roll center past "low = good, high = bad". This is why I will continue to base my camber gain around what is the highest importance to me: chassis stability at high speed. Minimal tire scrub is a huge factor in this.

    All this being said, this is not to say that your roll center could not be tweaked from increasing the height of the spindle along the KPI line. As already stated, low roll center is good. High roll center is bad. This is about as much thought as I put into it when factoring it into my design. The way I check camber for bodyroll is very simple. I put one side of the suspension at ride height and the other side of the suspension at full compressions. I then draw I line from the center of each tire (contact patch) and check to see if the fully compressed tire and the simulated ground are not past 90 degrees.
    Last edited by flyerrider; 09-30-2013 at 05:05 PM.
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    Re: How to design and build A-Arm suspension
    #25
    Senior Member deaner's Avatar
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    How about some thoughts on rake,achermann,caster,ect...
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    Re: How to design and build A-Arm suspension
    #26
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    Rake is good, rake adds caster, too much caster and you'll get the wheel ripped out of your hand . So only add enough rake to not have too much caster. IMHO 10 degrees of both is good (k.I.s.s.)
    Ackerman- yes, on front steer you'll run the steering arm into the rotor before you have too much in a short wheel base car.
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    Re: How to design and build A-Arm suspension
    #27
    Super Moderator minibajaman's Avatar
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    Quote Originally Posted by JGSturbo View Post
    Rake is good, rake adds caster, too much caster and you'll get the wheel ripped out of your hand . So only add enough rake to not have too much caster. IMHO 10 degrees of both is good (k.I.s.s.)
    Rake and caster are independent variables, they are only directly related if you design things as such. It is not that hard to separate them. But if I were going to make a generalized blanket recommendation, I agree with 10 deg of each.
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    Re: How to design and build A-Arm suspension
    #28
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    You give a particular suspension 10 degrees of rake and you'll add 10 degrees of caster so imo they are very closely related.
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    Re: How to design and build A-Arm suspension
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    Senior Member deaner's Avatar
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    How about front steer vs. rear steer (fore or aft of front "axle", not rear of car)
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    Re: How to design and build A-Arm suspension
    #30
    Super Moderator minibajaman's Avatar
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    Quote Originally Posted by JGSturbo View Post
    You give a particular suspension 10 degrees of rake and you'll add 10 degrees of caster so imo they are very closely related.
    No. While this can happen, there isn't any reason it needs to. Sometimes the numbers work out well, like 10 deg rake and 10 deg caster. But if you want 10 deg of rake and 5 deg of caster like I have on my buggy, this is also achievable. This has been debated many times before, no real need to do it again.
    https://www.minibuggy.net/forum/suspe...vs-caster.html
    https://www.minibuggy.net/forum/suspe...questions.html

    Quote Originally Posted by deaner View Post
    How about front steer vs. rear steer (fore or aft of front "axle", not rear of car)
    To me this is more of a packaging issue, both for rack placement in the chassis and steering control arm placement on the upright. Many prefer front steer because it puts the tie rods in extension rather than compression while turning and keeps the rack away from your feet. I have used rear steer for small buggies because you can put the rack behind your feet to keep the overall vehicle length short, and I haven't had any issues with tie rods bending.
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