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<blockquote data-quote="Rokcrler" data-source="post: 630942" data-attributes="member: 48"><p><strong>Re:</strong></p><p></p><p>Here is the 75% rule explained by Jessie Haines:</p><p>75% Rule Explained!</p><p></p><p>For probably 15 years, I've been hearing about the 75% rule when it comes to suspension geometry: "your upper links should be 75% the length of your lower links". But why? Never once have I heard an intelligent explain of why. I'm guessing that at some point, someone read it in an engineering book somewhere, it got printed in a magazine, and it was then know as the rule! I can't say for certain the theory behind the 75% rule, but I believe I have a pretty solid theory on the basis for it (for a rear suspension). </p><p></p><p>I'm not going to spend hours explaining everything about anti-squat, but the change in AS throughout suspension travel is why the 75% rule was created. With your uppers shorter than your lowers, as your suspension droops, the AS will decrease. As the suspension compresses, your AS will increase. That means as the suspension droops, the geometry changes will DECREASE the forces pushing your axle down, away from the frame. As the suspension compresses, the change in geometry will INCREASE those same forces. This means the suspension will be less likely to unload as it droops. It also stiffens the suspension as it gets closer to bottoming out. </p><p></p><p>This sounds great in theory! The reality is, the 75% rule shouldn't be at the top of your priority list when you're building your suspension. Things like packaging, pinion angle change, and caster change may be more important in your situation. One example, on an Ultra4 or race truck, pinion angle change should be a higher priority. When the suspension goes full droop at 70 mph, you really don't want your pinion angle dropping and causing vibrations. In other situations, your driveline angle may bind at full droop, and you could remedy this by changing the length of the upper links. </p><p></p><p>Because AS forces are reversed on a front suspension, the 75% theory doesn't really make any sense on a front end. I can only assume the "rule" was based on rear suspension only. Designing a 4-link is almost always a compromise. You just need to figure out what's important to for your situation.</p></blockquote><p></p>
[QUOTE="Rokcrler, post: 630942, member: 48"] [b]Re:[/b] Here is the 75% rule explained by Jessie Haines: 75% Rule Explained! For probably 15 years, I've been hearing about the 75% rule when it comes to suspension geometry: "your upper links should be 75% the length of your lower links". But why? Never once have I heard an intelligent explain of why. I'm guessing that at some point, someone read it in an engineering book somewhere, it got printed in a magazine, and it was then know as the rule! I can't say for certain the theory behind the 75% rule, but I believe I have a pretty solid theory on the basis for it (for a rear suspension). I'm not going to spend hours explaining everything about anti-squat, but the change in AS throughout suspension travel is why the 75% rule was created. With your uppers shorter than your lowers, as your suspension droops, the AS will decrease. As the suspension compresses, your AS will increase. That means as the suspension droops, the geometry changes will DECREASE the forces pushing your axle down, away from the frame. As the suspension compresses, the change in geometry will INCREASE those same forces. This means the suspension will be less likely to unload as it droops. It also stiffens the suspension as it gets closer to bottoming out. This sounds great in theory! The reality is, the 75% rule shouldn't be at the top of your priority list when you're building your suspension. Things like packaging, pinion angle change, and caster change may be more important in your situation. One example, on an Ultra4 or race truck, pinion angle change should be a higher priority. When the suspension goes full droop at 70 mph, you really don't want your pinion angle dropping and causing vibrations. In other situations, your driveline angle may bind at full droop, and you could remedy this by changing the length of the upper links. Because AS forces are reversed on a front suspension, the 75% theory doesn't really make any sense on a front end. I can only assume the "rule" was based on rear suspension only. Designing a 4-link is almost always a compromise. You just need to figure out what's important to for your situation. [/QUOTE]
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