Need a little help figuring something out

[UMI Tech]

Hey Ron.

Would we ever want to adjust anti-dive on the upper a-arm or perform any other magic up there? I realize there isn't much room to work with but if it would be beneficial...



I'm envisioning our local street stock divisions where that type of adjustment is common.

[Ron Sutton]

Hey Ron.

Would we ever want to adjust anti-dive on the upper a-arm or perform any other magic up there? I realize there isn't much room to work with but if it would be beneficial...

I'm envisioning our local street stock divisions where that type of adjustment is common.

Hey Ramey!

As you know, the anti-dive percentage is a measure of the mechanical leverage the control arm assembly has over the spring under braking. Through testing, I've been all over the place with both anti-dive & pro-dive. For those reading along with us that don't know what they do ... anti-dive makes the spring act stiffer under braking & pro-dive makes the spring act softer under braking. If a car had zero anti-dive, the front end dive (compression) under braking would be solely based on spring rate & shock valving. By running anti-dive in the front control arms, we're making the car dive slower, and therefore less.

For those that don't know this, in addition to being a savvy suspension engineer, Ramey was crew chief in oval track racing.

Also as you know Ramey, in oval track, racers play with different anti-dive percentages ... and even pro-dive ... side to side. I've seen (and tested) running moderate anti-dive on the RF (say 40-50%) & small amounts of pro-dive (10%-20%) on the LF ... to get the front end to dive & stay flatter on corner entry under braking (oval track - turning left). The only thing I don't like about running pro-dive in those situations is we have caster loss on the LF ... which is the opposite of the goal. So after tons of testing, I landed on running small amounts of anti-dive on the LF (20% +/-) and more on the RF (40-50%) ... and of course run a LOT of spring split. (Spring split means we're running different spring rates left & right.) So instead of running springs of 325#/375# L/R with pro-dive on the left & anti-dive on the right ... I preferred 250#/375# L/R ... with a small amount of ant-dive on the left for caster gain. Of course other crew chiefs that also won races ran pro-dive successfully, but with more KPI split in the spindles. There's a lot of different ways tot skin that cat.

For road course track days, autocross & overall performance street performance, we obviously want everything to match side-to-side. We want the same spindle KPI, same anti-dive, same caster & camber. So the only question is how much anti-dive. I know autocross & road racers that run stiff front springs are all over the map ... from 20% to 70% ... depending on their theory. Personally, I like my high travel cars to run moderate anti-dive, typically around 40%, and soft enough front springs to hit my travel target under braking. This makes the springs act softer when not under braking ... which provides several benefits ... smoother ride, less harshness, etc ... but mainly more grip.

A key thing is that the anti-dive be the same on the left & right. I just did the calcs on a SCCA racer with a 3rd gen Camaro ... and the anti-dive was dramatically different side to side. It was 20% on LF side & 41% on the RF side. It came from GM that way. Typical production tolerance variances. That creates more diagonal roll on RH corners ... making the car looser ... and less diagonal roll on LH corners ... making the car tighter. Gotta fix that.

While I am a fan of guys testing to learn what stuff does on practice days & track tests ... I'm not a fan of using anti-dive as a "tuning tool" at race events. Some guys do tune with anti-dive to fine tune their spring rate ... if they need the front end to dive quicker or slower. I prefer to change shock valving to achieve that. In summary, if someone's anti-dive is different from side to side ... fix it for Pete's sake. If your anti-dive is too low ... under 30% ... I suggest increasing it to 40-50%. If it is too high ... over 55% ... and the car isn't "getting on the nose" hard enough ... I'd decrease it into the 40-50% range.