Another 91 vs 93 Suspension Difference: Anti-Dive

7782 Views 10 Replies 5 Participants Last post by Alex W, Apr 19, 2010

Alex W

Discussion Starter · Jan 5, 2010 (Edited)

While swapping to the 93+ front strut rod brackets in preparation for a spherical bearing front strut rod I noticed a difference between the 91 and 93 brackets that nobody talks about. Its tough to measure exactly, but it appears that the 93 brackets lower the front strut rod pivot point by about 1/2", maybe a little less. The effect of this is to raise the front suspension side view instant center from about 8.3" above ground level to about 15.8" above ground level. This will give the 93 suspension nearly twice as much (about 1.9x actually) anti-dive as the 91.

Without knowing the exact location of the CG we can't calculate the exact percentages for either suspension, but if (for example) the 91 has 20% anti-dive then the 93, all else equal, will have 38% anti-dive.

On the subject of anti dive, it is worth noting that increasing caster will decrease anti-dive on a McPherson strut suspension.

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phatvw

· #2 · Jan 14, 2010

Neat!

For me, the whole anti-dive concept has always been really difficult to visualize. Part of the braking load is transferred to the tires through the suspension pivots and control arms rather than the springs/dampers? Just sounds weird to me.

We always talk about spring rates and damping curves but there is a huge amount of tuning/engineering that goes into the suspension geometry. Makes me wish I studied mech eng instead of elec/comp eng in school.

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Alex W

Discussion Starter · #3 · Jan 16, 2010

Yeah, it took me a while to make sense of it too. Looking from the side of the car, it has to do with where the horizontal plane of the front "control arm" (the combination of the control arm and the strut rod) crosses the vertical plan through the center of gravity. Angling the strut rod down causes the horizontal plane to angle down, causing it to cross the vertical plane closer to the CG.

Basically, it works like a roll center, only for dive instead. Its rarely talked about, but you could define an "anti roll" percentage based on the height of the roll centers vs the CG in the same way that anti dive is commonly used.

I'm looking forward to actually getting to drive it, see if there is a noticeable difference. Unfortunately it will have been 5 months or so since I last drove the car when I get to try it... so it may be tough to tell.

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SuperMR2

· #4 · Jan 28, 2010

Alex W said:
On the subject of anti dive, it is worth noting that increasing caster will decrease anti-dive on a McPherson strut suspension.

Never knew this.....I never heard anything bad about increasing caster other than the fact it increase steering effort but made up for it in on-center feel.

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Blue Bomber

· #5 · Feb 25, 2010

Would adding 1/2" spacers to the 91 brackets between the body give the same effect?

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Alex W

Discussion Starter · #6 · Feb 25, 2010

Blue Bomber said:
Would adding 1/2" spacers to the 91 brackets between the body give the same effect?

Yep, it sure would

Although I have to say I much prefer the 93 design (when coupled with a spherical bearing type strut rod). Much more free movement.

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wrxwalsh

· #7 · Apr 14, 2010 (Edited)

would there be in gain spacing down the 93 brackets... and if there is a gain would it be more noticeable at higher caster angles?? i have spherical bearing adjustable strut rods.

Matt

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Alex W

Discussion Starter · #8 · Apr 19, 2010

wrxwalsh said:
would there be in gain spacing down the 93 brackets... and if there is a gain would it be more noticeable at higher caster angles?? i have spherical bearing adjustable strut rods.

Matt

As I mentioned in the anti-squat thread, there is such a thing as too much of these effects. I don't know exactly where that point lies, but like everything else in tuning (suspension or otherwise) its all a trade-off. Its possible that dropping the 93 brackets to gain more anti-dive would be beneficial, especially on a car with lots of caster (get you back toward the original 93 anti-dive level), but you need to be careful not to take it too far and be ready to go back if it makes things worse.

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wrxwalsh

· #9 · Apr 19, 2010

i realize that this is for r/c cars... when i was in the 6th grade i raced these things quite successfully for a couple of years... however the physics is physics

anti squat
http://www.rc-truckncar-tuning.com/Anti-Squat.html

anti dive
http://www.rc-truckncar-tuning.com/Kick-up.html

Matt

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wrxwalsh

· #10 · Apr 19, 2010

im not entirely sure what it changes but on my tension rod it came with spacers to space the rod end left or right of center in the mounting bracket. i assume its to that you can run a higher caster angle without binding the suspension but im not entirely sure... any enlightenment?

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Alex W

Discussion Starter · #11 · Apr 19, 2010

wrxwalsh said:
im not entirely sure what it changes but on my tension rod it came with spacers to space the rod end left or right of center in the mounting bracket. i assume its to that you can run a higher caster angle without binding the suspension but im not entirely sure... any enlightenment?

Yeah, thats the only reason I can see.

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