Antidive, how to adjust it... Options

[Mike T]

I am somewhat familiar with antidive characteristics with a double a-arm type suspension but was surprised today when I read on the Pelican board a discussion of increasing antidive on a 911 front suspension(similar to our 914) by shimming the front torsion bar mount down.

Has anyone played with that at all? What was your result? How much of a shim would be needed?

Mike T

[Joe Ricard]

Hmm, I just increase the rebound in my Koni's. and also installed 22mm torsion bars.
Keeps the ass end planted under hard braking so the back tires can help a little bit with stopping the car by slowing weight transfer.

I suppose somebody with good understanding of roll centers and stuff can add a BUNCH more.

[drgchapman]

The Smart Racing PDF catalog mentions this mod on page 43 in the ERP 935 front suspension upgrade kit. www.smartracingproducts.com
They raise the inner pivot point 20mm. This would effectively lower the car a little. Lowering the front pivot will raise the front of the car slightly.
This will change the geometry as well, presumably for the better.

[groot]

The benefits are certainly debatable. The downside is impact harshness and the inability for the suspension to keep the tire on the ground when facing a sharp bump (transition to concrete for example).

[Mike T]

The benefits are certainly debatable. The downside is impact harshness and the inability for the suspension to keep the tire on the ground when facing a sharp bump (transition to concrete for example).

Maybe that's why Staniforth says it goes in and out of fashion in the racing world. There are tradeoffs.

I have a V8 autocross car and I am fighting rear end looseness on hard braking. When I stop with the steering dead straight there is no problem but add any steering input and the rear tends to want to slew around.

I have adjusted rear Koni firmness and it seems to help with a firmer setting but then in transitions such as a high speed slalom the back gets looser.

The front bites well with no understeer.

This week I am evaluating the car. Alignment settings, bushing condition, etc.

Car Specs;

weight 2215 with driver
58% weight on rear
52% weight on drivers side
Koni reds front and rear
250lb rear springs
22mm torsion bars
22mm Welt front swaybar set about 1/2
no rear swaybar
-1° camber front
3/16" toe out front
?° camber rear
0 toe rear
23.5x12-15 Hoosier R25a rear on 14" wide wheels
22.5x9-15 Hoosier R25a front on 10" wheels
Weltmeister Polygraphite bushings front and rear(8 years old)
camberball upper strut mounts
turbo tie rods
welt bumpsteer spacers
lowered with front control arms and rear semi trailing arms parallel to ground

Mike T

[drew365]

If you're not suffering from understeer, you might be able to step up to 23m torsion bars up front to keep the rear end in place. Just a thought.

[Jeroen]

braking and turning at the same time is never a good recipe in any car, specially in one with a rear weight bias
you can use it to your advantage (trailbraking) but other than that, try to avoid it

you could also add some toe in at the rear
if you have 0 toe at stance, then unloading the rear will cause toe out and that will cause more/easier oversteer

from what I gathered, shimming the front of the a-arms effectively causes your front susp. to bind up (move less freely) and thus you get "anti dive"
but I'm sure you'll sacrifice in other handling areas

might be worth to look if you can shift some more weight from the back to the front (move parts around in your car)

[groot]

You have too much rear brake bias.

[Joe Ricard]

Mike T, I love your car set up. very similar to mine except you have that mad Horsepower yo!!

[Mike T]

[quote name='Jeroen' date='Jul 18 2006, 06:52 AM' post='731093']
braking and turning at the same time is never a good recipe in any car, specially in one with a rear weight bias
you can use it to your advantage (trailbraking) but other than that, try to avoid it

Agreed. It's actually not my intent to do that to any great extent. I wouldn't mind make the car a bit less sensitive to it.

you could also add some toe in at the rear
if you have 0 toe at stance, then unloading the rear will cause toe out and that will cause more/easier oversteer

I'm going to string the car this week to recheck my settings so I have a baseline to adjust from.

from what I gathered, shimming the front of the a-arms effectively causes your front susp. to bind up (move less freely) and thus you get "anti dive"
but I'm sure you'll sacrifice in other handling areas

That wouldn't be desirable at all. I'm still open minded on this idea.

might be worth to look if you can shift some more weight from the back to the front (move parts around in your car)

I intend to lighten the rear, budget permitting as that is a big chunk of cast iron back there. But adding weight at the front would increase the polar moment and perhaps make the car more stable. Probably not what you meant though.

I also have replaced the stock proportioning valve with a tee. Rear brake lockup occured to me but I haven't checked to see if it is happenng though.

I tend to over analyze things I think. Sometimes I miss the obvious.

thanks;

Mike T

[groot]

It may not manifest itself in rear lockup, but you may just decrease the amount of grip you have available for lateral acceleration (you know... the traction circle deal).

Do you have any rear steer issues? I'm asking because I did, beyond what's normally built into the 914 rear suspension. My right side rear outer suspension console was tearing away from the body. Caused some awful things to happen under heavy braking.

BTW... personally, I wouldn't go after antidive geometry... and certainly not to address your problem. It won't help your problem. Antidive (as you know) works by forcing your wheel to travel forward when it travels into jounce, creating a force that works against the braking forces. While it won't cause your suspension to bind if done correctly, it will not travel as freely as one would like it to during jounce events because it has to overcome that additional force.

[Mike T]

I replaced the stock prop valve with a tee when I did the V8 swap on advice from this forum.

I could have rear steer issues. The PolyGraphite bushings on there now are 8 yrs. old and may be worn. I've seen some pictures here of worn ones.

I felt antidive may keep the back from rising under braking and keep the back from rolling. I imagined the car squatting level on hard braking instead of pitching forward. Not really sure what it does now as I'm in the car at the time...

Mike T

[Demick]

Put in an adjustable brake porportioning device so that you can adjust the rear bias. Doing too much braking with the rear wheels will explain your symptoms.

You stated you replaced the stock prop valve with a T when you did the V8 swap, but you failed to state what brake upgrades you did. Please don't say you are running a stock brake system with those huge tires and a V8.

Demick

[byndbad914]

It may not manifest itself in rear lockup, but you may just decrease the amount of grip you have available for lateral acceleration (you know... the traction circle deal).


BTW... personally, I wouldn't go after antidive geometry... and certainly not to address your problem. It won't help your problem. Antidive (as you know) works by forcing your wheel to travel forward when it travels into jounce, creating a force that works against the braking forces. While it won't cause your suspension to bind if done correctly, it will not travel as freely as one would like it to during jounce events because it has to overcome that additional force.

(IMG:style_emoticons/default/agree.gif) on both accounts. I have an adjustable bias in my V8 car (I was 59.5% rear weight dist btw) and I was messing with the bias on the track. I added a "fair" amount to the rear to see the difference and damn near didn't make it thru turn 3 (a harder braking point on the track) at Willow Springs! Car was skating like a mofo, and wasn't locking up. You will just be loose as nuts when on the binders entering a turn with too much rear.

And the ass-end wanted to come around ALL the time on that car. Do you have an oversteer tendency? You mention no understeer, but didn't imply if that meant neutral, slight oversteer, or oversteer. Mine would OVERSTEER at any speed and the low polar moment is crazy. It is the only car I have ever spun and I have hung a few out there in full drifts before. Not the 914.

So I maxed out my swaybar and could get the car a little more neutral with throttle understeer when I had the balls to be on it and push out off the apex.

You need an adj bias valve and probably need a stiffer front setup. You state the front bar is about at 1/2 - does it help to max it? I would try that if you haven't. If the car gets better about sliding/drifting as opposed to snapping around, then you know you need bigger front T-bars (so you have adjustability in the bar again to tune to track conditions). Obviously if you start to push you don't need bigger bars. But if you don't plow, go bigger.

Don't bother with the anti-dive stuff - better to tune the car with springs and such. May want to consider coil-over fronts (RSR struts) to get higher spring rates. That is what I have. I was running 350 lb/in fronts and 450 rears before and the front was too soft - I maxed the Tarret bar as stated and have no rear bar. I am starting out 450/450 when the car is done and I have some 400s and 500s and 550s to tune the car at the track. Stiff setup, but with good tires (you have huge tires and soft compound at 25 btw so use the friction) it will work better IMO.

You autocross so I wouldn't expect you to go that stiff but you may need to be in the 300-350lb/in front to get that car to act right. Hard to say of course as it is all dynamic. I forget what the Tbars are equiv too, but maybe/probably not 350 lb/in.

Also, if you don't have one, get a tire temp gauge and set the camber correctly. You may not have enough rear camber for instance and tire temp gradients will tell you for sure. Doesn't matter what the value is frankly (I never pay attention to that) whether you tell me 1deg neg or 2deg neg. All I care is the tire temps are constant across the surface.

[brant]

I'm also going to bet that and adjustable bias valve will make the car feel much better..

I ran modified brakes with an adjustable valve for years
at one point, I decided to switch to a "T" and see if I was giving up anything. I really don't think the rear wheels were locking on my car, but they must have been on the verge of it... because the first Day of trying the new set up, I spun the car in a braking zone at 65mph and went straight through the corner while facing backwards.

damage was luckily minimal.

I think I contributed to the problem by using a tiny bit of subconscious trail braking.

needless to say, I finished the weekend and put the dang adjustable valve back in before the next event.

65mph backwards into a ditch is not a fun thing

brant

[Mike T]

Thank you all for replying. I'll try and answer all.

I measured the camber tonight and inspected the front Weltmiester Polygraphite bushings. I could pull the front bushings right off with little trouble. They didn't look excessively worn. What I did find was way more negetive camber on the drivers side than the passenger side. I certainly didn't set it up that way. I will have degree numbers tomorrow.

Nothing was loose when I dissasembled it. Don't know why there was so much difference right to left.

One thing to note is this car is originally from Texas and is completely lacking in any sort of rust at all. When I removed the backing plates years ago the original factory part numbers were still readable. Perfect hellhole. Perfect longs.There isn't even any surface rust on the control arms. Just factory paint. A rare find in the frozen Northeast.



Brakes: 19mm master cyl. Braided lines. Stock calipers, rotors. stock. Sorry but consider this. No Track use. No Steet use. Autocross only. SCCA autocross tends to be slower than PCA as I understand. These brakes are sufficicient for the task.



I have considered an adjustable bias valve.

No understeer. It's real good in that respect. Very responsive. Good controllability under hard acceleration. No power on oversteer to speak of.

Haven't maxxed the sway bar yet. Never been more than 1/2 way. It's as stiff as I've ever had it now. If I need to run it stiffer then I will consider going to 23mm t-bars.

the Sway-a-way website has a page where you can determine proper spring rates based on corner weights and motion ratios. I was in the general ballpark with my 22mm bars in front. When I had the V6 in the car I had 180lb springs and no particular tendancy to snap oversteer. After going to the V8 in 2004(added 130lbs in engine weight) I went to 250lb springs.

The car was completly different and was actually slower for a while. I made changes, tuned and eventually got faster thn ever but the snap oversteer is the last problem I can't get rid of.

Mike T

[groot]

Adding antidive will not keep the rear from lifted, but it can keep the front from dipping. The way to control the rear lift is through increased rebound control in the rear dampers.

[Mike T]

I understand that now. I'm learning a lot here. Thanks Groot.

I have camber numbers:

Left front -2.9° right front -1.25°

left rear -1.75° right rear -1.8°

-2.9° ?! WTF?

I'm sure I didn't set it like that. The rear numbers are just like I set them the last time I adjusted anything.

The front camber according to my records were set at -1.75° for both sides. It's like something shifted.

Could poorly installed roll cage bracing tweak the chassis bad enough to result in this? Warp it? I had some roll cage work done by a local circle track guy and he did a terrible job. I had a low front hoop and braces to the front shock towers done and he didn't put the bars where I wanted them.

Mike T

[groot]

I'm pretty sure your Hoosiers don't want that much camber. There're are plenty of threads on alignment opinions.

[Brett W]

Where is your ride height set right now, with you in the car? A severely lowered 914 will create all kinds of problems with the suspension geometry, although it looks much better.