kyoo

iTrader: (29)

Is there 'harm' in adding caster at the top?

JDavenport

iTrader: (33)

I'm also right at 6 degrees, perrin PSRS and CiroDesign plates. I don't autox, track only, but I really like the added caster. Car just plain grooves better on turn in. Little heavier steering, but not bad IMHO. Seeing better tire wear as well (I think the dynamic camber helps).

mouseIX

No harm.

kyoo

iTrader: (29)

gotcha - reason for the preference then?

JDavenport

iTrader: (33)

Actually, If you are trying to stuff wide tires under your Evo, the PSRS moves the tire forward in the wheel well. Then you can use a plate (like the Cirodesign) to move the wheel back towards the center of the wheelwell and minimize clearence issues. All while adding even more camber.

I don't take credit for this, RTErnie clued me in.

chrisw

iTrader: (2)

Adding caster by using camber/caster plates will affect your roll center on the front end. Changing the caster on the lower arm does not affect the roll center.

You can easily reset the roll center with a proper alignment, but it's technically incorrect to say there is no harm done. One change simply has a bigger side affect than the other.

kyoo

iTrader: (29)

interesting, thanks for that info

ratt_finkel

For those worried about steering feel. The only time I've noticed a detriment was when I installed the plates backwards for LESS than stock caster. The steering was super twitchy. The above stock caster feels totally normal to me. Even on big sticky R-comps.

EVOizmm

iTrader: (10)

Did any of you notice and increase in bump steer after adding significant caster?

kyoo

iTrader: (29)

i'm not a suspension guy but i think it makes sense for there to be some changes in bump steer? idk a guru should step in for that

kyoo

iTrader: (29)

found this reading a motoiq article:

"
Too much positive caster is felt as mid turn and later understeer and or bumpsteer. You must use caution using more than 8 degrees or so positive caster, especially with wider tires."

"
The same moment that creates the self-aligning torque can contribute towards torque steer in a FWD or AWD car. This is why FWD and AWD cars often have less positive caster and KPA in their geometry.
Excessive caster and/or KPA can result in understeer because tipping the tire on edge and lifting it extremely can cause the corner weight on the outside tire to drastically increase, artificially increasing weight transfer. Extremely wide tires exaggerate this effect. Sometimes when running much wider wheels and tires on certain cars like a 300ZX, you want to actually reduce the positive caster.
"

JDavenport

iTrader: (33)

OK, But with the offset PSRS and the Cirodesign plates I am at 5.8-5.9 total degreees caster.

I understand what is being said and referenced in the MotoIQ article. But they are referring to amounts of caster you cannot obtain with the CT9A chassis unless you start moving mounting points or wholesale replacement of the OEM suspension design.

After installing the PSRS with offset and Ciro plates, I had the car re-aligned with 3.6 degrees camber front, 2 degrees rear and zero toe all around. At my next event (Road Atlanta) it was the first time I have driven the car hard (TT) and not seen the normal abuse of the outside edge of the front tires. I put on a set of stickers for first session Sunday morning, set a new personal best, came in and the tires showed no signs of the outside edge scrub I have fought since I started tracking this car.

The 3.6 is extreme, but I am fighting coilovers that are not spung high enough to deal with the grip of A6's. Somebody wants to buy me a set of AST 5200's, I'd be your friend for life . So I rolled the dice on the 3.6 to counteract some of the body roll, and it combined with the added caster worked great.

kyoo

iTrader: (29)

thanks for that info - i was just chiming in a response to what someone said about added bump steer and just other general implications of caster that i saw (though they may be larger amounts of caster than what the evo can obtain)

what's curious to me about that article are 2 things:
one is that they recommend 3-4 degrees of caster for fwd/awd cars, but it seems like the sole reason for that is to battle torque steer? i dont think that's too bad of an issue in the evo.
second, and more important, is that the article talks about how the steering and turn in gets sharper with added caster, but everyone here notices the numbness of it.

Construct

iTrader: (4)

The difference in steering feel with added caster is hard to explain. I wouldn't call it numb, because there's still plenty of feedback and precision. The real difference I noticed was more of a dead zone in the middle. Small steering inputs away from straight ahead don't result in as much turning as you might be accustomed to, but once you turn the wheel past that dead zone everything feels just fine. The dead zone isn't even that big, but it may feel like the steering responds slightly slower than before as you must turn the wheel past that dead zone before the actual steering begins at a similar rate to what you expect.

It makes more sense if you look at it this (overly simplified) view: By increasing caster, you're tipping the steering axis further away from vertical. As a result, each degree you turn the wheel results in slightly less steering angle, but slightly more camber angle. You're trading one for the other.

JDavenport

iTrader: (33)

Well, I haven't noticed any torque steer issues. I was at 332 awhp and 340-ish awtq I believe. That will be coming up to get me closer to max in TT3.

I wonder if people are equating the heavier feel of the wheel to numbness. As it comes from the factory, the Evo does a very light easy feel to the wheel. It does feel heavier off-center with the added caster. But I personally think overall (seat of the pants) the car feels much more eager to turn into a corner. To me it took some of that pushy, ready to understeer at the drop of a hat feel out of the car on turn in.

And with no other changes I easily bested my previous times at Road Atl without breaking a sweat. But the real deal maker for me was coming into the paddock after running a sticker set of tires very hard and the tires showed none of that outside edge of the shoulder abuse these cars tend to generate. That means my tires will last longer, more money to spend going to events!