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Add a rear swaybar! Right? Sure, perhaps. you're removing grip from the rear though and you might not corner any faster, though it helps for balance. But i think i may have found the root cause of our evo's understeer problem.
Bump-Toe. As the wheel travels upwards during cornering, the wheel will toe in severely, increasing tire scrub. My hunch is that this is a major contributor to severe understeer at the limit. Just check out this graph as an example.
When you're approaching a turn under hard braking, the front will have 4+ degrees of toe in which would severely scrub the front tires, reducing their available traction on turn in. Sure, having a heavy front bias doesn't help, but the implications of what i've just been able to measure is astounding. I'm seriously shocked that this hasn't been discovered before.
So, what do we do now? .....
Well, whiteline makes a kit, right?? Sure. And it would work if you ONLY used the tie rod end and NOT the extended balljoint and kept your ride height STOCK. The whiteline rod end in their RCK kit is designed bring it back to stock-like geometry when used with their extended balljoint. Which is a hilarious 1/4 inch longer than stock.
(stolen from EVOlutionary)
So what the hell do we do? Leave our car at a stock ride height and use only the rod end in the whiteline kit? Pshh..... maybe if i was building a rally car.
Well, perhaps this will work......
I'm having this machined right now and will be pairing with some rod ends. Hitting Texas World Speedway on April 22nd so we shall see
Before i hit the track, i'll be measuring and zeroing out the bump-toe curve, so hopefully this issue will be history after i'm done developing this part. And you wanna know the best part of it all?? The cost of this entire kit will end up costing me less than a rear sway bar Score!
I'm not necessarily disagreeing with you, but I haven't noticed this severe understeer at the limit, even when stock. Once you throw stiff coil overs and sway bars into the equation you are limiting the front suspension compression, at that point any bump steer will be mild at worst.
I'm not necessarily disagreeing with you, but I haven't noticed this severe understeer at the limit, even when stock. Once you throw stiff coil overs and sway bars into the equation you are limiting the front suspension compression, at that point any bump steer will be mild at worst.
Mild? Compared to what? Taking a look at an E46 (non M) bump steer curve and the dang thing is flat. Besides, limiting travel isn't the best answer, but for us it is seemingly the only answer. I'm trying to find real solutions here, not bandaids. That's all.
I don't know how close attention you pay to your alignments, but try driving with zero (0.00) static toe, and try driving with 1 degree toe in on all 4 corners. I can tell you that it's terrible. Now immagine if under hard cornering, your tires toe'd in 1 degree less. I assure you, the difference is equally dramatic.
Mild? Compared to what? Taking a look at an E46 (non M) bump steer curve and the dang thing is flat. Besides, limiting travel isn't the best answer, but for us it is seemingly the only answer. I'm trying to find real solutions here, not bandaids. That's all.
I don't know how close attention you pay to your alignments, but try driving with zero (0.00) static toe, and try driving with 1 degree toe in on all 4 corners. I can tell you that it's terrible. Now immagine if under hard cornering, your tires toe'd in 1 degree less. I assure you, the difference is equally dramatic.
I'm not disagreeing on the theory, but in practice; I have driven this car in autocross in many forms from stock to fairly modified. Understeer has never been an issue, in fact the car rotates beautifully and exhibits mild oversteer at the limit.
It seems like you are trying to solve a problem that in practice isn't really much of a problem. Especially once you figure that front compression when properly modified is so small.
I am all for clever fixes to improve front end grip. And if you have success with this please share, I hope my comments aren't taken the wrong way. I am genuinely interested. It's just when I saw "Evo" and "severe understeer" in the same topic, I got defensive
I am all for clever fixes to improve front end grip. And if you have success with this please share, I hope my comments aren't taken the wrong way. I am genuinely interested. It's just when I saw "Evo" and "severe understeer" in the same topic, I got defensive
No problem , and i'll definitely be sharing what i find. And you're right in that a properly setup evo doesn't have problems with understeer, but therein lies the problem. What if we didn't have to sacrifice compliance in order to achieve balance and oversteer? What if we can use lower rear spring rates and soften front compression for compliance and have a balanced chassis? What if we don't have to crank up a rear sway bar and risk 3 wheeling? What if?
As it stands, my evo right now is super balanced, but i also have some serious rake and the front end is over-lowered to force the rear to load up more than the front. I'd like to be able to get my front roll geometry much tighter so i don't have to run a 20k spring to combat body roll. I'm at 14.5k front springs right now and i'm getting plenty of roll on street tires, fwiw. I'm throwing on some full slicks soon and am concerned with the increased roll those would create.
Essentially, i'm just trying to find the proper solution. Not a sub optimal solution that requires bandaids to fix their inherent shortcomings. I mean heck i'm just traveling down this path on a hunch. That's what this is all about right? Investigating assumptions in pursuit of the fastest Evo possible, right?
Let’s review SAI as its rarely discussed….. SAI = steering axis inclination. On a macstrut setup, we often increase our camber via the tophat. This adjustment, though it increases caster, is actually increasing SAI. Steering axis inclination has a few effects…. But let’s review that in graph form.
Mods on the evo above in the stock graph: None
Mods on evo X in graph below:
Stock LCA
Whiteline RCK
Perrin offset PSRS with stock dive geometry
Super Pro Poly front LCA bushing
Vorshlag top hats at max caster & max SAI
MCS 2 way Coilovers. 6 inch total stroke.
Mods on Evo X for graph below
Exact same as above with the following exceptions:
Stock SAI (camber) - This is the camber curve of a stock evo with the camber bolt flipped for a -2 static camber.
You can see that by maxing out your steering axis inclination (and dialing out some camber via the camber bolt) You're DOUBLING your camber curve. Yay! Unfortunately its still pretty pitiful. S2000 camber curves gain like 3.5 degrees of camber over the same distance. Also, it seems the LCA begins to go beyond 90 degrees (that's bad) when lowered and compressed at the stock SAI settings.
This thread isn't for discussing optimizing the camber curve (though i have something in the works for that), but i'm just throwing it here for reference.
Last edited by hispanicpanic; Apr 8, 2016 at 06:37 AM.
This is very interesting. I'm looking forward to see that find. With body roll, that bump steer curve could easily put you in a spot where the outside tire is turned in farther than the inside tire, which is the recipe for understeer.
And good god that camber gain is terrible, Only .5*. No wonder these cars need to run so much static camber...
And that's gaining positive camber right? So on full compression, if your static camber is -3, your compressed camber will drop to -2.5.
The rear goes in the opposite direction (gains negative camber on compression)
Correct me if I am wrong. Also OP, the charts are awesome, keep them comin.
I tried organizing the chart specifically so people wouldn't accidentally interpret this but i couldn't figure out how to do it lol. But no, the right half is a gain on negative camber, and the left half is a loss on negative camber.
Originally Posted by letsgetthisdone
This is very interesting. I'm looking forward to see that find. With body roll, that bump steer curve could easily put you in a spot where the outside tire is turned in farther than the inside tire, which is the recipe for understeer..
Exactly. Most street tires can only achieve 6-8 degrees of slip angle before they begin to loose traction. Now, 3.5 degrees of that is taken up by ****ty geometry so you're left with less grip potential.
Last edited by hispanicpanic; Apr 7, 2016 at 01:10 PM.
I posted about this like 4 years ago. No biggie though.
Was this measured or predicted through a model?
Any other parts? Camber plates (setting)? Hub Camber? PSRS? Precision Steering Kit? It all matters here.
The numbers I have follow the same trend, but are about 1/2 the total value you came up with, more inline with the RCK numbers in stock form.
The RCK on the VIII/IX along with the PSRS and PSK actually has a HUGE impact on this. Like literally flat lines it for most of the curve... At least in my model. I haven't measured it directly though. I'm actually kind of curious if some of the improvements noted with the PSRS is actually due to how it impacts the bump steer curve.
The calculated data you describe absolutely coincide with what my seat of the pants feel suggests ....
So you are suggesting that the PSRS (offset?) in addition to limiting toe change as a result of the solid bushing also has a geometric effect on limiting toe change/bumpsteer as the suspension moves through its arc?
So how does the additional caster/moving the suspension arm do that? Would you care to explain?
Thanks
Originally Posted by 03whitegsr
The RCK on the VIII/IX along with the PSRS and PSK actually has a HUGE impact on this. Like literally flat lines it for most of the curve... At least in my model. I haven't measured it directly though. I'm actually kind of curious if some of the improvements noted with the PSRS is actually due to how it impacts the bump steer curve.
I posted about this like 4 years ago. No biggie though.
Was this measured or predicted through a model?
Any other parts? Camber plates (setting)? Hub Camber? PSRS? Precision Steering Kit? It all matters here.
The numbers I have follow the same trend, but are about 1/2 the total value you came up with, more inline with the RCK numbers in stock form.
The RCK on the VIII/IX along with the PSRS and PSK actually has a HUGE impact on this. Like literally flat lines it for most of the curve... At least in my model. I haven't measured it directly though. I'm actually kind of curious if some of the improvements noted with the PSRS is actually due to how it impacts the bump steer curve.
This was all measured.
The stock evo is 100% stock. The modified evo is an X with the following
PSRS offset, stock dive geometry
Whiteline RCK
Super Pro front LCA poly bushing
Vorshlag top hats at max caster & max Camber via tophat, min camber via hub. This is about a 3.3 degree static camber setting.
It's also important to note that this is on MCS coilovers, which have a much shorter stroke than a stock shock absorber setup. I measured 6 inches of stroke because that's literally all the shock has in it.
Apr 6, 2016, 12:23 PM
, and i'll definitely be sharing what i find. And you're right in that a properly setup evo doesn't have problems with understeer, but therein lies the problem. What if we didn't have to sacrifice compliance in order to achieve balance and oversteer? What if we can use lower rear spring rates and soften front compression for compliance and have a balanced chassis? What if we don't have to crank up a rear sway bar and risk 3 wheeling? What if? 
