[Guest_1u53r]

SAI is the "Steering Axis Inclination". On a strut based car, if you draw an imaginary line from the lower ball joint to the upper strut mount, the angle that line makes with vertical (as viewed from the front of the car) is the SAI.

The main effect of SAI is to cause the car to 'lift up' a bit when the wheel is turned. This has the effect of strongly centering the steering while rolling down the road. There are other self-centering alignment topics, such as caster. If there is an unequal SAI left to right, then one side wants to center itself more than the other and the result is a persistant pull. Futher, adjusting SAI affects the scrub radius (the scrub radius is where that imaginary line hits the ground and how far that is from the center of the tire's footprint). If the scrub radius is unequal, then the car will tend to pull towards the side that has the larger positive scrub radius.


In relation to SAI, you may also see "Included Angle" listed. This is the sum total of the SAI and the Camber, so if the SAI is 15 and the camber is -1, the included angle is 14.

[flatthump]

Does anyone have more than the smaller ring cut on the bumpstops?

I can feel my car hit them now I think. I havent been to the track in a while but I was at thunderhill this weekend and noticed it much more. Under braking(especially with bumps) I couldnt really predict when the front end was going to start pushing.

Towards the end, I was braking earlier so I wouldnt have to experience this annoying push. Should I shave some more off the front bump stops?

[austinpike]

Originally Posted by Cadence

Does anyone have the dimensions of the stock and Prodrive bumpstops or photos of the two side by side?

[Cadence]

Originally Posted by austinpike

Thanks! I looks like the Prodrive is approx 8mm shorter than stock.

If the Prodrive bumpstop is also softer then it would make sense to trim approx 10mm from the stock bumpstops.

[stretch]

Is the entire first portion of the ProDrive bump stop hollow, ie it has a very low spring rate?

Does anyone have more than the smaller ring cut on the bumpstops?

I cut the biggest progression off only. Cutting of the smallest, softest part isn't going to be as effective because you're removing the section that isn't too firm.

[XDNBC]

I just got my 2007 STI Ltd and this discussion is very informative.
Questions: is "droop" the same as "sag" [a motorcycle term]?
There does not seem to be a preload adjustment on the stock struts- are there preload adjustments on coilovers? If so, you can use this to allow softer springs with higher preload to prevent bottoming the suspension.
Also- as long as the spring length is slightly longer than the shock shaft extension, it wil fit tightly on the perches and not come loose.
If the stock springs are progressive [I actually like this feature for rough roads and whoops] what are the rate changes? Or do you consider them progressive because they use the bump stops which have different rates than the springs.
On motorcycles, the spring rate is considered ideal if it just reaches max. compression at the most enthusiastic bump you actually experience, and just tops out on max rebound when the road drops away. Cars should be considered in the same light. Compression and rebound damping should eliminate wallowing and wheel skip for a given road- which is why it is good to have it adjustable. And- you can decrease understeer when AutoX by stiffening the rear compared to the front [and also putting more air in the front tires, and if you have time, increasing rear swaybar stiffness.
Since I will be using my car as a daily driver, Solo 1 and 2, and occasional fun track day car, I am very much interested in this conversation.
Keep it going.

[Guest_1u53r]

"Droop" refers to the suspension being more extended... like if you jack up the car, the wheel will "droop" down. This is the opposite of "bump". When talking dampers, the "rebound" damping will limit the rate of droop and the "compression" damping will limit the rate of bump.

There is indeed no preload adjustment on the stock struts, but most coilovers do have a preload adjustment. In fact, some of them achieve their lowering via adding preload. Better shocks will have preload and height as separate adjustments. The thing is, most aftermarket dampers (coilovers, different struts, etc) will not have this problem with the bumpstops so the systems that offer that adjustment don't really have the problem to correct anyway. Still, some coilovers have more bump travel than others. I think the Whiteline G4's are known to have a lot of travel.

Regarding springs that fit on the stock struts, they are ALL slightly progressive that I've seen. Whether they accomplish this progressive nature via closer coil windings or tapering the wire thickness, well that changes. There ARE some non-progressive springs to fit on the stock struts, but they are essentially sleeved coilover style springs which just slide over the stock struts (Ground Control).

With motorcycles, I imagine the spring rate issue would be a simple as you describe it, but on a car, the spring rates have another job besides just preventing bottoming-out... They control the car in roll, which in turn affects how much camber is lost at the wheel when the car is cornering. Stiffer springs that allow less roll will give the car more grip because the tire is able to stay flatter on the road and keep its contact patch nice and planted. Further, balancing the front and rear spring rates will have a drastic effect on the way the car handles... Since a tire's grip is reduced as load is increased (here, grip is defined as the ratio of traction:vertical load), putting a stiffer spring on the front means the front suspension is resisting more of the car's total roll and thus the front tires would be more loaded. This will tend to create an 'understeer', except that that is not the whole picture.

There are many things happening when change the roll stiffness, especially when you change it at just one axle. Usually the benefit of reduced roll (and thus more optimal camber) outweighs the tire-overloading effect and the net effect is faster cornering.

Additionally, in the same way the springs have more jobs on a car than on a moto, the dampers do as well. The stiffness of the shocks themselves play a major part in the way the car transitions into and out of corners because the shocks will effectively add spring rate to the car during the initial steering input.

[Boxer]

Excellent thread.

The stock front bumpstop news may help explain what I'm experiencing during certain turns involving less-than smooth surfaces. Aside from the more aggressive Sure-Trac front differential on my '04 it seems that the front just gets bounced off track after hitting, to me, minor imperfections on the road surface. This would indicate high spring rate to me.

It's getting progressively worse (along with the "bobblehead") as mileage increases (~37.5K, currently). Could these bumpstops be "wearing out" or am I just suffering from the, overall, soft stock suspension bushings wear?

[stretch]

More than likely your struts need replacing. That could explain all your symptoms, and at 37k miles, your struts are certainly worn. Since you have an '04, I'd recommend some Koni's in WRX strut housings.

[austinpike]

Originally Posted by stretch

Is the entire first portion of the ProDrive bump stop hollow, ie it has a very low spring rate?

Not the whole first portion, but the first 10mm or so is pretty much hollow. Then it is practically solid after that.

[stretch]

OK, then the first 10mm of the ProDrive stops are almost certainly much softer than the first 10mm of the OE stops. I figured they would be; Prodrive said their stops were softer and more progressive, whereas the stock ones are firm on even the first progression.

[thrlskr]

Originally Posted by stretch

I initially commented that my car felt squishy, but that's not the case. I made that comment after having only driven a half mile test over a few speed bumps. I've now been commuting for a few days and know the car's new personality. It certainly isn't squishy. Large bumps are absorbed much better, but small bumps are still about as prominent as stock. Overall, I would describe the car as feeling normal, which is a good thing. It feels like a premium coilover setup set to one of its softer settings. Bobblehead and jarring bumps are gone. Again, remember that I installed winter tires, Group-N tops, Koni rear struts, and trimmed (or removed) my bump stops all at the same time, and any of these could be to credit for this.

Stretch-

Do you still have the OEM Rubber mounted on some wheels?

If so, and it safe to do so in your climate, would you mind putting them back on and going for a drive? This way we could remove sidewall compliance from the equation.

I have recently mounted up some 255/40-17 Azenis on my OEM wheels and was really suprised how much smoother the car road. The bobble head is still present but the jarring bumps have almost been elliminated. I can only imagine that any winter tire would have a much more profound effect.

Perhaps, your tire change has been responsible for a significant portion of the improvement that we may be attributing to trimming the bumpstops.

[flatthump]

Originally Posted by stretch

OK, then the first 10mm of the ProDrive stops are almost certainly much softer than the first 10mm of the OE stops. I figured they would be; Prodrive said their stops were softer and more progressive, whereas the stock ones are firm on even the first progression.

Hmmm. I have a pair of the prodrive bumpstops but opted to use the 'cut' stock ones instead.

If I remember correctly, a cut stock bumpstop was about 10-11mm shorter than the prodrive ones. SO I figured I would gain that measurement in jounce travel.

But now im thinking the transition from my pink spring rates to the cut stock bump stop rates is to drastic. Maybe if I put the prodrive's in, there will be more of a 'progressive' transition from spring to bumpstop??

Any thoughts?

[wdb]

Originally Posted by flatthump

Hmmm. I have a pair of the prodrive bumpstops but opted to use the 'cut' stock ones instead.

If I remember correctly, a cut stock bumpstop was about 10-11mm shorter than the prodrive ones. SO I figured I would gain that measurement in jounce travel.

But now im thinking the transition from my pink spring rates to the cut stock bump stop rates is to drastic. Maybe if I put the prodrive's in, there will be more of a 'progressive' transition from spring to bumpstop??

Any thoughts?

I have one: my head hurts! Just what I needed, another variable to consider when choosing suspension components.

At the moment I am happier than ever to have left a large part of this particular decision making process in the hands of the engineers at Prodrive.

[stretch]

Originally Posted by thrlskr

Stretch-

Do you still have the OEM Rubber mounted on some wheels?

If so, and it safe to do so in your climate, would you mind putting them back on and going for a drive? This way we could remove sidewall compliance from the equation.

I have recently mounted up some 255/40-17 Azenis on my OEM wheels and was really suprised how much smoother the car road. The bobble head is still present but the jarring bumps have almost been elliminated. I can only imagine that any winter tire would have a much more profound effect.

Perhaps, your tire change has been responsible for a significant portion of the improvement that we may be attributing to trimming the bumpstops.

I thought this too, but in my experience tires help more with the small bumps, not the big ones.

FWIW, my "winter" tires are currently all-season Falken Ziex 512's until I get the Blizzak's mounted. I bought the wheels used, tires came with. The Ziex's are pretty soft, but not typical winter tire soft.

But now im thinking the transition from my pink spring rates to the cut stock bump stop rates is to drastic. Maybe if I put the prodrive's in, there will be more of a 'progressive' transition from spring to bumpstop??

Any thoughts?

Yeah, give me your Prodrive stops.

Seriously, though, I'd of run the Prodrive stops, but I wouldn't make a big effort to replace them. Your right about the transition probably being more sudden with cut stockers, but so long as you're not riding on the stops, I think you've won half the battle. If (when?) you do change 'em out, report back!