[Guest_1u53r]

Originally Posted by stretch

Ah, good discussion.


Is geometry really to blame for a drifting car, or the alignment?

Well, some would say that SAI is part of the alignment topic. It is not adjustable stock and most people never think of it so it's largely ignored. There are certainly a LOT of factors that cause a pull/drift. Cross camber, cross caster, thrust angle (which presents more of an aerodynamically influenced pull), unequal scrub radius, etc.

However, if ride height (thus strut length) is equal left-and-right and camber still differs with your camber bolts in the same position, wouldn't that imply that the frame is slightly off-center and your SAI is already not identical left-and-right?

It might imply that. It might also imply that your camber bolts are slightly different in tolerance, the holes in your strut aren't drilled exactly the same as the other side, you may have a slightly bent upright, you may have a slightly bent strut, etc. I guess what I am saying is that I would not try to correct an unknown imbalance with a known imbalance.

[Cadence]

I no longer have any bobblehead, but I trimmed my bump stops and installed Group-N tops at the same time. I can't say for sure which did the trick, but I'd bet it was the tops.

I think we all agree that the bobblehead is an undamped suspension motion and the STi struts have plenty of damping so if your car feels better now it is probably due to the Group N mounts.

Has anyone else found that firm or soild front upper mounts reduce or eliminate the bobblehead? I want it gone....GAWD it is annoying

[Guest_1u53r]

I have solid mounts and no, the bobble is not gone.

Keep in mind that although the struts have a lot of damping, they do not have the kind of damping they would need to handle a several hundred lb/in ramp up of the bumpstop.

[Cadence]

Originally Posted by nhluhr

I have solid mounts and no, the bobble is not gone.

Keep in mind that although the struts have a lot of damping, they do not have the kind of damping they would need to handle a several hundred lb/in ramp up of the bumpstop.

Interesting......because so many people have reported less bobblehead after changing to stiffer springs. The combination of stiffer springs that are lower and therefore closer to, or sitting on the bumpstops, should make the bobble worse if the front struts don't have enough damping

[Guest_1u53r]

Well, being completely objective in my observations, the stiffer springs and tops take out some of the motion and replace it with another motion. It's honestly been awhile since I've driven on anything BUT the solid fronts and groupN rears, so maybe I'm just noticing an overall stiffer rate....

[thrlskr]

Originally Posted by Cadence

Interesting......because so many people have reported less bobblehead after changing to stiffer springs. The combination of stiffer springs that are lower and therefore closer to, or sitting on the bumpstops, should make the bobble worse if the front struts don't have enough damping

I can't comment on stiffer spings but I have solid mounts up front, RCE Camber plates, bobble head wasn't reduced in the slightest.

[stretch]

Then WTF did I get Group-N tops for? Damn the Internet and its poor recommendations, the bump stop mod did the trick for free!

On the bright side, now we know better.

I have a few notes I'd like to share now that I've been riding on my new setup for a while:

Re-lubricating the stock struts has made them utterly silent. It's awesome, there are no creaks or pops in the entire front of the car even over big bumps. I just stuffed them with Durablend grease when reassembling them, nothing fancy. They made some noises prior to reassembly.

The rebound resistance of the stock shocks feel about like Koni's somewhere around 1/3 turn from full soft or 1-1/2 turns from full firm. Bump resistance feels similar.

My setup is not perceptively overdamped. They feel pretty spot-on critically damped. My struts have only 14k miles, so wear should be present yet minor. With bump stops I would have a minor second oscillation of the spring (only noticeable after going slowly over speed breakers), but it is now gone. This may explain what the entire bobblehead thing is, as nhulur suggests. The stock struts are underdamped for the stock combined spring rates.

While the stock struts may be able to handle a minor increase in spring rate and still feel critically damped, it would require the springs not put the car on the bump stops. No such springs exist, thus I'd bet all current lowering springs leave the car underdamped. Again, I think this is the conclusion Prodrive came to long ago. They use spring rates barely over stock with shorter, softer bump stops and an almost non-existent drop. My findings suggest that Prodrive has it right (that is, if you feel you must lower your car- really I don't see a compelling reason to justify them over stock w/ trimmed stops).

With the bump stops trimmed and my effective spring rate lower, it is my hope the the stock struts will last longer. They need to last until replacements are available!

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.

[Cadence]

Originally Posted by stretch

...

My setup is not perceptively overdamped. They feel pretty spot-on critically damped. My struts have only 14k miles, so wear should be present yet minor. With bump stops I would have a minor second oscillation of the spring (only noticeable after going slowly over speed breakers), but it is now gone. This may explain what the entire bobblehead thing is, as nhulur suggests. The stock struts are underdamped for the stock combined spring rates.


With the bump stops trimmed and my effective spring rate lower, it is my hope the the stock struts will last longer. They need to last until replacements are available!

These are some very interesting findings. My lingering question is........... with the bumpstops trimmed what happens when the front runs through all the available travel? Will the piston in the strut bottom out because the bumpstop is shorter or will the spring coil-bind or ?

[stretch]

As far as bottoming out, there are still three progressions of the bump stop to absorb the impact. I'm not really sure what purpose the firmest portion of the bump stop serves. It seems to only take up space. Its spring rate is very high, approaching infinity very quickly. The other three progressions, once compressed to their limits, will do pretty much the same thing. So I'm really not sure what that last stop accomplishes nor what I've lost by removing it. It seems like a no-brainer to lop it off and gain the suspension travel instead; it lets the strut absorb more impact so the bump stops don't have to.

The fronts will never coil bind. The coil only winds around four times- the spring would have to compress to about two inches long for it to bind.

[Cadence]

Originally Posted by stretch

The fronts will never coil bind. The coil only winds around four times- the spring would have to compress to about two inches long for it to bind.

I hear what you are saying but maybe I am not expressing my point well.

Imagine you jump your car WRC style - when you land it will fully compress the suspension, at which point something in the front suspension will move as far as it can and the travel will run out. What will it be? The bumpstop compressing before any metal to metal contact occurs? or will the damper piston bottom out in the housing? or will the spring coil bind? These seem to be the only three options and one of them has to happen when all the travel is used up.

My worry is that the stock length bumpstop is sized to ensure no metal to metal ever occurs and by cutting some off things could get nasty.

[Guest_1u53r]

I love threads like this where actual useful conclusions are hypothesized.

[Cadence]

Originally Posted by stretch

Again, I think this is the conclusion Prodrive came to long ago. They use spring rates barely over stock with shorter, softer bump stops and an almost non-existent drop. My findings suggest that Prodrive has it right (that is, if you feel you must lower your car- really I don't see a compelling reason to justify them over stock w/ trimmed stops).

Does anyone have the dimensions of the stock and Prodrive bumpstops or photos of the two side by side? I would hope that Prodrive wouldn't make the bumpstops so soft and short that any harmful bottoming could occur

[stretch]

Originally Posted by Cadence

Imagine you jump your car WRC style - when you land it will fully compress the suspension, at which point something in the front suspension will move as far as it can and the travel will run out. What will it be? The bumpstop compressing before any metal to metal contact occurs? or will the damper piston bottom out in the housing?

Any amount of bump stop should prevent metal on metal. The shorter bump stop makes the bump stop's rate go towards infinity more suddenly, but the difference will be very slight. The spring rate does not start rising as early in the strut's travel. That slight difference may result in a decrease in total OMGWTF jump landing capability- I just can't answer that. The bump stops may have been designed just for that, rallying. Such a progressive spring rate certainly isn't desirable for road racing.

In any event, I'd expect lowering the vehicle (decreasing suspension travel) to be a much greater overall liability for OMGWTF rally jumps, and I haven't read of any lowered cars (with trimmed stops, Prodrive stops, or uncut stockers) having exploded. Yet.

[psuLemon]

Originally Posted by nhluhr

I love threads like this where actual useful conclusions are hypothesized.

I agree very much. This may be a dumb question but what is SAI?



<--- still suspension noobs

[stretch]

Steering Axis Inclination. It is the same thing, I think, as Kingpin Inclination. In this discussion, we're referring to the angle of the strut away from vertical.

Scrub Radius was mentioned earlier in this thread, too.