markdaddio

iTrader: (1)

Well I finally got the Ohlins coilovers installed this past weekend, and the car seems to really benefit from stiffer spring rates and lower cg. But I can't help but come to the conclusion that the rear coilover mounting setup is a big issue. The lower mount for the rear coilover onto the lower arm is a fairly soft rubber bushing. It seems to me that the tires' motion is undamped for any vertical motion that this bushing allows. I also can't help but conclude that the whole point of running a stiffer and better quality damper is very much lost with this bushing arrangement. I am also running a pillow ball mount for the upper mount. So I am left with the task of fixing this issue. Does anyone know of any replacement bushings available for this lower mount, and better yet, is there anyone who has fabricated replacement lower arms with pillow mounts other than Nagisa? (3 month wait to get those) One of my major concerns with stiffening up this mount is that the entire rear weight of the vehicle is supported by this bushing. I am worried about the aluminum stock arm failing due to the shock loads that would be applied with a stiffer bushing installed. Any other logical input to this?
Mark Daddio

Evo8RA

iTrader: (11)

You have a good point as to whether the strength of the stock arm will stand up to a stiffer bushing. Here's a link to someone who sells a polyurethane lower rear shock mount bushing. They may have some experience with them installed on an Evo. The bushings are shown as part of a kit, but can be purchased separately.

http://www.worksevo.com/store/index.php?cPath=37


Mark

nils

iTrader: (56)

interesting read...

^^those look very similar to whiteline bushings...

SS RX7 r2

iTrader: (1)

Hi Mark,

I think that the stock arms are probably over designed for street use compared to the stamped lancer arms. I was talking to my friend Wolfgang H. last week about his Group N Evo VII, and he mentioned that they routinely bend the rear arms under severe impacts, leads me to believe the arms may be forged. I don't know how much excess stress a solid bushing would add though.

When is the first national event that you're going to bring the car out?

Rick

marksae

iTrader: (12)

Changing bushing material should not require you to get a stronger lower arm. If the arm is supporting 500 lbs of force w/ a rubber bushing, it will also be supporting 500 lbs of force w/ a solid bushing.

What the lower rubber bushing does, is delay the time it takes for the motion to get transferred from the wheel up to the shock. The rubber bushing material needs to compress before it starts compressing the strut. Changing the bushing to something stiffer will mean better response at the expense of more noise and vibration being transferred into the cabin.

I would not go so far as to say that your whole rear suspension setup is being wasted due the slop in the rubber bushing though.

marksae

iTrader: (12)

Yes, there should be a good factor of safety designed into all the suspension components. If we were really running near the limits of the arms, they would show signs of fatiguing after a few hard track days.

markdaddio

iTrader: (1)

Thanks for the input, and lead on the works bushings. I got their upper mounts but didn't see those available. I would imagine that those are going to make a huge difference. As for my first national event with the car, it will probably be in June. I do plan on running the Westchester event at Orchard Beach on April 2nd as a first tune up event. Has anyone run those bushings yet, and have any input as to the effect?

Mark

markdaddio

iTrader: (1)

Marksae,
I thought that the delay in time for the moton to get transferred from the wheel to the shock is critical to the effective amount of instantanious force applied to the lower arm. The longer the period of time to spread the applied force out, the lower the instantanious force would be at any given instant due to the bushing deflection decelerating the inpact to the lower arm over a period of time. Kind of like hitting a piece of metal with a sledgehammer, and then covering the metal with an inch of fairly soft rubber and hitting it with the same force. It would deform easier without the rubber to decelerate the impact. Maybe I am wrong here, could you clarify. Additionally my concern on the lower arm is that it was designed for maybe a 400 lb/in spring rate with rubber mounts top and bottom, and I am running 700-800 lbs/in and looking to solid mount it.

Thanks,
Mark

marksae

iTrader: (12)

Mark, you’re thinking of a situation where the shock is a solid linkage and does not compress. In that case, then bushing material would definitely affect how much load gets transferred into the lower arm.

In the case of your rear suspension, the lower shock mount on the lower arm is a hinge, meaning that any type of load it sees will get transferred through to the linkage attached to it, which is the shock. Since your shock compresses, it should damp any instantaneous force that gets transferred from the wheel. If you draw a free body diagram, you’ll see that the load the arm sees is the same regardless of bushing material.

93esp

iTrader: (4)

In a steady state condition, you are correct that the bushing material will not affect the load on the lower arm. However in a dynamic situation the instantaneous load applied to the arm will be greater as the rubber deformation does dissipate energy. I would not be too worried about the rear arm breaking by changing the bushing. It would also be fairly simple to adapt a spherical bearing into the lower arm mount - any machine shop could make a sleeve to press into the arm that would hold the bearing and a couple bushing ends to mate to the shock.

Eric

Silencer

iTrader: (4)

Any stress evidence on yours Eric from last year? (cracks or silght bends)

BTW, how's your car coming?


Dave

markdaddio

iTrader: (1)

Yes, I think Eric understands what I am getting at as far as load distribution over time. Eric, have you run a heim joint setup, in conjunction with a pillow ball upper mount, without any problems with bending the lower arm? I guess that is the question at this point for me. lol.

Thanks,
Mark

93esp

iTrader: (4)

I haven't run that setup as it wasn't legal in ESP.... I didn't see any ill effects on the rear arms running rear springs stiffer than 800# with the stock lower shock mounts however. Assuming I get around to it I will be running spherical bearings in all the suspension joints for SM.

Eric

markdaddio

iTrader: (1)

I believe that I am going to go with a urethane bushing mount for that location, and see what happens. I guess heim jointing the whole rear suspension would be the best setup though.
Mark

chrisw

iTrader: (2)

Let me know how it turns out.

With the multilink suspension we have in the rear, I am skeptical that heim jointing would gain you any significant advantage, unless your counting the weight you will lose in the wallet. (is that another form of corner weight balencing )