MacP strut design

[Jrud]

I'm currently working on designing a front spring and strut package for a MacP car (S13 240SX). I'm trying to keep costs down and would like to go with a bilstein take apart shock. I know that I need to account for side loading and bending in the strut. And I was considering using a shock in an inverted setup with a steel or Al sleeve on the bottom that would slide with the shaft and go around the body of the shock to help deal with these loads.

I was wondering if anyone has any insight on this? I've seen several other setups for this car that don't use an inverted strut and don't know how they take into account the side loads. I mean that could do it internally but based on the size of the shock and the shaft displacement it doesn't look like there's any room for an extra internal guide. The shaft diameter might be a little larger in the front on some setups, but is that enough? I also seem to remember that the stock struts for this car seem to apprear very similar to the rear shocks in terms of shaft diameter.

This setup would be used for a track car, so loads would be an issue, but long-term (more than a race season) durability isn't as important. I just don't want to have something that will need to be rebuilt after every race.

I haven't done any of the loading calcs yet, I'm still in preliminary stages for this design and am really just trying to gather more information.

So I guess my main question is would there be any problems using a shock in a strut configuration? I would like to make an inverted strut to help with the side loads, but is it necessary? And does anyone know what type of extra support is designed into OE or aftermarket struts versus similarly sized shocks?

Tim

 

[GregLocock]

The design loads on a track car are less than on a production car. I suggest you check with Bilstein, the side loads that can be developed in a MacP setup are far in excess of those that a conventional strut sees. According to my sources additional linear bearings are used both at the upper seal, and at the piston. That being said I'm sure Bilstein make a take-apart for a MacP.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Jrud]

Well I just got off the phone with a rep from Bilstein and he said they they didn't offer a take apart strut cartridge or insert. He also seemed to think that the only difference between they're strut cartidges and their regular shocks was the way the nitrogen is filled because the struts don't have an external schrader valve. So this leads me to believe that I might be able to use one of their take apart shocks in an inverted setup with a housing to hold the damper.

I'm going to keep looking around at other shock companies, but I would like to keep costs down that's why I'm trying to stay away from some of the other big names.

I wonder if I could fit a bearing and seal into the bilstein damper. I know it wouldn't be as good as having one on the piston as well, but it might help out enough especially with the shock inverted.

I'll take a look at everything when my brother's shocks get here. I'm using the same shock for his car that I would use for the MacP setup if I decided to try it, so at least seeing his I would have a better idea of what loads it shoudl be able to take and what else I would need to do to use it as a strut.

 

[GregLocock]

Sorry, in my post above it should read :

The design loads on a track car are less than on a production car. I suggest you check with Bilstein, the side loads that can be developed in a MacP setup are far in excess of those that a conventional /shock/ sees. According to my sources additional linear bearings are used both at the upper seal, and at the piston. That being said I'm sure Bilstein make a take-apart for a MacP.

Try a European rally car parts dealer, I'm sure their experience with the Focus and many other small cars will be much more relevant.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[pmwltd]

I built a setup for an Audie 4000 a number of years ago that used a Bilstein cartridge that uses the outside diameter of the cartridge body as a bearing surface. We honed the ID of the strut housing and installed bronze bushings. A hole was drilled in the bottom of the strut housing and the strut rod was double nutted to the bottom of the housing. The large diameter of the cartridge body then takes the side loads. This car is driven strictly as a race car in SCCA.

Having said that, there are many strut cars using coil over conversions and various front strut cartridges with strut rod diameters down to 5/8". A good resource that I have used on many ocations is Jay Fizer at Koni NA
859 586 4100 X334. He can tell you right off what will work with your setup.

Good luck




Dave

 

[Jrud]

Dave, that sounds like what I was planning on doing. I would like to make a new piece to replace the stock strut mount and this piece will be threaded so I can adjust height and preload here. The shaft will be bolted in at the bottom and I was thinking of using a bearing at the bottom. I haven't decided what type of bushing I would use between this sleeve and the shock body.

I mean this is mainly for a race car, but I would also like to have something that would be strong enough to handle occasional street driving. I guess more in terms of durability.

Thanks for the contact, I will give Jay a call, I've been meaning to give Koni a call about their strut setups.

Tim

 

[Jrud]

Just another question on the same thing. What material do you think would be best to use for the lower strut mount and for the bearing surface between the lower piece and the shock body. Dave, you said you used bronze, what would other good options be. I would like to use an aluminum tube for the lower part but wouldn't be against using steel. I just want it to be as light as possible. Let me know if you guys have any opinions on this. I really appreciate the help so far. Thanks.

Tim

 

[NakedTestMonkey]

I have an S13 as well.

If you are going for functionality and affordability then I would not overlook the Koni 8611 shock. They have just started integrating digressive valving as standard (was an option before) into the shocks so they would be effective for dual usage. They offer multiple adjustment choices as well.

They are affordable as well, and with proper modification any 60mm spring can be used such as Eibach's race springs, or if you like to wait forever you can use Hyperco.