Trick Rear Suspension

Our neighbor just came in to show us his friend's company which is down the street that does a pretty trick rear suspension setup for S13s.

http://www.fatcatfabrication.com/id3.html

The engineer in me is cringing a little. I hope they did some FEA on that tube. With no center support, that thing is gonna flex and could lead to fore-aft buckling...

Is it going to flex? The lower shock mount will only see forces acting in line with the shock. The pivot point for the rocker will get a fair bit of load, but it's close to the mounting point.

But more importantly, what does this accomplish? I'm guessing that's a strut suspension, so I hope that pushrod is very solid.

The whole arrangement seems to try and mimic open wheel cars, but with no aerodynamic benefit, what's the point? The center of gravity is higher, and they have ruined any storage space they might have had for absolutely no reason.

That bar would be better off straight anyway, it's doing basically nothing structurally to the shock towers bent like that, and it had better be very strong. The pushrod, as Keith pointed out, must be gigantic to not buckle too.

Bling Bling, I guess. I will never get this or neon lights under cars. -Brian

Looks like Photoshop to me. Never mind, after looking at the pics on the website, I guess they looked less photoshopped. I'm still scratching my head trying to figure out the benefit here.

CGLockRacer wrote: The engineer in me is cringing a little. I hope they did some FEA on that tube. With no center support, that thing is gonna flex and could lead to fore-aft buckling...

Same thing is going through this engineer's brain also. My question is why? One of the advantages of a pushrod suspention is packaging. In this case it's taking up the entire rear of the car. I don't see the point. Also, did I miss it, or is there no picture on the website of how the assembly attaches to the control arms? That would be something I would want to see.

tuna55 wrote: The whole arrangement seems to try and mimic open wheel cars, but with no aerodynamic benefit, what's the point? The center of gravity is higher, and they have ruined any storage space they might have had for absolutely no reason.

Sprung VS unsprung mass. Inboard mounted suspension reduces unsprung mass, which is a terrible evil thing that will kill you in your sleep.

Whether or not this makes a difference on a streetcar is up for dispute.

edited for stupid mistake.

It would be a lot better if the shock mount gussets were 1 piece spanning the bend in the tube but otherwise I'm not scared by the design. it's really pretty slick from a simplicity standpoint. I'm pretty sure the S13 has a dual a-arm or some type of Non-strut rear. There are non-bling advantages to push-rod suspensions. The motion ratio is much easier adjusted than on a standard suspension, the unsprung weight is less, shock options become more off-the shelf less custom aplication. Then there are all kinds of advantages when designed from the begining. It really for <$1000 doesn't seem like a bad solution.

Looks like a 240SX does have a double wishbone rear, so that's a lot less load on the pushrod. Good.

jpaturzo, I don't see the drop in unsprung mass. In fact, I see a possible increase. The way I see it, anything that moves with the suspension is going to be unsprung, with the exception of the spring itself - that's considered "semi-sprung", I believe. So, the pushrod and the pivot are added to unsprung mass, as is the shock shaft. The shock body is no longer unsprung, however, so it might be about even. However, if dropping unsprung weight was the goal, turning the shock/strut upside down in the stock location would be more effective.

Attachment point to the suspension should simply be the stock shock mount. No reason to change that.

Motion ratio would be easy to change, sure. But why? You do have the option of making the suspension rising or falling rate if you want. It's certainly going to be easy to adjust the ride height :)

I'm not seeing the advantage.

Cons: More weight, more parts, more cost, more space consumed, Pros: You've got some relatively cheap fully adjustable coilovers from a motorcycle and it's easier to show off your bling.

Bryce

How much does this really reduce the unsprung mass though? You've removed the long factory shock, so that has removed some mass, but you've replaced it with a smaller shock inside that still contributes to the unsprung mass (the resevoir is sprung whereas it was unsprung in the stock configuration so that helps offset things, but the end connected to the turnbuckle and the spring itself is still unsprung). You've also added in the pushrod, so that cuts down on your unsprung mass savings, as well as the turnbuckle (although it's more a rotational inertia contribution than a straight mass inertia issue). If you're building an all out track car and don't mind giving up the space inside, I guess this works, I just don't think it will give you much of an advantage over a nice coil over shock. Can you get us any numbers on the new vs. old unsprung mass? I'm sincerely interested (not in purchasing, just the engineering aspect).

I'd think that removing the rear seat on a track day would yield the same performance upgrade (although if the 240SX shown is his daily driver/demonstration car, I can see why he kept the interior and assume that many customers with hard core track cars will already have gutted the interior.)

It is a cool idea though. Nice out of the box thinking. The "army jeep" lawnmower on their web site was pretty sweet, as were some of the headers.

Bob

Keith wrote: jpaturzo, I don't see the drop in unsprung mass. In fact, I see a possible increase. The way I see it, anything that moves with the suspension is going to be unsprung, with the exception of the spring itself - that's considered "semi-sprung", I believe. So, the pushrod and the pivot are added to unsprung mass, as is the shock shaft. The shock body is no longer unsprung, however, so it might be about even. However, if dropping unsprung weight was the goal, turning the shock/strut upside down in the stock location would be more effective.

The rule of thumb is anything that is attached to the lower wishbone on one side (unsprung) and terminates on the body (sprung) is counted as 2/3 unsprung weight. So in a regular install this would be 2/3's the weight of the spring and damper assembly.

Now replace the spring and damper with a solid rod that runs from the wishbone to the bellcrank as pictured above. So now you are only considering 2/3 the weight of the rod. The spring and damper are now mounted completely inboard and count towards sprung mass.

I still don't know if there is a huge benefit on a streetcar, but it's definitely different. only count towards sprung mass.

I see a huge advantage. Your passenger can adjust your suspension while you drive! j/k

I'm going to have to do more reading. I really don't see how a part that moves with the suspension can be anything but unsprung mass. The shock, in this case, is attached to the lower wishbone via the bellcrank. So the pushrod is 100% unsprung as it's not attached to a fixed location on the chassis. The bellcrank is questionable, although I think that because it simply pivots (like a control arm does, really) then it's also 100%.

You can apply the 2/3 rule of thumb to the shock in this case, but more accurately you'd count the weight of the shaft only wheras you'd count the weight of the body only in most applications. For example, switch from a steel to an aluminum body shock (all else staying equal) and all of the weight loss will come off the unsprung weight. The spring will stay 2/3 in all cases.

jpaturzo wrote: The rule of thumb is anything that is attached to the lower wishbone on one side (unsprung) and terminates at a fixed point on the body (sprung) is counted as 2/3 unsprung weight.

There, I think I filled in the blank. The pushrod doesn't terminate on the body. It terminates on a pivot. It's a linkage. The shock terminates on the body.

Big difference between knowing how to fab parts and understanding whether you should fab it...

Keith wrote:

jpaturzo wrote: The rule of thumb is anything that is attached to the lower wishbone on one side (unsprung) and terminates at a fixed point on the body (sprung) is counted as 2/3 unsprung weight.

There, I think I filled in the blank. The pushrod doesn't terminate on the body. It terminates on a pivot. It's a linkage. The shock terminates on the body.

true, but the shock body isn't really unsprung in this case, because it is stationary, except for some rotation due to the range of motion of the bellcrank. so what i would count as unsprung would be the rod from the lower arm to the crank, the crank itself, the mount on the shock shaft, the spring perch, and the shock shaft. also, due to the lever-arm, the rate of motion on the shock shaft may be reduced, and therefore has a lower effective unsprung weight.

That's the same list I'd come up with Strizzo. And as I said earlier, turning the shock upside down in the stock mounting location would provide a greater loss in unsprung weight, as then you'd only have the shock shaft and the parts bolted directly to it. That's why I mounted the front shocks on my Locost in this orientation.

The shock is effectively inverted in this pushrod setup, but that's not inherent to a pushrod design. The packaging may not be possible otherwise however.

Keith wrote: That's the same list I'd come up with Strizzo. And as I said earlier, turning the shock upside down in the stock mounting location would provide a greater loss in unsprung weight, as then you'd only have the shock shaft and the parts bolted directly to it. That's why I mounted the front shocks on my Locost in this orientation. The shock is effectively inverted in this pushrod setup, but that's not inherent to a pushrod design. The packaging may not be possible otherwise however.

1) agreed

2) seems plausible. i think i remember my friend that had a 240 had to cut holes in the carpet inside the hatch in ordre to reach the adjusters on his shocks.

A lot of coilover kits for the 240 will invert the rear shocks anyways. So most of the shock body/reservoir and what not is already sprung weight, what is left as unsprung is the shock piston, shaft, spring and mount for the lower control arm. This setup probably has a similar mount for the lower control arm, and a heavier shaft...I get the feeling that the added mass of a rod to support the car is more then a spring, I am betting there is an increase in unsprung mass.

I also don't like the random kinked bar with no center support.

But hell, we all know push rod suspension is for race cars... simple logic will tell you that any car with push rod suspension MUST be a race car.

Strizzo, you are right, the top of the shock towers is covered so if the adjuster is placed there some holes need to be cut.

You guys are missing the point.

You have to think like jjrrrrifters.

It's better because it LOOKS better.

Not sure by looking at just that picture but one advantage to a set-up like that would be super-adjustable spring rates and ride heights if done right. All you would need are a couple of slots for mounting the shock and the push rod to the pivot point and you could re-ratio the struts to the rear suspension by several percentage. Adjustable length push rod means easy height and corner weight adjustment too, and if not done by a doofus all of it done without even getting dirty under the car.

Not that it would have been my way of doing it, but I can understand at least a couple of reasons for doing it that way.

Some shocks can't be mounted purely sideways as well, so the kink might have been necessary for the shocks to stay oiled properly....?

Kind of hard on the suitcases on a trip, or the bread on the way home from the grocery store. You know it makes the car faster because it blue and shiny and matches the neons and the light up brake peddle.

It does look kind of cool though. I wouldn't buy one, but I would bounce the bumper a few time just to watch it work.

All that trick E36 M3 in the rear and it's probably still got the ox-cart mac struts up front.

[/wishbone snob]

looks like ricer eye candy to me.