Push rod rear end

[RBbugBITme]

I know I've seen pics of a 240 with a pushrod conversion a while ago. Just wondering if anyone on here has considered it/done it? I'm actually surprised I haven't seen at least a few more since it seems like an easy job for a DIYer if you don't care about loosing some cabin/trunk space and know what you're doing.

Anyone think about it?

My thoughts...
-Reduced unsprung mass and probably improved polar moment of inertia
-Improved geometry to better utilize those expensive shocks/springs
-Can't see it being very expensive if you can weld

[90kacoupe]

sorry for the noob question but are u refering to a canti-leaver suspension or how ever u spell it.

[RBbugBITme]

Negative, its the style you'll see on most formula style cars. A rod is located where our rear coilovers are and that actuates a bell crank. On the other side of the bell crank is the coilover.

Seen here...
http://img.photobucket.com/albums/v364/iwantawd/cars/IM000472.jpg

[90kacoupe]

yea thats what i was referring to.. sorry if im getting my terms wrong.. im just trying to figure it all out.. but yea i have always wanted to do it.. since it would get alot of weight off ur suspension and u could adjust ride height with out having to mess with the coilovers setup.. and it would really open us up to different coilovers

[90kacoupe]

after a quick search it seems like those are also referred to as cantileaver suspensions.

[Def]

I'd be worried about the strength of the pivot and load paths without adding excessive weight up higher in the chassis. In which case it kind of seems counterproductive.

[RBbugBITme]

The setup I saw had it built into his roll cage with some extra support around the shock towers so overall its not that much weight compared to the full roll cage and the stiffness is there. Everything's a tradeoff but I'd say its worth design time. Until there is a design you don't know how much it weighs though or where that weight is being distributed.

[sr20goofus]

The setup I saw had it built into his roll cage with some extra support around the shock towers so overall its not that much weight compared to the full roll cage and the stiffness is there. Everything's a tradeoff but I'd say its worth design time. Until there is a design you don't know how much it weighs though or where that weight is being distributed.


i have photos of this suspension setup on a 240 on my comp at home (pics tomorrow night). The guys that do this use small coilovers from motorcycles with external reservoirs. But onc eyou add in the metal bracketry to support said design, i dont think it will benefit anythign besides you being too lazy to jack up the rear of the car and mess with the suspension.

In the end your front suspension will still be less than ideal while teh rear is workign its ways towards a well engineered design. Why only go "all out" on half the car? Save yourself alot of effort and just stick with what works.

[racepar1]

Too much work, not enough benefits. I really don't see how the car would be any better with a pushrod style shock setup, other than the availibility of super gangster racing shocks that is. Cool idea, but in the end there is really no point.

[veilside180sx]

The 5 link in the back of the 240 really is not a bad design, and there are other places to seek restitution first before attacking it.

[Epstein]

Everytime I think about exotic setups like cantilevers or double wishbone conversions, I just go back to the JIC S15 and how successful it is in Time Attack without all that.

[RBbugBITme]

In the end your front suspension will still be less than ideal while teh rear is workign its ways towards a well engineered design. Why only go "all out" on half the car? Save yourself alot of effort and just stick with what works.

Well it was just a conversation starter. I wasn't going to bring up my thinking about a possible pull rod setup up front. I know, I should head over to the looney bin!

[a_ahmed]

I say do it. Ignore the rest haha.

[Epstein]

No I don't think you're looney for bringing up the conversation.... I think these guys are looney for trying it.

http://i67.photobucket.com/albums/h298/JDMITRCinci/Heraldo/catilevers.jpg

I've found a whole lot more talk about it, and I know there's a pic of a green hatch with it actually put together.

[a_ahmed]

the us 'silvia' something 240 forum had a race car in the making but the whole site died/project died... It was a white hatch

[racepar1]

I believe it is also better to have the shock to compress directly in a straight line than it is to remote mount it with a bell crank. The dampening should be more effective. For the front a pull rod suspension is also useless. I think the best front suspension solution is to make a custom dual a-arm set-up with a regular racing shock, like the front suspension conversions for the mustangs. For the rear just make a custom upper mount so that you can use a similar racing shock and re-arrange the geometry 0f the 5-link set-up a bit to get it back to where it should be on a lowered car. Anything beyond that will just end up being a lot of work for nothing. Using motorcycle shocks is also a bad idea as they are not designed to handle the loads that a car can produce.

[RBbugBITme]

A pull rod set up up front would inherently mean a custom double a-arm arrangement. The point of the pull rod over a conventional double a-arm again is to move mass closer to the center of mass of the car and remove mass from the unsprung structure.

The shock on the rear/strut on the front do not compress in a direct line. The bottom pick up points move through an arc just like on a bell crank.

Obviously this would require some extensive chassis work and isn't for the weekend autoxer/daily driver. Keep that in mind, I'm trying to talk about taking the old 240 chassis to the next level. Of course its had good success with what its got but it also has more potential.

Does anyone have opinions/thoughts on the force based roll center concept and designing for roll centers below the ground plane?

[racepar1]

I have yet to see a 240 that I would consider to have fully explored the capabilities of the stock suspension design. To fully explore the stock suspension design's capabilities would require mapping out the suspension in a suspension analyser program and setting up all the angles and curves to as close to optimal as poissible. That is out of the realistic reach of pretty much all 240 enthusiasts as it would take an experienced suspension engineer to decide what exactly "optimal" would be. Even if you did all the mapping yourself it would not be cheap for a good engineer to go over it with a fine tooth comb. And then if "optimal" meant having to move pick-up points, which it almost certainly would, it would not be cheap or easy to find a fabricator competent enough to get it right.

I really don't think it would be possible to gain enough to make a push/pull rod suspension set-up worth it in the end. Now a dual a-arm front suspension, designed as simply as possible, and altering the rear geometry to bring it back to where it should be on the other hand would be worth it and much more realistically attainable. It would still require some pretty hardcore engineering, but to a much lesser extent than the push/pull rod idea.

[RBbugBITme]

I'd be surprised if there wasn't someone other than me on here with access to some type of program like you speak of. I'm sure Wiisass does. The only key thing missing really is tire data. Besides, your basic MS Excel is capable of doing a lot for suspension design. It just takes a solid handle on some college level math. How did Carroll Smith ever design suspensions without a computer program?

Again, this isn't for the majority of 240 enthusiasts and if someone is capable of modifying the front suspension into a double a-arm then incorporating a push/pull rod into the design at that point would be childs play.

This is getting way off topic now. I'm not discussing if it should/could be done, I've already started working on it. I'd like to get down to some theory with other suspension enthusiasts and hopefully get some learn'in done outside of a book.
Hopefully someone can put their two cents in on the roll center questions. Does anyone have any other info on force based RC's than this? http://www.neohio-scca.org/comp_clinic/hand_out_reprints/Vehicle%20Dynamics2007.pdf

[racepar1]

I like you man! You're nuts and it's cool as hell. Now why would you use a pull rod suspension up front rather than a push rod set-up? Pull rod set-ups are VERY rare. Even CART, IRL, formula atlantic, formula mazda, and F1 cars use the push rod design rather than the pull rod design.

[RBbugBITme]

Packaging was the initial thought. Thats far from a final thought but if I were to use the strut tower hole thats already there, there isn't much room for a bellcrank. The front is certainly tricky and this will take at least a year before I make any significant progress. I'm at the beginning stages of a complete redesign of the FSAE suspension due to some painful chassis regulation changes.

[racepar1]

Packaging was the initial thought. Thats far from a final thought but if I were to use the strut tower hole thats already there, there isn't much room for a bellcrank. The front is certainly tricky and this will take at least a year before I make any significant progress. I'm at the beginning stages of a complete redesign of the FSAE suspension due to some painful chassis regulation changes.

You could mount the shock longitudinally along the framerail and just cut a new hole. That would keep the weight lower in the chasis, lessen the length of the pushrod, and the framerail would be a good solid place to start for mounting the bell crank.

[RBbugBITme]

That's one of the options I was looking at.

[a_ahmed]

This is uhm... a program, that works for video games and apparently real life thingamajigs bwahaha

just kidding well check it out ;)

http://www.vehicle-analyser.com/

I'm a big fan of LFS

[sr20goofus]

http://i42.photobucket.com/albums/e331/sr20goofus/1822_194.jpg



NON-Nissan....

http://i42.photobucket.com/albums/e331/sr20goofus/hatch.gif

http://i42.photobucket.com/albums/e331/sr20goofus/130_0702_09_zextreme_dimensions_200.jpg


Honda Front....
http://i42.photobucket.com/albums/e331/sr20goofus/1248420-R1-040-18A.jpg

[Wiisass]

I've said it before, I don't think it's worth it. It could be cool, but the time/money/frustration vs. effectiveness is low compared to spending the time and money on other things would be a much better investment.

There are benefits of an inboard damper setup, wheel/tire packaging, aero concerns and a better selection of installation ratio. I don't think there's enough to gain in the wheel/tire packaging unless you're totally modifying the spindle, which would just add to the cost and complexity. Aero concerns are not applicable. The better selection of installation ratios and the different possibilities of dampers is a good reason, but still not enough. And since I don't think that motorcycle dampers would be suitable for this task unless you got the car super light, like sub-2k race weight, then you're still going to be spending a lot of money on sweet dampers whether they're inboard or normally mounted.

And there are more gains to be made in other places than in shock/spring packaging.

And the unsprung weight argument isn't as valid as most people think. The conventionally defined unsprung weight will be lower, but the damper and spring still need to move with the chassis/wheel movement, so the actual intertia of the overall system may not be any lower than with a conventional setup. And the inertia is more important than the measured weight, so that argument may not hold up too well.

I think a double a-arm setup in the front would be easier and more beneficial. I've been wanting to do this for a while, but just haven't found the time to really get into it. But I don't think it would be that hard, even using mostly stock parts and then some easily fabricated parts. But depending on how you do it, it wouldn't really bad too bad at all, as long as you know what you're doing.

I would just focus on the main flaws in the suspension and prioritize. I'm sure when you do, you will realize that shock/spring packaging is further down on the list and that an inboard shock setup would be more for showing off than anything else. I mean the 240 installation ratios aren't too bad, you can fit more than big enough wheels/tires depending on offset choice and fender modification/replacement.

But if you're doing it, then I guess go for it. Maybe it will come out awesome and be worth the work or maybe you'll be working on it for 3 years until you decide to just junk the chassis because you've gone too far and it's not worth finishing. Either way, I want to come see it.

[RBbugBITme]

A lot of the future plans will depend on what kind of money I end up making. It would be better and more fun to just build a whole new car with a tube frame chassis but piggy backing off the 240 is basically the "poor" mans way of getting it done.

I don't think there's enough to gain in the wheel/tire packaging unless you're totally modifying the spindle, which would just add to the cost and complexity.

I couldn't imagine going through this massive project and restricting myself to the stock uprights/spindles. That to me is insanity.

And the unsprung weight argument isn't as valid as most people think. The conventionally defined unsprung weight will be lower, but the damper and spring still need to move with the chassis/wheel movement, so the actual intertia of the overall system may not be any lower than with a conventional setup. And the inertia is more important than the measured weight, so that argument may not hold up too well.


You're going to have to sell me on this one a bit more. If you were to make a free body diagram of one corners suspension it would end at the bell crank. I don't see how the mass of the coilover would be included. You're also still reducing the polar moment of the entire car by moving that mass inward.

[Wiisass]

So basically a clean sheet design using pretty much nothing stock. If you're going that far, I wouldn't even waste the time on the 240 at all. Just build a full chassis and do it right instead of wasting time with a 240 at all.

And even if you are modifying the spindles/uprights, it's just going to add a lot more time and money to the project.

Trust me, I'm the same way as you are, you want to do everything yourself and design it all and build it all, but I also want to drive it all. And building for 3 years and then driving for a little and then breaking it or deciding it sucks or whatever is just bound to happen with stuff like this.

I'm all for doing what you're talking about, but I would build it out of a better chassis or something that looks cooler. Once I have the time/money/space to do it, I want to build something that looks like an AC Cobra or something along those lines, using just the body from Factory Five or another replica company, but with my chassis and suspension design and engine choice and everything else. It will give me a much more open packaging envelope and endless possibilities for, realistically, not too much more work than you are talking about.

Oh and another thing I thought of, if you do a lot of work to get more wheel/tire under the fender, you're going to run into packaging issues if you're trying to keep a damper anywhere in the wheel well, unless you don't plan on turning much.

And for the intertia vs. weight, you're free body diagram is wrong. Why are you stopping at the bellcrank? The only part of the bellcrank that is grounded is the pivot point. The end of the shock that attaches to the chassis is grounded and that sees force. How would the shock not see force based on wheel travel?

Like I said, it's not weight that it adds, but you still have to move the same amount of the spring/damper that you would have to move before. Depending on damper and spring choice, it could be less, but it still needs to move when the wheel does.

Tim

[RBbugBITme]

As soon as I posted that I thought it was wrong. Starting from a clean slate would be ideal but then you're really talking about a skyrocketing budget.

[RBbugBITme]

I didn't feel like making a whole new thread but here is a very cheap program for those that are starting out on suspension analysis. This is pretty much exactly what you can do with excel though.
http://www.auto-ware.com/software/mac/mac.htm

I hear the webinars are quite good too and I'm hoping to get in on the next round.
http://www.auto-ware.com/webinar_home.html

[Darkgrey240]

Ryan,

Is Drexel going to be competing in California next week? If so check out University of Hartford's pull rod set-up. It's quite interesting to say the least.


John

[RBbugBITme]

Feel free to shoot me over some pics if you have them but no we won't be going to Cali.