PVC suspension simulator
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Author:  JackMcCornack [ October 30, 2007, 1:30 am ]
Post subject:  PVC suspension simulator

So I was messing with my 2k7 body, and plugged in the front suspension, and noted that despite my careful calculations the wheel didn't line up exactly with the fender. Grrr. I decided to sleep on the problem, but instead woke with a cry of "Eureka!" and whipped up some adapters that let me fit 1/2" Schedule 40 PVC pipe 'twixt the spindle carriers at the wheel and the rod ends at the chassis. That took a few hours, but then Voila! With my pipe snipper in one hand and my tape measure in the other, I made control arm mockups that fit the way I wanted them to, in about twenty minutes.

Then I remembered I wanted to shorten the wheelbase of the Kubota car by an inch, because the engine is so light and the car will be so slow, so I popped the pipe sections out of the adapters and snipped some new ones for a book frame, except with the wheels set back an inch. That took another twenty minutes. You might want to take a look at this...it could be quite a time saver.

[edited 11/2/07: how-to-use comments moved to bottom of page, with photos]

Author:  locostv8 [ October 30, 2007, 2:42 am ]
Post subject: 

Great idea.

Author:  JagLite [ October 30, 2007, 10:59 am ]
Post subject:  Control arm tuning

Fantastic Jack!

A super time saver and inexpensive way to test and tune the suspension.

Author:  chetcpo [ October 30, 2007, 11:26 am ]
Post subject: 

Can you use a different offset wheel instead of re-fabbing the control arms?

Author:  JackMcCornack [ October 30, 2007, 1:43 pm ]
Post subject: 

Thanks for the comments. Here's how to make the adapters.

For the chassis end, bore a section of 1/2" PVC schedule 40 water pipe to fit the OD of 1/2" CPVC hot water pipe--either ream to .622 for a press fit, or drill it out to 5/8" for a glue fit--and cut it into 3/4" long sleeves. Cut 3/4" inner sleeves from the CPVC and press/glue them together.

BTW, I have no idea why PVC and CPVC (PVC is white, CPVC is slightly brownish; I think the C stands for chocolate) have different dimensional standards, but they do. This double-sleeved bung will be .475" ID, and stiff enough to tap for 1/2-20 threads. Don't waste your time trying to thread the CPVC without the PVC outer reinforcement; the single wall CPVC is too flexible to grip the outside and tap the inside, and if you try it, you may end up with the tap so stuck that you have to cut the CPVC sleeve off of it. Guess how I know that?

If you're tapping this with a lathe, use a four jaw chuck. Even double-sleeved, a three jaw chuck will distort the part too much for good threading...and then it's back to the hacksaw to rescue the tap again.

Now glue the threaded double-sleeved bung into a 1/2" PVC slip-slip joiner (ah, the colorful patois of the plumbing trade) and you're done. Make right and left adapters as needed, and mark them appropriately with a felt pen.

If you're hand tapping the adapters, it would probably go best if you wait 'till you've glued them into the joiners; I'll probably try it that way next time I do a run.

The pipe, joiners and glue are available at hardware stores from big-box to mom-and-pop. Materials cost is about 30 cents per adapter, but you may have to buy your pipe in 10 foot lengths, so you might have ten bucks wrapped up in the adapters you need, plus your labor. But compared to the cost of steel, and to the labor of cutting/welding/fitting/swearing/uncutting/grinding/rewelding/refitting, it's a bargain.

Author:  JackMcCornack [ October 30, 2007, 2:42 pm ]
Post subject: 

chetcpo wrote:
Can you use a different offset wheel instead of re-fabbing the control arms?

Sure I can, but that would change the scrub radius and Locosts are notorious for their sensitivity to scrub--plus the added expense of new wheels. I could change the body, too, but new control arms seemed quickest and easiest, and in the long run, cheapest. Using the PVC suspension simulator, I can get it how I want it before sparks fly.

On the outboard end, use 1" .083 wall round and/or square tubing; you'll get a nice firm fit. Use ERW rather than DOM; the welded ridge inside will help locate the PVC tube.

For the upper adapter, the 1/2" schedule 40 PVC threads nicely to 16mm or 5/8", depending on your upper ball joint. On the adapter shown I've used a 3" steel tube with a 2" threaded PVC bung (16mm x 1.5).

The lower adapter has an end like the usual Kinetic rectangular control arm for Miata lower ball joints, with a 2-1/2" long section of square tubing welded between the top and bottom plates. Yep, just me an my welding duck at two in the morning.

The lower shock mount bracket is left free, to position and affix another day. Note there's a 1/8" hole in the bottom of the bracket; that's so I can hold it in place with a pop rivit.

Also note I used two bottom plates instead of a bottom and top plate. That's 'cause the inner ear of the Miata ball joint mounts to the bottom of my control arm, so by using two bottom plates (and drilling the rectangular section accordingly, and making the curved cutaway on the end go all the way through) I can flip the ball joint other side up, and use this same adapter on the other side of the car.

The adapter for the rear arm on the lower control arm assembly is a 1/2" PVC sli-slip joiner with a 1/2" schedule 40 PVC plug in the outboard end, drilled for a 3/8" bold and milled to 1" thick...but it's a 20 cent part and it's just an adapter, so I think a flap wheel on an angle grinder would do as well as a mill, long as it's 1" thick when you're done.

Author:  e30sho [ October 31, 2007, 10:34 am ]
Post subject:  cpvc

Great stuff Jack. Thanks. FYI: Cpvc is rated for hot water, pvc for cold only. Building code does not let you run pvc inside a house, anywhere. I'm guessing that the two are not interchangeable to prevent DIY's and/or rouge contractors from mixing the two up.


Author:  jdgar0649 [ October 31, 2007, 10:30 pm ]
Post subject: 

Jack! What a FANTASTIC solution! I happen to have bunches of PVC lying around--I can't wait to get the length right without threading up endless variations in steel. Wooo-Hooo!

Author:  chetcpo [ October 31, 2007, 11:00 pm ]
Post subject: 

That's a good mockup tool, but making "after the fact" changes to control arm lengths without changing your suspension pickups will change your roll center location, (and RC stability) camber curve, etc. since you have shortened your swingarm length and changed the angles of the control arms.

That being said, if you are using a chassis with pre-selected and fixed control arm pickups not of your own design and you need to make quick changes to fit your bodywork, this method is ideal. As I said it's a great mockup tool, but IMO shouldn't be relied on soley for suspension design.

*Zips up flame suit and prepares to wait longer on those fenders*

Author:  scrabblegod [ October 31, 2007, 11:10 pm ]
Post subject: 

PVC is great stuff.
When I built my V-8 Fiero, I mocked the whole exhaust up in PVC and took it to the muffler shop to have it bent to the peoper shape.

They would not fo the exhaust on the car due to the lack of converters.


Author:  JackMcCornack [ November 1, 2007, 3:19 am ]
Post subject: 

chetcpo wrote:
That's a good mockup tool, but making "after the fact" changes to control arm lengths without changing your suspension pickups will change your roll center location, (and RC stability) camber curve, etc. since you have shortened your swingarm length and changed the angles of the control arms.

Good point, that. And one of the reasons I've made bolt-on upper pickup brackets (see first couple photos on this thread). And <that> being said (here I go putting a "flame me!" sign on my own back), there are a heck of a lot of Locosts out there--including commercial kits--that are copies of copies of chassis, with no apparent compensation for different spindles or tracks (and thus different ball joint locations), than the British escort from whence they sprang.

By the way, these PVC control arms, with a spot of glue (at the chassis end) and some pop rivits (at the spindle end), are plenty strong and solid enough to do range-of-motion camber and caster checks on the car. I sure wouldn't (and don't) rely on it for suspension design, but it's a nice way to check my calculations.

Author:  chetcpo [ November 1, 2007, 9:43 am ]
Post subject: 

JackMcCornack wrote:
<snip> I sure wouldn't (and don't) rely on it for suspension design, but it's a nice way to check my calculations.

Sorry if it sounded like I was suggesting you did. I wasn't. The only reason for my comment is I didn't want to give the impression to any newcomers that this was the best way (to simply build it out of PVC and make changes till it appears to work) to develop a suspension design. It is however hands down the best way to test your predetermined design that I have ever seen.

Author:  JackMcCornack [ November 1, 2007, 1:27 pm ]
Post subject: 

No worries, mate, and I'm sorry if I sounded cavalier about the suspension design and testing process. I consider dang near everything I make for myself to be a prototype for future production, so I don't mind cutting and changing when it would be beneficial, and if after this car is "finished", a repositioning of pickup points is in order, I'll do so.

And if fine-tuning this particular car's track was the only suspension problem I was thinking about, I wouldn't have bothered trying to sleep on it, I would have just tacked together a new set of control arms--I've got plenty of tubing and plenty of wire and enough gas, and as always, my trusty welding duck. The other problem was (to me) considerably more significant:

The problem is, pickup point positions on homebuilt chassis vary widely in fore-and-aft position as well as other axes, and though fore-and-aft location changes don't affect the geometry any, they sure change the shape of the control arms...and Kinetic sells control arms. I wanted a way that Kinetic customers with scratch-built frames could double-check their control arm dimensions before we cut and weld the finished product for them. It looks like this is a useful technique for people building their whole car from scratch, so I've passed it along for general use.

To see an extreme example of different fore-and-aft locations, compare the first couple photos on this string with the later photos. On the first photos, it's a chassis set up for the forward lower control arms to be in line with each other (it looks sort of like a beam axle, but that's not why it was done that way). The front control arm tube is much shorter than the rear tube, but the suspension geometry is identical to an A-shaped control arm with equal length tubes.

Author:  JackMcCornack [ November 2, 2007, 11:58 pm ]
Post subject:  Using the PVC Suspension SImulator

The basic technique, with photos, no less:

1. Insert rod ends and clevis, with jam nuts, in the adapters, with 1/4" of thread above the jam nut. You don't need to measure; they're 1/2-20, 20 threads per inch, so thread them all the way in and then thread them out five turns. Thread the upper ball joint, with jam nut, into the upper control arm adapter, with 1/2" of thread outside the jam nut. Install the lower ball joint into its adapter as appropriate (note with the Kinetic control arms for Miata spindles, "as appropriate" means with the inner tongue of the ball joint bolted from the bottom, with a .20" spacer between the tongue and the tube...but if you're doing something different, "as appropriate" will likely be different too ).

By starting with 1/4" of thread on the rod ends, you'll have +/- 1/4" adjustment with the recommended 3/4" of thread remaining in the control arm. No of course that doesn't matter any right now, of course you're not going to drive on it, but we're simulating the real thing so why not be as accurate as we can? With 1/2" showing on the upper ball joint...well, you're going to want negative camber eventually, so you're sure not going to adjust the upper ball joint out...you have +0" and minus 1/2" adjustment available.

Hang the chassis adapters in the suspension brackets with 1/2" bolts--no need for nuts or spacers at this stage.

Install the upper and lower ball joints into the spindle (with the wheel attached).

Put the wheel where you want it. Shim the wheel (or the chassis) if needed to set the ride height, measure for wheelbase and track, use a carpenter's square to set the camber 90 degrees from the floor, and just eyeball the steering for now (it should be aimed so the car goes straight).

2. Measure from where the PVC pipe will bottom against the lower ball joint, to the edge of the lower front chassis adapter. Add 3/4" (the inside depth of the adapter) and cut a piece of 1/2" PVC Schedule 40 pipe to fit--in the photo, the measurement is 11", so the tube is cut to 11-3/4".

3. Insert the freshly-cut tube into the adapters. Press it in firmly. You can do this without moving the wheel if you remove the chassis adapter from the chassis (pull the 1/2" bolt out of the suspension bracket and rod end) and reinstall it when the tube is in place.

Author:  JackMcCornack [ November 3, 2007, 12:30 am ]
Post subject: 

4. Repeat with the rear tube of the lower control arm--this one has PVC slip-slip joiners on both ends, so the tube length will be 1-1/2" longer than the measurement edge-to-edge of the adapters.

5. Use a spare piece of PVC tube (or whatever's handy) and an angle finder alongside the upper and lower ball joints to set caster. Measure what's needed for the forward tube of the upper control arm, cut the tube and insert it firmly into the adapters. Check caster again, measure for the rear tube, cut and install the rear tube.

6. By gosh, you ought to be pretty close. You can get it perfect by adjusting the rod ends and upper ball joint, just as if they were real control arms. Once they're perfect, glue the tubes into the PVC adapters and rivit the front tubes into the steel tubes of the spindle adapters, and use the assemblies as your master when you make your control arms.

And that's it. With the components glued and rivited, the assembly is plenty strong enough to install your shocks (rivit the lower shock bracket to the lower ball joint adapter) with springs removed, and move the suspension through its paces. A similar system could be made for the steering tie rods (and I'm sure I will; I haven't had time to get to it yet) to set ackerman and bump steer.

By the way, if your lower ball joint mounting system allows (as does the one shown here), flip the simulators over and <without reajusting them> use them to fit the other wheel to the other side of the car. If there's a discrepency you can't adjust out with a few turns of the rod ends, give your chassis some serious scrutiny.

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