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gblawson (Gordon)
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Tom, I would say that analogy only really applies if you consider the connection from the engine mount to the engine block to be a hinge. I would suggest that if this is considered a rigid connection that can handle the load as a cantilever, there should be insignificant spreading forces on the frame.

John, first off, awesome build so far!
Looking at the engine mounts you have, I'd figure you'd see about 2000 lbf tension/compression at the bottom/top of the connection from the rectangular tube to the flange. This is based on a stock Miata engine and a stock transmission in 1st gear and assuming the rectangular tube is a 1 x 2. A dump the clutch condition could increase these forces. Shear stress at the tube to flange connection is pretty small compared to the tensile/compression forces from the cantilever. I would say as long as your welds are sound, this should be OK. If you want some insurance, you could brace/gusset the tube to the upper and lower extents of the flange.

Keep up the awesome work. I sure is motivating me on my build.

Jack

Jack,

The tubing is actually 1x1.5"x16g HREW (assume 40-ksi yield strength). Using your numbers I come up with roughly 32-ksi stress at the top/bottom of the tube at the flange (been a while since I did any real analysis) - this does not take into account the added cross sectional area of the vertical tubes, just the top/bottom sections, as I know the stress goes to zero as you near the center of those sections. But I'm assuming the force is actually higher than what you listed since it's only 1.5" tall tubing, and not 2".

If you don't mind, could you run that calculation again for 1x1.5" tubing, assuming the length of the motor mount is about 9.5" from the mounting flange to the center of the pivot on the urethane motor mount, and the angle from horizontal is about 20-deg. Also, assume the motor is a bit warmed over, maybe something in the range of 200-brake hp, and 150-lbft tq.

Just wondering if I should add gussets or maybe re-make the mounts using thicker gage tubing. I know eventually I'll have more power and sticky tires, and although I don't tend to do burnouts, I don't always exercise a great deal of mechanical empathy. I was just looking at some pics of a duratec powered CSR and noticed the engine mounts have a much taller cross section at the motor flange than caterham's old style mounts did.

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- John

A piece of 1x2 tube would be more then 3 times stronger and I remember something like 27 KSI for hot rolled steel tubing. That's just memory though, and it's not worth what it once was! I'd worry about hitting bumps. Figure at least 3 g's for that.

On your rear uprights thinking you need a washer between the rod end and the aluminum? Are you going to Loctite or something to keep the pin from turning?

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Now that I think about it, the motor mounts I made for the SV were 1/8" wall 1"x1.5", not the 16ga that I used here, so I may want to re-make those, but I'd like to do some analysis before I do. Yield strength for mild steel is all over the map. I called the mfg and they quoted me 40-ksi for what I used.

As for the uprights, they are cast iron, not aluminum. I'm essentially clamping on them in the same way the inner sleeve of the stock rubber bushings do, with a similar surface area of contact. I made sure the shoulder of the "pin" was shorter than the width of the upright, to ensure the upright was in compression, just as in the stock application. I can't see it rotating unless the rod ends were to sieze.

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- John

John,

Using the numbers you provided, I come up with 27ksi stress at the tube to flange junction. The numbers I used for this is your 1x1.5x16g tube, 9.5" long from flange to center of bushing, 150 lb-ft motor torque, 3.16:1 first gear ratio, 250 lbs for the combined weight of the engine/transmission that is sitting on the 2 front engine mounts (I'd assumed most of the trany weight is on the trany mount), and 26" center to center between your 2 front motor mount bushings (I tried to error on the short side since this is worse but won't make a huge difference). I did not factor in the 20 degrees since at this angle it has a small effect since we should be shooting for a good factor of safety.
I ended up with mount loads of 334 lbs down on one side and 94 lbs up on the opposite side. This does not take into account any shock loads or loads from inertia from the vehicle moving.
I think the 27ksi is high. I typically use 35ksi for yeild on mild steel tubing, so this doesn't leave a great FoS.
Going to 1x2x16g tubing would reduce this stress to 17ksi. I would consider this to be marginally acceptable, but if it were me, I'd go for a FoS of 3 or better. The weight penalty is small compared to the consequences of the motor mount failing IMO.
Going to 1x1.5x11g would be marginally better than 1x2x16g at 15.4ksi.
Going to 1x2x11g would reduce the stress to 9.9ksi.
Going to 2x2x16g tubing would reduce this stress to 10.5ksi.
Going to 1x3x16g tubing would reduce the stress to 8.9ksi.
Going to 1x2.5x14g (didn't find this size in 16g) would reduce the stress to 9.5ksi.
Going to 2x2x11g would reduce the stress to 5.9ksi.
Alternatively, if you want to be creative (and I know you do!), you could make this a tapered tube by cutting 2 tubes on a diagonal length wise and welding them together as a taperred tube. You could then go as tall as your motor flange will allow (leaving room for the fillet weld) and tapering down to your 1.5" at your bushing.
Hope this helps.

Jack

Been giving this motor mount thing some more thought. Below is a picture of the CSR motor mount for the duratec motor. You can see how tall it is near the motor, and it tapers down near the frame. I'm going to do some stress calculations first, but I think if I gusset the mounts as shown in the cross sectional view below, I might be fine. It will end up being 2-3/4" tall at the mounting plate, adding material and better geometry to the highest stress part of the mount.

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- John

A closed section piece like a tube would probably be more efficient. Free flanges do strange things when loaded.

I'd vote for calling them good for now (they look great) and watch them once you start driving (light colored paint helps). If they are going to fail, I would think you could see cracks before the mount completely let go.

I made mine out of 2"x1"x0.125" tubing and I suspect they are way too heavy.

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-Andrew
Build Log
Youtube

John,

I think you are on the right track with the tappered approach.
Adding the gussets top and bottom as you've shown should make a big difference. I believe as shown these are cut pieces of tube fully welded on and not flat bar that is open front and back.

I tend to be conservative when it comes to things I don't want to fail, like in this case the motor mounts. For me, I'd use 11 gauge tubing and taper from the 2-3/4" down to your 1-1/2" at the bushing.
The weight difference will be pretty small in the grant scheme of things, and gives you a much large factor of safety and piece of mind.
I would recommend keeping the stress levels below the endurance limit, which for steel is 1/2 ultimate tensile strength and then you don't have to worry about fatigue failure.
A couple of other things to keep in mind are:
- residual stresses from welding will somewhat reduce the load capacity unless they are stress relieved.
- we have been talking about the stress levels in the tube at the flange, but of course the weld itself could become the limiting factor in the ultimate strength of the mount. How confident are you that you can get full penitration all the way around when welding a thin material to a thick material? Using 11 gauge would make this easier and give you more room for error.
- handling full engine torque in first gear is only relevant if you have the traction to transmit this amount of torque. That of course is dependant on tires, weight distribution and rear end ratio.
- no use making the engine mount stronger than the fasteners holding the mount to the block. I beleive the stock Miata uses much short mounts meaning the load on the fasteners and engine block is much less than with the longer mounts you are uses. I don't expect this to be a limiting condition, but not knowing the size of fasteners, spacing and grade, I can't be sure.

Keep up the excelent work!

Jack

I guess the cross-sectional view was a bit confusing. I gusseted the mounts and am much happier with the end result. I think it added a little over a half pound to the two mounts, but they're now probably twice as strong as before. I'm calling them good. I finish welded the mounts into the frame and everything lines up great - moving on to making control arms.

I also made the final measurement for the new driveshaft length - I'll be getting a quote for shortening the stock one, or making up a new one with serviceable u-joints later this week.

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- John

Looks good.

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-Andrew
Build Log
Youtube

I like that too. But I ain't no engineer (even if I stayed at a Holiday Inn Express).

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The Lethal Locost
The Lethal Locost 2 - Even More Lethalerer

Looks good John. That should make a noticable improvement.

I called up PowerTrain Industries. Apparently they make most of the replacement driveshafts offered. I gave them a call and asked about custom length Miata driveshafts. They said no problem. I think it ended up being about ~$250 to my front door. It's balanced, and looks beautiful, and has U-joints with zerks on 'em.

Been kind of busy taking care of a few things around the house and getting out on the bicycle for a few long rides this last week. I have managed to fit in some work on the Seven though. I try to do at least one little thing every day, even if it's just 20-minutes - then at least I feel like I'm making progress.

I've been focusing on machining components for all the control arms - alignment adjusters for the upper-front and lower-rear arms. Also some wider bushings for the rear shock attachments, and all the sleeves for the bushing end of the control arms. I also gave some more thought to adding washers between the rear uprights and rod ends (horizenjob's comment above) - I made some 1/8" thick ones out of 17-4, but this means I have to re-make the pivot pins with longer ends. Instead of the soft stainless steel that I made the first ones from, I ordered some 4340 CrMo - and I'll tap fine threads instead of the course threads I did before - just to make it that much stronger.

I also picked up the new driveshaft today - local shop did a great job making up a new shaft with serviceable U-joints. Fits great too. As I was installing it I was imagining all of the body panels in place, and I'm really happy with how easy it is to install the differential from above. Now it's time to make the driveshaft hoops and finish the cross bracing on the tunnel. It's a tight fit, but should work well with a minimum of wasted space.

The pedal assy and master cylinders also showed up in the mail last week. I'm getting a bit ahead of myself here, but I set them in place to see how they fit in the foot box and how high I would have to mount them for proper pedal height - also to think about how I'll mount the accelerator pedal. I'm planning on copying Birkin's style of pedal adjustment when I get around to it. Also set the wheel in place on a dummy shaft to see how the routing would work through the pedals. I may need to get a smaller reservoir for the clutch master to clear the hood radius, but at least it's an option.

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- John