Chuck's RX7 / CMC w/IRS challenge
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Author:  Pete B [ July 22, 2008, 11:32 pm ]
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Thanks for the pictures, Chuck, a lot of food for thought here.

The differential offset doesn't look so bad, but the intrusion of the starter into driver footroom is a bummer. I need all the footroom I can get. There are compact starter motors available, will have to look into them. Relocating the starter came to mind also, but seems like a lot of work.

Your front suspension looks great. The uprights are very clean, and the way you used that C channel was inspired. Much better shock angles and it causes the shock loads to oppose each other somewhat.

It looks like you're going to have to do something with the rear wishbones in order to be able to mount a shock. It also appears that the lower pivots are not in the same plane.

I've toyed with the idea of making uprights from sheet metal, a la the Haynes Roadster. Here's another approach, of welding on hunks of aluminum to make new pivots:


I don't care for the single shear on the upper bolt, but this car has been campaigned very successfully in autocross.

Author:  rx7locost [ July 24, 2008, 8:19 pm ]
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Pete, my lower pivots (rod ends, heim joints, rose joints, etc) are in fact in line. The photos are deceiving. I would be concerned about single shear on the rod ends, I'm not so concerned about the shocks being in single shear, although I haven't had to go single shear yet.

I have a photo of the starter position shot from above. With a little work, there would be less intrusion into the cockpit than I have shown earlier. My earlier efforts in cardboard were to minimize re-shaping the lower frame members that were modded by the previous builder. Of course the engine could be moved further forward. That would move the shifter further forward and for my 5'8" body, I'm already as far forward as I care to be.

Today I fabbed up a temporary wooden seat for fitting purposes. I easily fit into the drivers side. I'm a little concerned that I would have too much room for Autocrossing with a bench seat as in the book. On the passenger side where there is an inch less. Both I and my wife fit in. Ok, not at the same time. So I guess it will not be a problem there.I'd really like to find a pair of small british car seats (MG Midget?) with headrests and narrow them up. I'd be able to use the seat adjusters.

The trans mount and the front differential mount are tacked in the frame and are self supporting. I'm happy with the way they turned out. I will final weld these, and many more welds, when the car is dissassembled for frame-painting.

I had earlier reported having some bump steer issues. I have done some more testing and decided I can keep the rack inside the front of the frame by lowering the rack by ~3/4". I oringinally didn't think I had room to do this, but, as it turns out, I can do it.

Every day, a little bit more.

File comment: Top view of starter position
starter.JPG [ 54.19 KiB | Viewed 8982 times ]

Author:  Pete B [ July 24, 2008, 11:39 pm ]
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Im glad I'm wrong about the pivot points on the rear uprights. TBH, I haven't gotten a good look at mine because my donor is intact and on flat tires in the garage.

The starter position looks much better in this picture. In a worst case, it wouldn't hurt too much to have the engine forward a couple more inches.

Hey, I have MG Midget seats. I measured them to be exactly 17" wide. The pic from the eBay ad is below.

I will copy your transmission and differential mounts shamelessly. :)

seats.jpg [ 16.74 KiB | Viewed 8962 times ]

Author:  Pete B [ July 24, 2008, 11:42 pm ]
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Another thought: How about substituting a piece of 1/8" plate for the tubes at the lower rear passenger side of your trans tunnel to gain a little room?

Author:  rx7locost [ July 25, 2008, 8:06 am ]
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17 inches looks deal. I can deal with thtat. I will have to keep my eyes open for a local deal. Somebody in the Chicago area must have a pair for $cheap.

Go ahead and copy away. I take no responsibility if it doesn't work out for you. of course I will take credit if it does :P

I'll try to get some pics up re: the finished mounts.

1/8" plate will not work for me. My problem is clearing the inside wall from the differential flange. Currently, the flange is horizontally located between 2 vertical 1/4" tubes of the tunnel frame. Either way, the tunnel sheet metal would be in the same location. And that is what is needed to clear.

Author:  rx7locost [ July 27, 2008, 11:04 am ]
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I got the rack positioning finalized and welded up. I ended up with a slight toe-in on bump which can be dialled out with shims on the final assembly after paint. I also finished off the RH tunnel build offset 1 inch for the Rx7 differential.

One question: I'll be re-considering my rear suspension design today to allow me to use my existing rear coilovers. Something I figured would work out as I went along. Well, it didn't work out. So my question is: How much clearance does your rear tire have to the body? I'm currently at 2-1/2 inches when at road height. I have 205R50-15 tires so that would require 12 inch fenders. That looks too wide. My CMC rear fenders run 9" to 10" from the mounting surface to the outdies of the fender. They will have to be widened. I just want to see what the group has.

Author:  rx7locost [ July 31, 2008, 9:26 pm ]
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I’ve made some more progress this week. The mounting for the rack is complete. I welded a mounting bracket to the steel end of the rack housing to eliminate the rubber bushings. On the other side, I made a saddle bracket to fit over the top and bolt into the frame mount. As in other places, I have welded the nuts to reduce the loose hardware during reassembly and any repairs, I mean upgrades. This will be done wherever possible.

I have added pictures of the transmission and differential mounts. I will add gussets at a later date. Throughout the build, I will be tacking everything together first. Only when all the parts are fitted, and the car is disassembled for frame paint will I finish the welding.

I got no responses to my question about recommended tire clearances. During the integration of the coilovers, I shortened the upper and lower control arms to give 1-1/2 inch tire clearance. This looks much better and I may now be able to fit the CMC rear fenders with my 205 tires in the future. The rear shock’s upper mount fits on a bracket welded between the two ¾” sq tubes angled up from the outer rear corner of the frame. Currently it is mounted in single shear. I will add another bracket to catch the other side of the upper shock bolt. No need to do that at this time. The lower mount is temporarily tacked to the lower control arm as far out as it can go. A new bracket will be designed to run across all 3 tubes forming the lower control arm.

The upper control arm is a very convoluted design to clear the coilover. I wish it were easier but I am trying to make the best use of the donor 2nd gen Rx7. The only part I really don’t care for is the attachment to the rear upright. It is in single shear. I see that some others have been successful in this manner so I should be OK.

In summary, the entire rear suspension was rebuilt to move the wheels colser to the body and to allow for the mounting of the coilovers. I’ve now got frame self supporting on 3 sprung wheels. Tomorrow I will rebuild the left rear to match the right rear design, completing the rolling chassis stage. WOo HOo!

File comment: Mounted rack
P7300128.JPG [ 54.6 KiB | Viewed 8757 times ]
File comment: Transmission rear mount
P7250119.JPG [ 67.38 KiB | Viewed 8754 times ]
File comment: Differential front mount
P7250120.JPG [ 71.39 KiB | Viewed 8756 times ]
File comment: Rear view of suspension using unmodified 2nd gen rX7 uprights
P7300145.JPG [ 58.77 KiB | Viewed 8756 times ]
File comment: Rear upper control arm
P7300143.JPG [ 63.25 KiB | Viewed 8760 times ]
File comment: Rear coilover mounting
P7300144.JPG [ 61.08 KiB | Viewed 8763 times ]

Author:  Pete B [ August 1, 2008, 8:46 am ]
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I think you struck a good balance between snugging in the tires and preserving your wishbone geometry and having room for the shocks.

Re: the shock mounting, it looks flimsy to me, but then again I often see shocks in single shear and without much chassis triangulation behind them, especially in inboard arrangements. I think you did about the best you could under the circumstances.

It would be nice if some more experienced folk could comment.

Author:  chetcpo [ August 1, 2008, 9:34 am ]
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Nice progress!

Author:  rust_bucket [ August 1, 2008, 11:44 am ]
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I agree with Pete, single shear for a shock connection is asking for trouble. Even if it never actually breaks, you're going to get plenty of flex.

I remember the day I put a front shock tower brace on the miata. It was a night-and-day difference. You wouldn't think small amounts of flex would really change the feel but it does!

(I realize a front shock tower brace and a rear shock mount aren't exactly the same, but the principle holds...)

Author:  rx7locost [ August 1, 2008, 11:56 am ]
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Thanks for your comments. I do plan on adding a 2nd brace for the upper coilover mount, eliminating the single shear condition. It will be forward of the one shown and extend upward and meet the frame somewhere around the original CMC roll bar mount. Currently for the purposes of getting geometries worked out, I have only the one upper mount in single shear. The lower mount is also temporary and will be considerably beefed up. The photos are intended to show the geometries I have worked out and not necessarily the final construction. In the end, the only connection in single shear will be the upper mount on the upright. This cannot be avoided unless I weld on a chunk of aluminum to the upright.

It would have gone much easier if i started from scratch and wasn't trying to use the Rx7 uprights and the coilovers I got with the unfinished project. The Miata uprights look much easier to work with.

You know..... hindsight....

Author:  Pete B [ August 1, 2008, 12:28 pm ]
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The brace sounds great. Sorry I missed that before!

Author:  rx7locost [ August 1, 2008, 7:31 pm ]
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It is now on all four. This part of the build is now complete. Still a few things to go back over but it feels great to get to this point. It rolls, it bounces, it steers!

In major building blocks, what is the best next recommended step. Rear axle shortening? Scuttle? Brake pedals? Other?

Is anybody using the quick release steering wheel adapters found on ebay? The ones with the ball bearing lock? Are they any good?

One more question about the Rx7 hubs, are the bearings normally loose and become tight when the axle is installed and tightened down? There is a lot of wheel motion from that area but I don't have the axles in place yet. Or do I need to replace the wheel bearings?

Author:  rx7locost [ August 4, 2008, 5:53 pm ]
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I found the answer to my question about the rear wheel bearing slop. The Rx7 uses a split inner race. When the axle is tightened the inner pieces are in contact with each other. When there is no axle present, the inner races are allowed to separate giving a lot of slop. I cut my axles and installed the wheel ends. When I tightened the hub nut, all the slop disappeared.

Author:  Rotus7 [ August 6, 2008, 8:38 pm ]
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Wow you are really getting on with it. I'm a bit of a rotary freak. I have an 1982 Rx7 and a 1990 Rx7, also enough parts to build an S4 13B engine in the basement, and a running 12A bridgeport under the bench in the garage. An extra S4 gearbox and complete rear end from a 1st Gen Rx7. Just need to put together a Locost chassis. The problems with the 2nd Gen rear end makes me want to use the 1st gen rear end with 2nd Gen front uprights/spindles.

Your intake manifold:
maybe you could try to adapt an upper manifold from a TII Rx-7 to go on the S5 lower manifold. Reason is they sit much lower due to not looping around like a NA upper manifolds. They sit lower because above it the intercooler sits under the hood. Another advantage is then it is easy to just bolt on the TII turbo! This will also reduce the exhaust noise problem!

do you really need to keep these operational? Not sure you would notice much improvement in torque due to the Locost being so light. I hardly notice the difference when they open on my S5, although I do notice the VDI. To get the 6PI working you'll need an air pump (air horn motor?) of some sort to turn on at around 3700 rpm.

Height of your engine:
How many inchs does the sump hang below the chassis frame? Looks like you could lower the engine an inch.

Keep up the good work. I hope to be starting my build this time next year.

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