LocostUSA.com

Maybe this isn’t entirely relevant to Locosts but I’ll post it anyway. A guy wanted me to design a cradle to adapt a Gen 5 Camaro IRS to a rat rod. I came up with this concept – all the dimensions can be adjusted to suit whatever configuration he comes up with. I talked him into adjustable suspension so he can figure what drives best for his car. This is basically just a chunk of iron to get him started with ideas for his car. I still need dimensions for the pumpkin and I haven’t run strength numbers yet. It needs a diet also.

I also sold him on the inboard spring concept.

Of course I had to try to stuff this thing into the Generic Locost. With some finessing, it might work. You can get used Gen IV Camaro IRS assemblies for around a grand. They seem to be getting more common as people wreck their Camaros or upgrade their rear end. The hubs are 5 - m14x120mm, Wheel/Hat Pilot Dia. 2.63”/2.65”.

Bobber,
thanks so much for posting this!
I purchased a complete Camaro rear subframe assembly a while back to use in my locost project. what you posted is pretty much what i wanted to build/

I’m going to finish the design for the steel cradle for the rat rod guy and then adapt it to an aluminum design (I used to design marine stuff). My goal is to get the steel cradle down to 50 lbs (he likes a lot of lightening holes). Maybe I can get an aluminum cradle down to 30 lbs? I revised the front differential mount on the following sketch so it doesn’t protrude into the driver’s back, but I need pumpkin dimensions to compete. Would you have the dimensions on the attached sketch?

The dimensions of the cradle can be varied as you want. I made this one higher than necessary (12.75”) to fit his car. Let me know if I can draw it up to suit what you got. For reference, I’m attaching a drawing of the suspension geometry that this is based on. It’s based on custom uprights for now.

I will get the dimensions for you

John (Mongo)

Thanks, John.

I've got a preliminary design for the differential nose mount that I'll post later. This thing might work in a Locost after all.
How would you bolt a cradle to the tube frame of a locost? The differential mounting loads are up close to 20,000 lbs! But that's what happens when a car does a wheelie. Maybe I'm too conservative, but doesn't everybody lift their front wheels on the track?

For the mathies: 3000 lb car with 8 ft wheel base, wheel stand torque = 3000 lb x 4 ft = 12000 ft lb around rear axle. Distance between front and rear mounts on differential = 8" = 0.67 ft
Load on mount = 12000 ft lb / 0.67 ft = 18000 lb on each mount, crudely figured.

Here’s the Gen IV IRS cradle with the front mounts modified to fit a Locost without protruding into the cockpit as much. I tried to keep all the support directly below the differential mounting ears to keep it out of the way. I took an educated guess at the pumpkin dimensions and can adjust everything when I get more accurate measurements.

The upper suspension mounts still protrude into the cockpit and these can be modified after deciding on the upper control arm layout vs springs, etc. The suspension mounts can be moved anywhere along the tube (the coilover mount, as well).

The cradle still needs a diet. I think the 3x3x3/16 tubes are too heavy but I have to see how this thing will mount in a chassis before I downsize them. I understand the Camaro differential is 140 lb?


The file size is too big! I will trim and resubmit.

Here's the file.

Here's a detail for an Art Morrison Camaro IRS subframe. This would be ideal for installing in a Locost using spools like on the Exo-Hotrod posts. But this thing is very expensive.

This type of thing is appropriate for people modifying large street cars, but I'm not sure it's the right path for our smaller and lighter cars. I know that's what you designed it for.

When you get to out size cars there is no use for a 130 lb. diff., a unit from a Explorer weighs 75 lbs. and I can't imagine how you could possibly break one of those in our cars.

Finding ways to integrate an IRS diff with the chassis is what makes the most sense. The forward mounts of the diff are between the passengers and the rear mounts are only a few inches behind the bulkhead. In addition to this the rear mounts for the wishbones are difficult to provide, so it seems to make sense to use trailing arms ( or maybe leading arms? ) and then a lower wishbone with only a single inner mount and an upper radius arm are needed.

This was a frustrating area, it's difficult to do all these things. There was at least one Seven built at the factory with the setup I am describing so I think that lends additional support to this direction.

_________________
Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
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Actually I was thinking of this more as a joke. I’m afraid some rat rod attitude is rubbing off on me.

I mostly agree with everything you said. I can see the difficulty of installing an independent rear end in a Locost, which just wasn’t designed for one. A CHALLENGE!! So here’s a shot at designing a cradle. More practically I want to build on what Omaha Corvette did (thank you OC) and try to bring his spool design to a more typical Locost scale.

The attached sketch is just a concept for adapting any IRS differential to a tube frame using spools similar to those used by Omaha Corvette. I’m using square tube for the outer tube of the spool for easier welding. These spools are a nice way to tie hard connecting points to tube frames so I’m trying to use them for mounting an IRS differential. I’m using a cradle as a means of more easily (?) getting forces from the differential mounting ears to the tube frame nodes. I chose the Camaro diff since I’m designing a Camaro cradle for rat rod guy, and, why not? Right now this cradle is little more than a chunk of aluminum with differential mounting ears attached. It could be greatly improved but at this point, I have convinced myself that it is possible. This design has no suspension pick-ups but that could be the next step. I also need to get more accurate dimensions for the Camaro pumpkin.

Rat rod guy says he wants to use a modified fiberglass body for his rat rod. While he decides, I thought I would see how a Locost would mate to one.

John, can you get those differential dimensions? For a locost, I'm finding Horizenjob may be right in the advantage of tieing the pumpkin mounting ears directly to the frame pieces. If you can get me the dimensions, I have an idea to do this.
Meanwhile, rat rod guy found a piece of 15 inch deep x 3 inches wide aluminum channel that I'm trying to make into a cradle for him that will also carry his lower control arm inner mounts.

Here's a picture of a Lotus 7 with IRS. It's not the picture I was thinking of which I have saved and can't find right now - it seems my laptop is having filesystem problems or something

I will try to find the other picture after I get my laptop back from being checked out.

This cart is referred to as a Lotus 720 or 7/20 or I have also seen 7 1/2...

_________________
Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.