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Login to post Forums » Off-topic discussion » Locost transmission tunnel: WW-GRM-D

May 30, 2011 7:47 p.m. JoeyM SuperDork

Short version:
This is another one of those times when I'm asking, "What would GRMers do?" When building the front half of the transmission tunnel, would you guys
a) painstakingly fabricate something that closely follows the contours of the transmission
b) make something that's easy to fabricate, effective, but bulky/ugly in appearance.

Long version:
Yesterday I built the frame for the section of tunnel that will cover the drive shaft in my datsun replica.

I was hoping to fabricate the front of the transmission tunnel today, but frankly was a bit intimidated/stumped by how to follow the shape of the transmission. The idea I came up with late in the day was to not bother following the transmission contours. Instead, I could run tubes from the rear tunnel to the firewall braces like this to build a frame that looks something like this

Pros:
Simple to fabricate
probably simplifies mounting the gear selector (yes, I'm using a slushbox)

Cons:
Wastes space in the already-cramped interior
Ugly/kludgey because it doesn't closely follow the shape of the transmission.

Would you guys do that, or would you build something a little more complex an less bulky?
May 30, 2011 7:59 p.m. davidjs Reader

Not sure how much that adds to the stiffness of the chassis, but any bends you put in to more closely follow the transmission would mess up load transfer... (or require lots of kludgy bracing)
May 30, 2011 8:47 p.m. mad_machine SuperDork

I think this calls for a parts car. Something you can cut a trans tunnel out of and massage to fit
May 30, 2011 8:54 p.m. turbojunker HalfDork

Use a wheelbarrow. (not my idea)

http://chadcopeland.com/2009/06/transmission-tunnel-taking-shape/
May 30, 2011 8:58 p.m. fasted58 Reader

30 gal grease drum might be close
May 30, 2011 9:13 p.m. JoeyM SuperDork

mad_machine wrote:

I think this calls for a parts car. Something you can cut a trans tunnel out of and massage to fit

If use a sheet metal tunnel, there's no need to co-opt/modify/massage something to fit; I could just cut the tunnel out of the datsun station wagon that i took the drive train out of. I don't know if sheet metal tunnel without bracing will be strong enough, though. There's really not much to my frame ....just enough to support the sheet metal

I'm guessing that the frame will be too flimsy without a tube-based tunnel. I was trying to emulate a locost-style one but have it end at the middle of the firewall instead of all the way at the top.
May 31, 2011 12:12 a.m. mad_machine SuperDork

you never know. I know on the fiat 124 spider.. most of the car's strength is in the trans tunnel. You can actually open and close the doors on a spider that has had both sills eaten away by rust as long as the trans tunnel is intact
May 31, 2011 12:18 a.m. Wally SuperDork

I'm not an engineer but I like what you have with the tubing, and with a steel skin welded to it it should help nicely.
May 31, 2011 12:20 a.m. Wally SuperDork

What's holding the back of the car on? it looks like there is a piece missing between the front leafspring mount and the chassis above the axle.
May 31, 2011 3:59 a.m. carguy123 SuperDork

Wally wrote:

I'm not an engineer but I like what you have with the tubing, and with a steel skin welded to it it should help nicely.

While strong, a skin welded to the tubing leaves you with no topside access. Think of how much easier your life would be if you could easily remove the interior skin to work on things.

I see nothing wrong with what you are proposing. It's definitely as strong as a Locost and if you were to fasten it with removable fasteners you could get to it to work on things later if need be.
May 31, 2011 5:53 a.m. JoeyM SuperDork

Wally wrote:

I'm not an engineer but I like what you have with the tubing, and with a steel skin welded to it it should help nicely.

This is not a stressed-skin construction. That would not be safe since all the sheet metal in the body is from second hand sources with various thicknesses. Besides, that requires fabrication acumen that's currently beyond my abilities. (I've had to learn a lot just to do this style of construction.)

I'm planning to pop rivet the skin to the tubing....the second choice would be sheet metal screws, but I'm leaning against that option because road vibration and sheet metal screws may not mix well.

Wally wrote:

What's holding the back of the car on? it looks like there is a piece missing between the front leafspring mount and the chassis above the axle.

There are only two things holding the back end on:
1) the transmission tunnel
2) some 2"x3" tubing (1/4" sidewall; i.e. VERY stiff) that doubles as a door jamb.

I used the above photo of the frame because it was the last time I'd rolled the car out of the garage to photograph the entire thing without the skin on it. The 2x3 was added since then

Here's what it currently looks like with the skin on it:
You'll notice that I reworked the doors to be more datsun-like, and I added some steel plate box the outer surface of the twin chassis tubes below the door. (thus looking like a traditional 1930s frame rail)

[BTW, I hate when you guys change your avatars. I'm looking at you Wally. Right when I figure out who everybody is, it changes ]
May 31, 2011 9:17 a.m. Don49 Reader

You're overthonking it. Your rendering with the tubes going halfway up the firewall looks good to me. If you want to make it a little more aesthetic, use round tube for your top bars and have a nice rolled edge. Just a thought. Your work looks good so far.
May 31, 2011 9:25 a.m. Keith SuperDork

The tunnel's pretty important for strength, as noted. So I'd build it for that.
May 31, 2011 9:35 a.m. nocones HalfDork

I would do what you are propsing but with about half of the lateral connectors (the short pieces) and do triangles instead.. You need Triangles for strength not boxes. they are worse to fabricate but add so much strength they are worth the hassle.

I would also connect the "trunk floor" to the top of your front leafspring mount with a piece of your rectangular tubing. I would then run a piece from the front of the trunk to the door frame. This should add GREATLY to the strength in this area. As it is now I would be very concerned about what would happen if you were to get rear ended.
May 31, 2011 7:33 p.m. JoeyM SuperDork

JoeyM wrote:

[BTW, I hate when you guys change your avatars. I'm looking at you Wally. Right when I figure out who everybody is, it changes ]

AAAAARRRRRGGGHHHHH!!!!!!
May 31, 2011 8:57 p.m. Wally SuperDork

I can't help it. For somereason I have to keep changing them. If it helps I've only changed my name I think three times, and when I do I try not to change the picture at the same time.
June 1, 2011 12:56 a.m. mad_machine SuperDork

In looking at what you have done up in tubes (and plan to do) how about continuing your tubing fowards at the same level until you get closer to the top of the trans.. then angling it upwards towards the firewall. You really do not need a LOT of clearance around the trans
June 1, 2011 5:46 a.m. JoeyM SuperDork

mad_machine wrote: how about continuing your tubing fowards at the same level until you get closer to the top of the trans.. then angling it upwards towards the firewall.

Hmmm....interesting idea, it definitely has potential. I need to ponder the fabrication details for a bit...

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