IMSA GTU vintage racer build

[AydinZ71]

That is a cake job on a lathe. Since it is not rotating, you can get away with a mill  

 

Start with round stock. A fly mill could do the job if you vary the radius of the cutting head several times. Center hole can be done in a drill press. Thinking of 6061 AL? It machines easier than any other cost effective billet in my opinion. Steel is cheaper but you have to keep it constantly lubricated, which stinks up the place. Also heavy. 
 

eastwood sells a wicked mini-lathe for $750. Might find a used one around for half as much! 
 

let me Know if I can help! I used to design and draft fabrication drawings when I was a junior engineer. Spent hours working with our machinist to optimize designs for cost effective fabrication. 

[clarkspeed]

Thanks for the offer. Yes I plan on getting a small lathe maybe later this year. I am forever needing a special bushing or spacer. I just got my home built CNC mill running and it seems to cut 6061 aluminum very well. This thing is so cool and opens up another world for me. I will cut these top hats as one of my first projects on it. Check out The MPCNC website from V1 Engineering.

[clarkspeed]

Next up is the floor and interior structure.  I never noticed before until I started building this thing that the floor bottom on a 240Z is actually lower than the rocker panels.  This car only came with rocker panels, nothing more.  When I got ready to try and locate driver stuff I quickly realized that I couldn't just run a flat panel across the bottom.  Turns out I needed to drop the floor roughly 1-1/2".  Again all measured to the FSM datum points.  So that meant building a frame around the tub.  I used fairly thin square tubing since I already had plenty of structure above it. Again you can see the square tubing running fore and aft that represents where the bottom of the floor should be in relation to the chassis points.  

[AydinZ71]

Looks good!! Does your class require the outside dimensions of the car to be OEM? If I was building something nearly from scratch as you are, I would raise the floor bottom by a 3/4". After the exhaust and crossmembers, the floor pan support rail is the next interference point for lowering a car in my experience on the street car. Maybe I would mock it up and change my mind. Fun to kick around the idea. 

 

Another thought I had recently was to replace the heavy crossmember that runs behind your head. Its what has your rear hatch mounting points. it seems to me the method of stacking stamped sheet here made it heavier than it needs to be. I would replace it with a tubular crossmember and rebuild the hatch mounts. In EP we are not restricted on cage mounting points, so I would use the nearby cage (overhead loop) as the reinforcement. eliminating weight at this height will help lower the CR quite a bit.

[clarkspeed]

I am pushing the CG down as low as I can go. Exhaust exits side so no problem there. Flat bottom floor all the way. I think it will work ok.

 

I have been staring at that damn cross member for months. Started to cut it out couple weeks ago and declined. Do you think it weighs enough to make a difference? Looking closely at it, it didnt look like that much metal.

 

I am also staring at the drip rails. 

[AydinZ71]

11 minutes ago, clarkspeed said:

I am pushing the CG down as low as I can go. Exhaust exits side so no problem there. Flat bottom floor all the way. I think it will work ok.

 

I have been staring at that damn cross member for months. Started to cut it out couple weeks ago and declined. Do you think it weighs enough to make a difference? Looking closely at it, it didnt look like that much metal.

 

I am also staring at the drip rails. 

It looks heavy, but good point on whether it’s heavy enough to make a difference I bet you could shave out at least a pound or two if you replaced it with a tubular crossmember. You could always cut it out cleanly, weight it, and weld it back in if you change your mind. It’s not like your welds will be any weaker than the sporadic resistance spot welds 
 

Funny thing, the drip rails double as resistance to deflection in the vertical plane. Still, with the cage in there you already have so much added rigidity, I bet you can cut them out and seam weld cross. If I remember right, the drip rail is two 20-22 gauge sheets spot welded together so adding a weld along the length will be important if you cut the spot welds out. 
 

Are you restricted on cage mounting points in your class? I would tie the body to the cage in more places than the standard 8 if you could. For example, adding a rib along the windshield sill to the cage risers nearest the windshield. If you eliminate a large amount of independent motion between the tub and your cage, it will make a HUGE impact on tub rigidity. I still need to explore the idea of how rigid is too rigid. I suppose the number one sign of a body that is too tight would be unmanageable vibration. Steel flexing even in minute amounts will limit how bad vibration will be if you near the natural frequency of the structure. You might notice at one specific RPM the whole car starts feeling like a tuning fork LOL. 

[clarkspeed]

The car is mostly cage, it has very little rigidity left in the tub. Between the rust and everything I have already cut out, I am more worried about panels wrinkling up than providing any structure.  That is one of the reasons I have not cut it out yet. The front pillars are already tied to the cage, and it is connected to the rear main hoop above the side window.

Vibration is not normally an issue on a car car. You tend to rev right through it.

[AydinZ71]

31 minutes ago, clarkspeed said:

The car is mostly cage, it has very little rigidity left in the tub. Between the rust and everything I have already cut out, I am more worried about panels wrinkling up than providing any structure.  That is one of the reasons I have not cut it out yet. The front pillars are already tied to the cage, and it is connected to the rear main hoop above the side window.

Vibration is not normally an issue on a car car. You tend to rev right through it.

yeah good point on the vibration. I’m overthinking it of course, but that’s half the fun for me 😂

 

I am also getting some wrinkling on OEM 20-22 gauge steel from the heat of adding reinforcement and rebuilding rusted surfaces. Not too worried about it since none of them are outward facing (I’m much more careful on body panels). 

 

Your sweet steering column, what length did you settle on? I’m looking into that now. Greg runs OEM, but he is already down to the weight limit so not much incentive to change. He just bought someone else’s project for vintage racing. Another 240z. 

[clarkspeed]

Not sure on the length, I can measure it next time I am at the shop. It is a little on the long side because I need to collapse it quite a bit when installed so it only leaves a couple inches of "crush" zone. I will also try to get an estimate of the weight. It is a solid steel rod.

Greg's EP car is refined to a point. He spent probably 20 years developing it. He doesn't make any more major changes to it and uses a similar formula to build other cars for himself and customers. However it is surprising how much OEM stuff he uses and simple common off the shelf parts. 

[AydinZ71]

7 hours ago, clarkspeed said:

Not sure on the length, I can measure it next time I am at the shop. It is a little on the long side because I need to collapse it quite a bit when installed so it only leaves a couple inches of "crush" zone. I will also try to get an estimate of the weight. It is a solid steel rod.

Greg's EP car is refined to a point. He spent probably 20 years developing it. He doesn't make any more major changes to it and uses a similar formula to build other cars for himself and customers. However it is surprising how much OEM stuff he uses and simple common off the shelf parts. 

Thanks Clark! The length just helps since that is one of the few variables they are sold in. I could measure myself, and then il have to factor in the adjustability. 
 

I really appreciate Greg’s experience! It’s refreshing that he has a “tried it, wasn’t worth it” approach to A/M parts. I know if I ask him a question, the answer will be based on his actual first-hand track experience. Can’t top that. 
 

Have you looked into the Honda K-series motors at all? I have seen quite a few people migrating from the L to an RB, JZ, or LS for racing applications. I have seen a few SR’s but I believe the Honda k’s are lighter and post impressive numbers for an NA. 

[clarkspeed]

Maybe this gives a little sample of just how rusty this tub was.  These are some of my repairs along with patches over previous repairs.  Please don't comment on some of the poor welding.  Many of the welds were not clean on the backside and these are not structural.   If you notice, I cut out the entire storage box area.  I have the weight savings written down somewhere if I can find it.

[clarkspeed]

And here is the driver's door.  Car came with the door fully cut out and it was floppy.  I have seen these wrinkle up on other race cars and God forbid if someone accidentally leans on it.  Anyway the cage sits very close to the skin, so I got creative with a some bent sheet metal and 3/16 rod.  It is as rigid as a complete door now.  The upper hinge was milled out, but I ended up blasting out the lower hinge with my $400 plasma cutter.

 

I added a 3D printed inside door handle.  I can sell these if anyone is interested.

[AydinZ71]

Looking good!! My door was dissected like your by the perilous owner. I need to add reinforcement as well. Did you ruin the outside finish of the door, or were you able to weld the rods in a spot that kept the paint cool? 
 

On the sheet metal welding, I dunno man. I’m convinced you can’t make a weld on 20-gauge or thinner metal look good on a first pass with a MiG. It doesn’t seem possible. You also have to wait several minutes to let the sheet cool between stitches, so you don’t warp it all to hell. When the metal cools off, your first several snaps are going to be loud again. On a MiG, You are either burning through at high heat and get a flat puddle, or you are on low heat and making a Pearl necklace. 18-gauge plus, no problem! You can make a MiG weld look almost as good as a tig with thicker sheet metal and the right settings. Here is my hand at 20 gauge. The blobs are burn-through I repairs. No warping, and I managed to get a curved contour from flat sheet. 
 

You going with 2 component epoxy sealer for the primer? 

[grannyknot]

19 hours ago, AydinZ71 said:

 

On the sheet metal welding, I dunno man. I’m convinced you can’t make a weld on 20-gauge or thinner metal look good on a first pass with a MiG. It doesn’t seem possible. You also have to wait several minutes to let the sheet cool between stitches, so you don’t warp it all to hell.

I agree if you are using steel wire but this last winter I had some good success Silicon/Bronze Mig wire, melts at a much lower temp and is plenty strong.  On 18ga the parent metal will always break first in a bending test, it's also good the have compressed air there to cool it. I wasn't able to run a bead with it but spot/butt welds and jumping around on the panel until all the spots were joined up kept things very flat, really easy to grind as well.

 

[AydinZ71]

3 hours ago, grannyknot said:

I agree if you are using steel wire but this last winter I had some good success Silicon/Bronze Mig wire, melts at a much lower temp and is plenty strong.  On 18ga the parent metal will always break first in a bending test, it's also good the have compressed air there to cool it. I wasn't able to run a bead with it but spot/butt welds and jumping around on the panel until all the spots were joined up kept things very flat, really easy to grind as well.

 

 

Hey il give that a shot! Never tried it.

 

I don't mind the ugly welds on thin metal to be honest. I just grind it down later, and none of the work I'm currently doing is on the outside body. @Invincibleextremes also gave me a pointer not to grind the weld seam down entirely if its not intended to be an aesthetic piece. After some practice, I agree. Once I grind the weld seem so its its even (no more hills and valleys), I now leave 0.5-1mm of "proud" metal. I have noticed it adds additional rigidity to super thin sheet, and keeps you from having that " steel drum" or "tin can" sound. Its like a mini rib.   

[grannyknot]

18 hours ago, AydinZ71 said:

Hey il give that a shot! Never tried it.

Just remember that it is brazing not welding, all metal should be clean, like Tig clean, it likes to have a gap between the pieces being joined of about the thickness of the sheet. You want it blob up on the inside of the seam as well, it will take a bit to get use to it but it has some real advantages, just to find out how versatile it is I brazed together a piece of sheet metal to a piece of aluminum and stainless steel to that, certainly not something that you would use often but I was impressed.

Also, you can use 75Argon/25% Co2 but pure Argon gives you a better result.

[Ironhead]

On 5/25/2021 at 8:36 PM, AydinZ71 said:

Looking good!! My door was dissected like your by the perilous owner. I need to add reinforcement as well. Did you ruin the outside finish of the door, or were you able to weld the rods in a spot that kept the paint cool? 
 

On the sheet metal welding, I dunno man. I’m convinced you can’t make a weld on 20-gauge or thinner metal look good on a first pass with a MiG. It doesn’t seem possible. You also have to wait several minutes to let the sheet cool between stitches, so you don’t warp it all to hell. When the metal cools off, your first several snaps are going to be loud again. On a MiG, You are either burning through at high heat and get a flat puddle, or you are on low heat and making a Pearl necklace. 18-gauge plus, no problem! You can make a MiG weld look almost as good as a tig with thicker sheet metal and the right settings. Here is my hand at 20 gauge. The blobs are burn-through I repairs. No warping, and I managed to get a curved contour from flat sheet. 
 

You going with 2 component epoxy sealer for the primer? 

 

 

I don't pretend to be an expert, but I experimented a ton with this on my build....

 

What worked best for me:

 

When butt welding panels with MIG, make sure you have a small gap...like 1/32" or so...between the pieces.  If you don't do this, the weld will remain on the surface of the two panels, and when you grind it flush you will have poor fusion and a million pinholes.

 

Use .023 or .024" wire.

 

Go hotter (like 15-20% over what the machine recommends) rather than colder, but make the weld extremely quick....almost as quick as you can tap the MIG gun trigger.  If you are watching/guiding the puddle, you are going WAY too slow.  If you time it right you get good fusion, the weld flattens out well, but nothing has enough time to burn away.

 

Once you have adequate tacks to hold everything in alignment, continue the weld by building on the existing tacks.  This makes it much easier.

 

And...I agree with you....impossible to make it look super pretty.  But most of the time you will be grinding it flush anyway.  After MIG welding a lot of Datsun sheet metal, at least your other MIG ventures will seem a breeze in comparison...

 

For me, it was using TIG on the Datsun panels that I found totally impossible.  You need 100% perfect fit, and even then, TIG by nature is a slow process and I found controlling the heat adequately to be extremely difficult.  I know many experienced welders recommend TIG for butt-welding body panels, but I think they are mostly talking about 18 gauge stuff.  I would like to see them try it on a Datsun.  I mean, I know it can be done, but it requires exhausting focus and concentration.

Edited May 31, 2021 by Ironhead

[AydinZ71]

17 hours ago, Ironhead said:

 

 

I don't pretend to be an expert, but I experimented a ton with this on my build....

 

What worked best for me:

 

When butt welding panels with MIG, make sure you have a small gap...like 1/32" or so...between the pieces.  If you don't do this, the weld will remain on the surface of the two panels, and when you grind it flush you will have poor fusion and a million pinholes.

 

Use .023 or .024" wire.

 

Go hotter (like 15-20% over what the machine recommends) rather than colder, but make the weld extremely quick....almost as quick as you can tap the MIG gun trigger.  If you are watching/guiding the puddle, you are going WAY too slow.  If you time it right you get good fusion, the weld flattens out well, but nothing has enough time to burn away.

 

Once you have adequate tacks to hold everything in alignment, continue the weld by building on the existing tacks.  This makes it much easier.

 

And...I agree with you....impossible to make it look super pretty.  But most of the time you will be grinding it flush anyway.  After MIG welding a lot of Datsun sheet metal, at least your other MIG ventures will seem a breeze in comparison...

 

For me, it was using TIG on the Datsun panels that I found totally impossible.  You need 100% perfect fit, and even then, TIG by nature is a slow process and I found controlling the heat adequately to be extremely difficult.  I know many experienced welders recommend TIG for butt-welding body panels, but I think they are mostly talking about 18 gauge stuff.  I would like to see them try it on a Datsun.  I mean, I know it can be done, but it requires exhausting focus and concentration.

I agree with everything you have said  I tend to go even bigger on the gap, 1/16. For me, this keeps the butt weld from binding as it expands. There is usually a “pinch point” where the expansion can bind and will sometimes need the cut-off wheel to break loose. The first few tack welds can be tough as they tend to “pull” the metal closer together. 
 

You see criticisms from time to time that makes you second guess your method, but after having this much practice I realize it’s just the nature of 20-gauge and rotsun metal. It would be interesting to understand what the quality of their steel actually is (analysis), because it certainly welds much more poorly then fresh modern cold-rolled mild steel. Lots of slag, off-gasing, splatter, and even the finished weld tends to be “duller” than virgin stock. Not that any of this mattered to Nissan since it was mostly resistance spot welded. 

[JMortensen]

Leaving a gap on super thin sheet sounds like a recipe for blowing holes to me.

I butt them up tight and then use low heat and let the weld pool sit there a little longer. Usually get a little bit of protrusion on the backside, so I have no fears about penetration. The brake duct might not be a perfect comparison as the tube is a little thicker, but in the second one you can see the side that was welded (around the tube) and the backside where I welded a stiffening flange around the edge of the backing plate.

[AydinZ71]

@JMortensen looks good! I need to make one of those too. I am going to shop around for a spare (front and back) so I can fiddle with them on the bench. 

I don’t think there is anything wrong with not leaving a gap if your temp is high enough. You should get a nice Saturn shaped puddle right on the seam. My biggest issue with no gap is if the piece is bound on all sides, it tends to warp more since there is nowhere for the hot metal to expand. 

 

I suppose it’s like cooking. Something different works for everyone. I get a much flatter, even puddle when I leave a small gap. It’s all in the wrist movement and the duty cycle for me. I do Fraction of a second. Usually only burn through on OEM material where the material is so thin from corrosion. I swear it’s thinner than 20 gauge. I can fix holes pretty quickly though. Just build up the material around the hole, then fill-in.

 

here is a vid of my technique, and a pic of how I fill gaps that are too small to create a sandwich piece for l. Certainly still a novice!