maschinenbau UltraDork
5/6/22 11:23 a.m.

Got that low-profile engine mount figured out. Here's a nice start-finish for fabricating it:

With that taken car of, I can put the frame in "welding service postilion A"

See also, "Welding service position B"

I am taking care to add plenty of 1/4-20 weld nuts for all the various lines and hoses, cap the open tubes, gusset random things, drill drainage holes, etc. 

nocones PowerDork
5/6/22 12:53 p.m.

In reply to maschinenbau :

Oh I see you are using Speed Nuts.   The MG is 100% speed, but for the 360 I chose to please Carroll Smith and use all fine so I could pretend my GR5 is just like AN.  


Progress looks great as usual

Mr_Asa PowerDork
5/6/22 1:19 p.m.

I like the idea of tape to transfer holes.  I think I've done something similar but am not 100% sure.

maschinenbau UltraDork
5/6/22 1:44 p.m.

In reply to nocones :

Thanks. Not sure we're speaking the same language, but to me these are Speed Nuts:

And these are weld nuts (what I'm using). I put them in with 4 tacks welds.

gumby Dork
5/6/22 1:52 p.m.

In reply to maschinenbau :

1/4-20 is fast thread

nocones PowerDork
5/6/22 2:10 p.m.

Around 2000 Carroll Smith would always give FSAE teams a hard time if they used "speed nuts" or "fast thread" by using anything coarser then UNF.  I've heard it from other people not on my FSAE team so I had assumed it was more universal. 

maschinenbau UltraDork
5/6/22 2:54 p.m.

Ah, gotcha. I didn't do FSAE so I'm not a fine thread purist. Even worse, I've been mixing and matching fine and coarse, metric and English devil That engine mount is M10x1.25 into the head and M10x1.5 through the dogbone.

Besides, speed nuts make you faster right? I mean it's right there in the name! 

maschinenbau UltraDork
5/11/22 9:00 a.m.

More frame welding, reinforcing, boxing in joints, stuff like that. Here's the forward subframe mount, reusing sheet from the original Lotus frame rails.

Also added new seatbelt mounting points, since the originals were pretty rusty.

Almost ready for primer, but just a few issues to sort out up front. The big one is brakes. I intend to use the Toyota MC and booster, since my Lotus didn't have those. I have the original Lotus pedals, but they were heavily rusted. I think I can save them. The problem here is the pedals are bottom-hinged. To clear under the steering rack, the Lotus MC was mounted as low as possible. This means adding a booster has two problems: any added diameter protrudes below the floor, and even if it didn't, it would hit the steering rack. Lotus got around this by using something called a "remote mounted brake booster" or sometimes called a "remote brake servo" which is essentially a 2nd MC driven by the 1st one at the pedals, which then drives a 3rd MC using a normal vacuum booster. Interesting solution, but not for a Challenge budget. So let's see what we got here.


I plan to build a bellcrank hidden inside the frame box to allow the Toyota booster to mount higher. I can cut a window in the top of the "T" of the frame, where the drop-down hinge and lever will be bolted in. Another solution would have been flipping the pedals to hang-down mounting, but then the pedal hinge would be a few inches above the frame so it would have to mount to fiberglass or require some serious structural mods. I think I need a roughly 4:1 pedal ratio for this booster. I will re-drill the brake pedal for 2:1, which will drive a lever that also has 2:1 advantage, making it 4:1 at the MC. This is very similar to aftermarket retrofit kits for converting classic cars from manual to power brakes.

Any brake experts in the room may also be thinking "But will this even work? He's got stock Lotus brakes up front and Toyota rear brakes out back, driven by a Toyota MC. What about pedal effort and bias?"

The short answer is I think I got lucky with this combo of parts. The Toyota MC is a tandem 7/8". Booster assist - 4:1 probably? Front Lotus calipers are opposing-piston 48mm on 9.1" rotors. Rear calipers are sliding-mount 1.5" pistons but with 10.6" rotors. I plugged it all into this online calculator (link below) with some hand-wavy assumptions about friction, weight, booster assist, etc, and the target optimal MC size for front and rear is pretty darn close to 7/8". Feel free to check my work as usual.

SV reX
SV reX MegaDork
5/11/22 9:13 a.m.

Last time I checked, 0.875 was not "pretty darn close to 7/8". wink

maschinenbau UltraDork
5/11/22 9:20 a.m.

In reply to SV reX :

Ah, that screen shot was misleading. The calculation linked above gave 0.92" and .76" optimal MC sizes front and rear respectively. This all assumes a hopefully worst-case 70% rear weight bias. It's closer to 7/8" for both front/rear if you make the weight distribution more even.

dherr (Forum Supporter)
dherr (Forum Supporter) Dork
5/11/22 9:34 a.m.

Perhaps he was covering for "British quality" in the Lotus side of the equation as I would bet that the Toyota specs are spot on!

AngryCorvair (Forum Supporter)
AngryCorvair (Forum Supporter) MegaDork
5/11/22 11:15 a.m.

In reply to maschinenbau :

Front caliper: how many pistons pushing on each face of the rotor? You only count the pistons on one face when calculating effective piston area. 

maschinenbau UltraDork
5/11/22 11:47 a.m.

In reply to AngryCorvair (Forum Supporter) :

Front caliper has 1 piston on each side of the rotor. So my front caliper area is half what I thought? Hmmm...this might change things. Now my ideal MC bores are 0.65 and 0.76. If I play with pedal ratio and pedal effort, I can get 0.82/.96 which straddles my 0.875 MC. So my brakes will be biased rear, which might be okay given how much more rear tire and weight I will have. I definitely welcome more input and advice.

Updated brake power calculation:

AngryCorvair (Forum Supporter)
AngryCorvair (Forum Supporter) MegaDork
5/11/22 11:55 a.m.

In reply to maschinenbau :

correct.  i'll take a look at your calculator later today/tonight.

Snowdoggie (Forum Supporter)
Snowdoggie (Forum Supporter) SuperDork
5/11/22 12:12 p.m.

Wow. Watching and learning a lot from this thread.

barefootcyborg5000 PowerDork
5/15/22 3:26 a.m.

Great work as usual. Also, fabricating in flip flops has my approval. 

maschinenbau UltraDork
5/15/22 6:17 p.m.

Pedals: restored. 

Interesting that the brake pedal uses a bronze bearing insert, but the clutch is just a tube over a shaft. I was able to source an oilite replacement bearing. And also make new mounting brackets.


Feels much better now. Significantly less crunchy, and should stay that way since I added grease fittings. I may run a few passes over the clutch pedal where it's very rust-pitted, but it's stronger than it looks.

I also sourced a radiator. I tried to make the Toyota one work, but it's simply too tall to fit up front. I got a tip in another thread about Jeep XJ radiators being unusally short and wide - perfect! The pre-'91 Cherokees use a remote cap on a reservoir, while the later ones have the cap on the radiator itself. I opted for earlier, so I can put the cap at a higher point. It's not a racecar without Jeep parts!

Cleaned up decently, and doesn't seem to obviously leak.

Thanks to off-roaders, there is a huge aftermarket for upgraded all-aluminum 3-row radiators and shrouded electric cooling fans in case I need more capacity later. So I feel good about fabricating a permanent mounting point for this. Probably a fiberglass bulkhead and duct to control the airflow. I'll worry about that later.

Brotus7 Dork
5/15/22 6:26 p.m.

That radiator fits surprisingly well!  It should even clear the hood when it's opened. I looked for a while when shopping for a rad for my car and ended up with an aluminum Scirocco rad that needs the outlet moved.  It tucked under the nose, but I think your solution is simpler.  Keep at it!

AngryCorvair (Forum Supporter)
AngryCorvair (Forum Supporter) MegaDork
5/15/22 8:00 p.m.
AngryCorvair (Forum Supporter) said:

In reply to maschinenbau :

correct.  i'll take a look at your calculator later today/tonight.

I checked the numbers and they look correct. I think your booster ratio might be a little low, and your pedal ratio seems a bit high, but they're both within reason.

AngryCorvair (Forum Supporter)
AngryCorvair (Forum Supporter) MegaDork
5/16/22 7:05 p.m.

In reply to AngryCorvair (Forum Supporter) :

Yeah, well *i* checked the numbers and they're berkeleyed. I've never seen that "sliding multiplier" before and it threw me off. Your per-caliper numbers of 5.61 and 3.53 are correct, so you should enter 11.22 and 7.06 as the per-axle numbers in the brakepower calculator. Quick sanity check: 50% more front piston area, 25% less front rotor diameter, and 100% more rear weight should result in less front pressure required to lock.

Rigante Reader
5/17/22 8:14 a.m.

that XJ radiator is a great find

¯\_(ツ)_/¯ UltimaDork
5/17/22 4:24 p.m.

Wow, poking around I see some almost comically cheap aluminum versions of that radiator.  Very interested in how well it ends up cooling a 300hp V6 in a performance environment.

Recon1342 SuperDork
5/17/22 10:36 p.m.

In reply to ¯\_(ツ)_/¯ :

Well, since they're ostensibly designed to help cool a 4.0 liter cast iron lump at crawling speeds, I reckon it'll be pretty good at anything over 5 mph...


ETA: Angry arguing with himself over brake system specs may be the most GRM thing in this thread so far...

dannyzabolotny Reader
5/18/22 2:34 a.m.

Just read through this whole thread, I tip my hat to you, sir. The amount of math/engineering going into this is way over my head so I'm just here for the pics. Normally I think I'm pretty smart since I work on BMW's for a living, but when I read the crazy threads on here I'm just in awe. Can't wait to see this whole thing driving, seems like it'll be a total riot with that much power in a car that weighs so little.

maschinenbau UltraDork
5/18/22 8:43 a.m.

Thank you AC for the brake help! I am feeling better about the parts I hope to use. 

Here's a fun picture stolen from the Europa faceplace group. It's hard to visualize just how small this car really is until it's parked behind a Honda Fit...

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