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Driven Daily Hillclimb 2016 Build

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Most of the weekend was checking and double-checking all of my math. The main hoop and diagonal braces are cut, and one is welded in. I think I'm going to make some minor adjustments to my A pillar design rather than making what I have mostly-work. This is the most critical point to get fitment spot-on for everything, since everything after this will be measured against these first 'core' pieces of tubing.




A friend had a non-running ej205 (older 2.0liter Subaru WRX engine) taking up space in his shop, and I generally like having a spare engine on-hand in case something breaks on any of my engines. I just finished pulling the intake manifold off, and decided I'd compare that engine to the engine I'm putting in the Datsun.


This is a complete valve-to-valve comparison of the two. If anyone is confused as to why I'd use a rotary in my Datsun, accept that simplicity and compactness are really the only reasons.




Horizontally opposed engines are known for keeping their mass low and short (front-to-back). As you can see, the rotary pretty much wins in every direction here. I still need to weigh both packages to get a decent reference, but I can easily carry one but the other requires two sets of hands to haul around.



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That diagonal might not leave you enough room to squeeze the back of the seat in line with the main hoop. When you're designing a cage having the diag hit the top of the hoop and not be in line with the rear stays is not ideal, but it's one of the compromises that is commonly made on the Z because of the position of the seat relative to the door opening. I'd suggest getting the seat installed and checking before you weld it all up. Easy enough to fix with it just tacked in...

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That diagonal might not leave you enough room to squeeze the back of the seat in line with the main hoop. When you're designing a cage having the diag hit the top of the hoop and not be in line with the rear stays is not ideal, but it's one of the compromises that is commonly made on the Z because of the position of the seat relative to the door opening. I'd suggest getting the seat installed and checking before you weld it all up. Easy enough to fix with it just tacked in...

I was hoping you'd weigh in on this. I can see a few problems with my current X brace design.


One is that the X will be right in the rear-view-mirror space, and I'd much rather have it a little lower.


The second is (as you mentioned) the seat placement, which I can't know until I test-fit the cockpit together. My hope is that having the main hoop reclined will give me plenty of room for everything, but I won't know for sure until I mock it up. Going from the 'standard' pickup point to the 'notch' in the ceiling seems to give me a lot more space for a seat.


I don't plan to fix the body to the chassis structure completely, so I won't do final welds until I've tacked the whole chassis together. That means I don't have to do final welds on the top of the main hoop, because I can always pop the body off to do them again. More like a circle track car, where the body panels are just hung on the tube structure. It makes all of this testing a lot easier.


I'll try to get the seat into a reasonable place tonight, and we'll just see how this all looks together.

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I reclined my hoop until it was just about touching the dome light bar across the roof. I put a small screwdriver in between the roof and the hoop on the sides to make sure it was lined up and welded it in so there is about a finger's worth of space in there on mine, and I still needed the diag moved over so that the seat slid back into the plane of the hoop, and I'm 6' tall. If you don't have that dome light bar part of the roof in there, then you might be able to lean it back a little bit farther. Without going out and looking at mine, I think the issue there is the further back you get the lower the bar needs to be, so you may make space for the seat but wind up with a main hoop that is not tall enough to save your head in the event of a rollover. The other issue I can think of off the top of my head is interference between the hoop and the wheel wells. I assume you don't have the wheel wells anymore, but the concern is that the wheels might hit the hoop. I guess it depends on the diameter of your tires.


If the X in the main hoop is for stiffening, I would suggest an alternative: just run the diag to meet the rules, then do a lower X between the rear strut towers and the bottom of the main hoop. This connects the strut towers to the strongest part of the cage. It was a real PITA on my car but should be pretty easy to weld in on yours since you can put the floor in later.

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There are safety concerns with reclining the main hoop too far. The general 'rule of thumb' is 15 degrees back, from what I've seen. That sounds like the same position I'm looking at (relative to the dome light), and it clears the wheel wells perfectly. I'd like to retain the wheelwells, if only because they're handy for reference measurements. I also like having a 'hard stop' between where I expect the tire to travel, and where the cage structure lives. Right now, the main hoop basically rests on the wheel wells, and that's perfect.


I love the screwdriver-spacer idea. That's a brilliant way to keep rattles down.


I'm very concerned about head height. I'm shorter than you by a fair amount (5'6") and my floor is around 3" lower than the stock floor (and even more, if we're talking about stock seat rails), so I could pretty much place my helmet under the door-window opening. However, I need to place the helmet above 'see-over-dash' height, so I'm going to need to maximize the helmet space I have. In the past, I've added a 10-degree bend to A pillar bars just before they meet the main hoop. That tucks the A pillar bar up into the roof area, and buys me another 3" of headroom. You can see what I'm talking about in this photo:



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Because it's Saturday, I have an update on the build! I think I've solidified most of the fender profile at this point. I'd say it looks pretty good, and is ready for some final touches and blending to the bumper.




I don't really have a way to express how much math and thinking went into the cage so far, but here are the key points:


1. It's friction-fit.

I didn't even need to tack it into place, it's so tight against the body.



2. It's symmetrical.

... and level within 5mm from any point to any other point




3. There are no visibility problems.

The diagonal X is 16" lower than the roofline, so it won't interfere with my rear-view mirror.


4. It reclines 10 degrees.

Originally I was planning on 15 degrees, as that would fit the lines of the body and rear quarter windows nicely, but FIA regulations require no more than 10 degrees. I'd like to keep my options open as to what racing I do in this car, so I'm going with the most stringent sanctioning body I know of.



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Today registration for the Mt Washington Hillclimb race opened (Climb to the Clouds 2017). This has been my life goal for the past three years, since I purchased this car.


For those who aren't already familiar with it, Mt Washington is the closest hillclimb to my home (around two hours away). It's also twice as long as the longest hill we typically run (over 7 miles). It happens roughly once every three years, and is a pretty big deal overall.


The last time it ran I crewed for a friend and one of my racing mentors, Jamie Melhuish. It was quite an experience. In addition to learning twice as much course, we only get four practice runs. Each of those runs is only half of the course, so the first time we stitch the whole hill together, it's the first timed run. There are two timed runs. This is where a rally-style codriver reading pace notes becomes a massive benefit, but I'll be doing it solo. That means watching a lot of footage from past events, and spending a few days driving up at street-legal speeds to ensure that I actually know the course.




Right now, my registration is in, but that doesn't necessarily mean I've been invited to race. I would need to be selected from the list of applicants to be invited to race. In any event, the only way I can be ready for the event if I'm accepted is to continue as planned and assume I'll make it to the start line.


To say I'm excited is a gross understatement, like saying that an atom is "rather small."

Edited by Jesse OBrien
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To celebrate having registered, I spent the night re-mathing and re-bending the driver's apillar. Tomorrow is the passenger, and the weekend is coping and tacking, then adding rockers, windshield, and dash bar. So far, everything is 1.75" DOM.


For my own future reference, this is 20" straight, then a 47 degree bend, followed by 24" straight, 13 degree twist on a 42 degree bend, then 24" straight (which will be trimmed and coped to fit the main hoop). I may add one more slight inward bend to meet up with the main hoop. 





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Are you planning to have the floor pan lower than stock relative to the bottom of the rocker panels?  If so, why?


If I were to redo the chassis on my own car, I would do the following:


1. Relocate the frame rails outboard and adjacent to the rocker panels (instead of following the path of the stock floor supports).


2. Move the driver seat, steering column, and pedals inboard a few inches.  This will provide additional head room for the halo bar, provide more space between the driver and the cage for a side impact, and improve the left-right weight distribution.


3. Raise the floor as high as possible (even with bottom of rocker panels) provided that head height is not an issue.


I love my car, but would love it more if I had made the design decision stated above.  If I redo mine again in the future, I will do as stated.


I hate the way my frame rails hang below the rockers. 


BTW, I am 6'0 but have a long torso (I have the legs of someone who is 5'6" but the torso of someone who is 6"5".  Head clearance is an issue.


Edited by 74_5.0L_Z
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I'm planning to put the seat as low as necessary while still being able to see over the dashboard, to ensure that I have as much space in the cockpit as possible. The kevlar floor will go under the seat. I'm open to alternative opinions there.


I'm skipping frame rails altogether, and going tube frame instead. I don't like the deformation characteristics of square/rectangular tube at all, and hillclimbs almost always end up with some sort of rock or tree intruding into the floor in an 'off-road excursion'.


I was also planning to keep the seats as far inboard as possible, without having shoulder interference. The only downside I've seen from that is that ingress/egress gets more difficult, as you have to climb further to get over the door bars. I'm willing to make that sacrifice.


I'm really curious as to why you want the floor higher. No obvious benefits come to mind, but it's entirely possible that I'm missing something critical here. Now is the time to ask questions.

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My pet peeve is the frame rails hanging below the stock floor.  At ride height I have less than 2.5" of clearance (ground to frame rails).  So my frame limits how low I can set the car. 


For reference, I have about 3.5 inches of clearance between the ground and the bottom of the front cross member which is approximately in line with the floor pan. 


I autocross and run track days, so the lower the better as long as the suspension geometry is correct and nothing scrapes the track.  If the frame rails didn't hang down, I would feel comfortable lowering the car an additional inch.

If the floor, cross member, and engine were raised, then I would lower the car even further.


If I were starting with a clean slate like you, I would build everything around the desired ride height.  As with everything I've done to my car, I would do it differently if I did it again.

Edited by 74_5.0L_Z
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The plan is to have the 'floor' mount to the top of the round tube in the floor, and the aero undertray on the bottom of that same round tube.




Hillclimbs really require some extra 'slop' in ground clearance. Spring rates generally need to be a bit softer as well, to accommodate the frost heaves and overall inconsistent surface condition. As a basis of comparison, a road race Miata would run something like 450lb/in springs. My hillclimb times stopped improving when I went over 225lb/in. I had 6" of ground clearance between the bottom of the rocker and the road surface, and still found myself scraping on certain hills.


Boring Stuff

For this, I hope to be around 20% lighter, so if I keep the same spring rates I should (rough theoretical math) have around 20% less compression, and can get away with being 20% lower. That would be around 4.75" ground clearance from the ground to the aero undertray. Assuming 1.75" tubing, that'll put my floor 6.5" off the ground, which happens to line up with where the bottom line of my fender is on the chassis table (assuming the table surface is ground-level).


As a general rule of thumb, rallycross ride heights are pretty dead-on for US hillclimbs.


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I think Dan is on the right track here. Other people are trying to get the cars lower, you're making the floor lower instead. That's not to say you can't make a taller car fast. There is that 4WD V8 Z in Australia that runs really high ride heights and is very quick, but 800 hp and 4WD can overcome a little bit of weight transfer.

In general though, the lower the car the less weight transfer and roll you get, so in theory traction should be better. If you look at what Cary (tube80z) is trying to do with his build, he's gone dry sump and small bellhousing/button clutch so that the drivetrain can be lowered, and then I think he was talking about raising the floor as well to get the whole thing lower yet.

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Sorry, I'm miscommunicating what I'm planning. My seat will go where it needs to be for the driver to comfortably see over the dash and have as much headroom as possible. The 'floor' (covering the drivetrain/exhaust and giving the driver somewhere to put their feet) will be right under the seat base. That floor will be mounted to the rocker/sill tubes, and the underbody aero will be mounted to the bottom of that, where the stock floor would normally go. The weight of the chassis is all going as low as possible, without building lower than the stock floor.


Essentially, the stock 'floor' will be replaced with the flat undertray, and the stock frame rail hang-down will be eliminated entirely (because the tube structure is above the flat undertray). 

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Hahaha that's one of my least favorite corners of our whole Makerspace. The floor is crap, one of the walls has loads of leaks, there's always a breeze going through that spot, and it desperately needs a few coats of paint.


I didn't really plan to get anything done today, just clean up the shop a bit in preparation for the weekend. Man, was I wrong. I nailed the passenger side A pillar so nicely, I decided to make another driver's side replacement ... but twisted the wrong way, which resulted in another perfect passenger's side A pillar. Then I made one more driver's side, and went so far as to add a small bend on the back of it to get it to fit up with the main hoop just so.
Tomorrow, I get to break out the welder, tack it together, and start in on reinforcements.
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Finally, everything's aligned and tack welded into place. I spent most of the day fine-tuning fitment so it's symmetrical on both sides and so I have as much headroom as possible, but the results are worth it.




All told, I bent 6 'not quite right' A pillars throughout this process, so roughly 40' of material that didn't make it onto the car. It'll all get used, as door bars or similar.



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