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Learning how to build Lotus Seven replicas...together!
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PostPosted: Thu Jan 26, 2012 11:42 pm 
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The only specific direction I've seen in print with respect to build table flatness is "make it as flat as possible."

We all get the idea, but how flat is flat enough? Like most, I have a wooden build table top and wood, especially layered wood like ply, tends to have variations and voids. My best tool for determining flatness is a 72" box level. It is extruded aluminum with a ground, unpainted bottom edge. It's pretty flat, but not perfect although most likely better than any piece of RHS I'll use in the chassis.

My ply top has a very slight crown side to side, slightly more than 1/16" & certainly less than 1/8". Using the larger figure that puts the slope at less than 0.5% over 24 inches since it is about in the in the middle. I've figured out a way to get most and probably all the crown out, but I'm wondering how flat some experienced chassis builders made their own tables.

Thanks,

Lonnie

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886


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PostPosted: Fri Jan 27, 2012 12:22 am 
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Well MDF is flatter than plywood, but how flat is your base? If it's crowned then so will your wood.

MDF also burns easier and puffs up or buckles easier when it gets wet.

And then there's the impossibility to build your chassis 100% flat, true and accurate.

I'm sorry that I'm no help, but the advice of build your chassis and then attach your suspension based not upon the chassis, but upon what it needs to be comes to mind here.

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PostPosted: Fri Jan 27, 2012 1:28 am 
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Hi, I'm new to the Locost site, but have extensive experience in building Formula Ford frames which is about the same. When we build FF frames we use two steel "I" beams, about 8" tall, placed parallel to each other, and leveled as perfectly as you can get. These things are heavy, so once you have them set, they usually don't move. The reason these are used, is that when you weld thin wall tubing (0.090"), they have an awful way of bending and twisting from the heat. So we clamp the tubing down to the "I" beams and weld, and let the tubes cool as straight as possible. The "I" beam acts as a conductor of heat and cools the tubes faster, and since it is tightly clamped to the straight and level "I" beam, it tends to come out without distortion.

Now with that being said, I have noticed that many Locost builders use a MDF table of sorts, which I assume would be ok if you are just tack welding with a mig or tig, but to get the full penetration you need for a quality weld, I have to believe you will burn through the table, not to mention the toxic fumes released from the MDF material. No direspect to the thousands of Locost builders, and I fully understand the need for something inexpensive to build on, as an investment in steel "I" beams for a one off project would be too impractical. The short answer to your question is a flat as you can get it. The MDF will not offer you the stiffness you need compared to the "I" beams, but don't fool yourself. No matter how straight you build your chassis, it will tweak and twist. Soon as you install the engine, or the first pot hole you hit, or after a year of aggressive driving, they will all tend to diveate from true. All you have to do is to adjust your suspension to compensate. FF are notorious for twisting, and in a course of a season the frame can be off as much as an inch. Back in the 1970's, Lola came out with its T340 formula Ford, and it started to win all it's races. Later it was attributted to it's frame dubbed as the 5th spring. The twisting actually made the car better to drive. So I wouldn't worry too much about the flatness, just do the best you can and enjoy the ride.


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PostPosted: Fri Jan 27, 2012 7:42 am 
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Lonnie
You should try to build your frame within a 1/16" or at a min the suspension attachment points. If you already have a 1/16+ error built in the table it is going a PITA to set up the brackets to those dims. I think a sheet of MFD would be a wise investment and if you can cough up a few more bucks you can get a sheet with a white mica surface that makes it a little easier to draw the layout line of the surface.
Dave W


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PostPosted: Fri Jan 27, 2012 10:35 am 
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Do the best you can and move on. None of us (that I know of) are looking to bump the current F1 champ off his podium, so it's just not that important. Yes, flat is best but don't kill yourself over it.

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PostPosted: Fri Jan 27, 2012 11:38 am 
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+1 what KB said. Besides, the only parts that come close to being important are the pickup oints in the 4 corners. What happens in between that is practically unimportant. Even the corners may be corrected depending on the control arm pickup style you choose. By adjusting the coilovers, you can balance out a lot of imperfections of warp.

The only dimensioned chassis drawing I've got for a sports car had a 1/4" and 3/8" tolerance on the diagonal measurements (squareness). Squareness is not affected by your undulating table surface.

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PostPosted: Fri Jan 27, 2012 11:45 am 
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KB58 wrote:
Do the best you can and move on. None of us (that I know of) are looking to bump the current F1 champ off his podium, so it's just not that important.


Oh no! Now you want me to give that up too? :mrgreen:

I'm going to do the best I can with the plywood. I can improve it a little more without too much effort so, I'll go ahead with that. I'd feel badly if I didn't do the best I could. Based on what I see in other build logs, 1/16" isn't way off, but I was very curious about what others in the more experienced group of builders had done and considered to be sufficiently flat.

That said, I do like the idea of the twin beams. I've seen an example or two in the "Ultimate Build Table" thread in this sub-forum and if one was building a number of chassis that would be the way to go. As a teenager there were a variety of builders withing 5 miles of my home (Nadeau Bourgeault, Joe Huffaker, Marvin Webster and two brothers in Sausalito [name?]) and I got to see a number of race cars being built over several years by different builders. Some builders are ultra-precise and some are not. I tended to like the cars the precise guys built.

The input about the race car chassis moving throughout the racing season is a little sobering, though. That I didn't expect.

Cheers,

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886


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PostPosted: Fri Jan 27, 2012 11:50 am 
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Lonnie-S wrote:
The input about the race car chassis moving throughout the racing season is a little sobering, though. That I didn't expect.

Cheers,


It may make you feel better to know that the SR-71 changes dimensions every flight. .............

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PostPosted: Fri Jan 27, 2012 11:59 am 
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rx7locost wrote:
+1 what KB said. Besides, the only parts that come close to being important are the pickup oints in the 4 corners. What happens in between that is practically unimportant. Even the corners may be corrected depending on the control arm pickup style you choose. By adjusting the coilovers, you can balance out a lot of imperfections of warp.

The only dimensioned chassis drawing I've got for a sports car had a 1/4" and 3/8" tolerance on the diagonal measurements (squareness). Squareness is not affected by your undulating table surface.


Thanks for the input.

Most Locost builders are using their build tables to install the suspension pick-ups by building some kind of simple jig and then permanently affixed them by welding. What would be an example of the adjustable pick-up points you mentioned? Do you have any photos?

Since this is my first build I haven't yet become confident in what can be adjusted out and what will be permanently fixed by build errors. I like the idea of adjustability not just for correcting errors, but also so that the suspension parameters (anti-dive, caster, etc.) can be altered as you develop the car performance wise. I expect to spend a considerable amount of time after the build getting the handling as good as possible.

Cheers,

.

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886


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PostPosted: Fri Jan 27, 2012 12:04 pm 
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oldejack wrote:
It may make you feel better to know that the SR-71 changes dimensions every flight. .............


But, not permanently, right? With all the heat it experiences you'd expect dimensional changes. Bit it should return to it's pre-flight dimension afterwords, right?

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886


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PostPosted: Fri Jan 27, 2012 12:20 pm 
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Yo, Lonnie-
I've never seen any numbers on just how "out" the frames were, but in blogs about rebuilding old race cars such as Mallocks (you knew I'd get that in there, right?); various models of Lotus racers-7, 11, 19; TVR, Triumphs, etc, etc, they commonly send the frame to somebody to be "straightened". These were not "wrecked" vintage racers, just vintage racers. I'd guess that tweaking of the frames during normal usage is not uncommon.

That said, there's no reason not to do the best you can to level/flatten your build surface. Also, my experience with MDF was that it starts out nice and flat, but moisture attacks it very quickly. It won't stay level long if there's much humidity in the environment.

Good Luck!
:cheers:

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PostPosted: Fri Jan 27, 2012 12:25 pm 
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Lonnie-S wrote:
oldejack wrote:
It may make you feel better to know that the SR-71 changes dimensions every flight. .............


But, not permanently, right? With all the heat it experiences you'd expect dimensional changes. Bit it should return to it's pre-flight dimension afterwords, right?


I'm basing the statement solely on what I'd heard from my uncle Bill when I was young but, yep a permanent measurable change. .......
Some of his tales were pretty wild but a lot of the projects he worked on are still classified so it's hard to say for sure.

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PostPosted: Fri Jan 27, 2012 12:49 pm 
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Quote:
What would be an example of the adjustable pick-up points you mentioned? Do you have any photos?



I didn't mean to imply the pickup points themselves were adjustable after the fact. And I have no pictures. Any of the normally used "U-brackets" that are mounted on a vertical surface could be relocated up or down to compensate for 1/8" or even more here or there. Perhaps a shim added if they were mounted on a horizontal surface. This may be why some builders use a fixture to locate the pickup points rather than just welding them to the frame via dimensions. But a lot of this is moot unless you know you are working from a known set of geometries. Keeping adjustability in your control arms and coilovers will get you along way toward your goals.

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“Any suspension will work if you don’t let it.” - Colin Chapman

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PostPosted: Fri Jan 27, 2012 2:44 pm 
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GonzoRacer wrote:
Yo, Lonnie-
I've never seen any numbers on just how "out" the frames were, but in blogs about rebuilding old race cars such as Mallocks (you knew I'd get that in there, right?); various models of Lotus racers-7, 11, 19; TVR, Triumphs, etc, etc, they commonly send the frame to somebody to be "straightened". These were not "wrecked" vintage racers, just vintage racers. I'd guess that tweaking of the frames during normal usage is not uncommon.


Hmmmm, maybe I can take my own chassis to be straightened prior to setting up the suspension and before it hits the road. :lol: I bet it would cost a fortune though.

GonzoRacer wrote:
That said, there's no reason not to do the best you can to level/flatten your build surface. Also, my experience with MDF was that it starts out nice and flat, but moisture attacks it very quickly. It won't stay level long if there's much humidity in the environment.

Good Luck!
:cheers:


Thanks for the encouragement, JD. I used MDF for the mock-up, but decided on ply for the real chassis for the reasons you just mentioned. I live near the ocean and we do get moisture and fog on a regular basis. Even though I built indoors, the MDF definitely suffered over time and sagged where it wasn't directly supported. My ply isn't marine grade, but all the lamination glues they use now days will stand up to moisture. It seemed like a better bet.

Cheers,

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886


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PostPosted: Sat Jan 28, 2012 4:48 am 
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KB58 wrote:
Do the best you can and move on. None of us (that I know of) are looking to bump the current F1 champ off his podium, so it's just not that important. Yes, flat is best but don't kill yourself over it.


+1 and as RX7 alludes to the last things I fully weld are the shock/spring mounts after setting their final heights.

FWIW I have a slab of steel 3 meters x 2 meters x 50mm thick but not everyone can have one of those.


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