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The Birkin tunnel is quite narrow. It uses a 1/8 inch plate hoop over each end of the drive-shaft and then a GRP cover as the tunnel. This GRP tunnel probably contributes zippo to chassis torsional rigidity, but a formed steel tunnel welded to some 1x1 longitudinal members that were then bolted to the floor might well offer a marked contribution to torsional stiffness.

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Warren
Isuzu Pickup/SR20DE, +401 COLD frame
Build Log: viewtopic.php?f=35&t=11601

I was thinking of hoops made of maybe 12 ga. and over the u joints because that is where the drive shaft might come apart.

ok here we go again

I think I've got most of the frame done. in theory Still need oz mod in front end and I couldn't get round tube to work for the back of the car so ill put the rest of the bars in later. Fuel tank as measured looks like about 15 gallons. I removed the floor brace in the foot well area. switched the tunnel to just sheet no bars and installed the floor. used 1x2 for forward rear end mounts.

So once again look it over and tell me what you think. Please



thank again Harold

oh yah sorry for taking so long racing starts next week.

It's vastly improved, Harold. Your choice of triangulation elements is much better in this version. It looks like it's going to be incredibly strong.

Without doing a really careful look-over and knowing the size and gauge of each member, I don't feel I can give you any truly meaningful input on the structure, so take the following just as casual observations.

1) I'd make the gas tank smaller and leave some crush space (empty space or crush-able foam, etc.) around it for safety. Even if you only got 10 MPG, a 15 gallon tank would give you 150 mile range. With a V8 and say 16-18 MPG (pretty conservative estimate, I think, for a 1600 lb car) a 10 gallon tank would give you 160-180 mile range.

2) I don't quite understand your second set of tripod mounts in the engine bay. I'm thinking the forward ones are your motor mounts. The rear ones are for what purpose?

3) I know you are more concerned with strength and stiffness than weight, so I will accept that eliminating members (like say under the cockpit area) is not a design goal for you at this point. That area will be incredibly strong.

4) At that outboard joint at the lower rail, where all your triangulation meets from the side rails and the cockpit floor, you are going to be doing a LOT of welding in a small area. Unless that is done very carefully, there will be danger of causing the chassis to deform. You could spread that triangulation out differently and get the same structural effects. Also, you might want to take a look at the triangulation style done by cheapracer in his new design and/or Justin with his more traditional Locost. I'd be very tempted to do it in that simpler style now that I've been through the process once.

5) I'm not sure how a Locost will look with square corners on the rear. I'd be tempted to mock that up in real life using wood and butcher paper before committing to it, but that's just me.

Cheers,

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

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

None of this is structural, but ..... following on Lonnie S, the fuel tank may also be too big in the fore and aft dimension as drawn if this is a live axle car. It can be a bit awkward manoeuvring that axle in and out of the space and you might have to do it after the car is finished. Don't forget that you will need space for the Panhard bar (or whatever you use for lateral location) in that same area. The live axle Birkins get around that aspect of space management by running the Panhard bar ahead of the axle, below the differential snout. That, of course, is only feasible if other bits of geometry shine in your favour.

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Warren
Isuzu Pickup/SR20DE, +401 COLD frame
Build Log: viewtopic.php?f=35&t=11601

Hello Hello


Lonnie-s

1. Good call on the fuel cell didn't even think about the pan hard bar. I think Warren Nethercote is right ill figure how big It can bee while building and look in to pan hard bar or J bar in front of the rear end. The tank is going to have to be as bigggggggggg as possible. Last time I had this engine as a 351 in a street car it got 8 miles to the gallon.

2. The second set of tripod mounts are for the rear of the motor as I don't plan to run a tranny mount just a mid plat type mount.

3. I thought about eliminating more of the floor braces but I wasn't sure what affect it would have. using the floor as a stressed skin I think I could with no real loss in strength but until I get a second opinion they look good to me.

4. I hadn't real thought about worpage from all the welding in one point. I think with some planning along the way it'll be ok. Good point

5.your right about the back of the car. I don't think it would look good as drawn. me and this stupid computer don't always see eye to eye. in other words it wouldn't do round or I couldn't do round so I drew it square for reff. The end product will be rounded.

Warren Nethercote

Thanks for chiming in. Not having built one of these yet my concept of space is fuzzy. I keep thinking of car stuff that is way over sized for the area this chassis allows. Thanks the tank after the car idea, sounds good. I haven't seen a birkins sit up yet but once I'm sure I beat this horse for all its worth I will be headed over to the live axle stuff to try and get that stuff right. hopefully I can learn a little more be for diving into the front end.



thank Harold

Lonnie-s

sorry Lonnie it looks like I skipped right over your first comment and went right to the list. I like lists any how all plating, tunnel, floor, rear corners, rear bulk head and foot wells are 16 ga. All uprights, braces in lower frame and base of frame are 1x1 16ga. Top rail, hand rail and top braces are 1.25x1.25 16ga . The only "odd" pieces. Is the 1x2 in the rear corner of the cockpit. I intend to use them for the front mounts on the 4 link. as pictured below. so it looks like its all 16ga and if its not the top of the cockpit / engine bay it 1x1. basically





Thanks again
] Harold

Harold,

Some Birkin build instructions are posted on-line. Search on Birkin Group 4 build instructions and you should find a pdf which shows their Panhard rod running ahead of and under the pumpkin snout. I could not attach it here because it is a pdf.

_________________
Warren
Isuzu Pickup/SR20DE, +401 COLD frame
Build Log: viewtopic.php?f=35&t=11601

Here are two of the photos.




Cheers,

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

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

Hi yall


Been working on this thing for a while. every time I think I've solved a problem I create a new one. I'm really starting to think all things cant be solved until I have a real chassis. this cad program is cool but every thing takes for ever.

Any how. haven't found any goo d write ups on four link angles. any thoughts?

thanks Harold

There is actually a lot of written material on 4-links, but a lot of it is for circle track or drag racing applications and even for off-road too. The road-going or road-racing design materials are out there too, but mostly in books. I'll list a few below and you can look them up on Amazon.

I keep telling myself that after I do my own design (coming shortly) I'm going to write up a brief, illustrated document called "Everything a Locoster Needs to Know About 4-Link Rear Suspensions." In truth, I doubt I'll have the time, but you never know, it could happen.

Some Relevant 4-Link References
=====================

1) Advanced Race Car Chassis Technology - Bob Bolles - HP Books
Recent, mostly circle track, but pretty exhaustive coverage. Probably best represents current methods.

2) Race Car Vehicle Dynamics - Milliken and Milliken - SAE International
THE mother lode with respect to suspension design. Very technical. Expensive. Not required, but extremely interesting if you're a true suspension wonk, or wonk wanna-be.

3) Chassis Engineering - Herb Adams - HP Books
A little dated now, but decent basic information

4) The Sports Car and Kit Car Suspension and Brakes High-Performance Manual - Des Hammil - Veloce Publishing
This is a straight forward, very clear book that actually gives you practical goals to shoot for in your design. It is aimed at builders of 7-type sports cars in particular. It's not quite as general as 1 or 2 above in theoretical coverage, but very useful for all aspects of our cars including springs, shocks, set-up and testing for the owner/builder.

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

If you muddle through all those books and Staniforth's you may come up with the same conclusion I did and the way a number of locosts are built. That is, equal length arms parallel to each other and parallel to the ground. That is pretty simple. The complicated bit is the Panhard rod which should be as long as possible and adjustable so that you can minimize bump steer. A Watts link would be better but is complicated to attach to either the axle or the chassis.

You might want skip the CAD and start building in wood a la Kurt B. There are certain parts of a car that are very time consuming to model (all that stuff in front of the driver) and wiggling a steering shaft around them is a real trick.

Stalled but not forgotten. long time but im still out here. knew thing change thing and ive been on stall.

Very cute, and quite a "diversion" too.

Cheers,

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

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