LocostUSA.com

Well, almost immediately after posting the above pictures I went out to trial fit the diff. Oops it didn't fit - I had most of the relative measurements correct, but everything was too low - now where for the drive shaft to go and no room for suspension travel.

So here is a photo after cutting out the top suspension mount frame



And here is a photo of the new rear suspension frame. The top mounts/diff hanger are basically 2.5" higher.



Talking with the various Miata locosters, my dimensions are very similar, hopefully tomorrow I will get the rear suspension hooked up. If I do I will post some pictures with dimensions. If I don't it might be a while before you hear from me.

One concern I have is the diff placement. When I position everything in the frame, the diff sits quite far back. It sits quite far back in the Miata rear subframe too. (near the rear top suspension mount) As far as I can see from the photos, Mr. Tanner's diff sits forward, near the front top suspension mounts. Does anyone have any experience of where the Miata diff should go?

Thanks

I have no experience or expertise for that matter but that won't stop me from throwing in my $.02. The diff I am using is the piece of junk out of my 90 model donor. The one pictured in Kieth's book is a 94+ Torsen model which I think has a longer nose/snout thingy. How much longer I don't know but if you are using an earlier diff like I am the fact it is shorter seems to make it "appear" to sit futher back. His mounting plate is up inside his tranny tunnel whereas it would be difficult to get mine that far forward without raising it way up or cutting the lower crossmember. (which I don't consider an option- but I've seen it done)

I'm out there in the garage trying to locate my diff today, I'm on a "queit time" break for the kids to get a nap.

I gave this a lot of thought last night and decided that you could put that diff about anywhere back there within reason and as long as you get it at about the right hieght you can't go wrong. The half shafts don't have to be exactly perpendicular to the car as far as I can tell.

You should see the butt ugly ghetto PPF replacement I'm working on.


Also- Are you using stainless? Your steel sure is pretty.

Thanks Chetcpo,

I will just hang the diff where it looks right - I can always redrill the plates.

The steel is standard stuff from the local metal mart - but it sure is shiny

Well I had a couple of days off work so I got a bit more done.

First job - off to Princess Auto to get a warm work coat and gloves. Canada is lovely but the garage gets a bit cold.

Following various paths of investigation I finally welded my rear suspension brackets to the frame. I am a little concerned that I may have them too high now, but at least the diff is in the right place. My top wishbones will have to be slightly narrower than the original ones. The brackets were fabricated to allow the use of the Miata bushings - but I think I may go with rod-ends after all.




The next job was the engine mounts. I put blocks under the frame to get the height of the diff output shaft the same as the gearbox output shaft. At this height the engine sat so low the original mounts were below the bottom frame rails. This was not the case on Keith build - at least not by the pictures in his book. I spent a while fiddling and came up with this solution.



With the engine in place I then constructed the transmission tunnel


Now I am back to the front suspension. Time to post in the Suspension forum again.

You sure got your engine in there a lot lower than I did. Mine might be too tall for a factory scuttle and nosecone, you should be fine. How did you locate the diff. Are you building anything PPF-ish?

looking good.
you are a little ahead of me. I'm still jigging up for may chassis (want everything laid out before I begin welding)

Where are you located? I'm in the great white north myself. If we're close by we'll have to have a get together.

Chetcpo: I am planning on some form of PPF - I will have to see how much room I have in the tunnel.

Violent Blue: Not too close, I am in Ontario.

I had an attempt at designing the front suspension today.

Basically I measured the Mazda suspension and subframe, did a full size drawing and added the locost frame (mine is a McSorley +4). I then extended the arms to reach the locost. I don't see any reason why this won't work - please let me know if you see any reason why it will be a superb failure.







I am going to post this in the suspension section too. Sorry if double posting upsets anyone.

Adam

You have the general idea, but you need to set your roll center first and then work back to the control arm bracket location and angles. Then plot the camber "camber curve" and verify roll center movement. Front suspension design can not be exampled in a couple of typed pages. They are several good book on it. I suggest you try to find the book Race and Rally car design and suspension, and excellent book. It will still take a week or two to of plotting all the possible lenghts and locations to get a reasonable design.
It's all part of the Locost journey Dave W

You can't simply "extend the arms" because it changes the wheel geometry. Like Davew said, you *start* with the desired roll-center, then work backwards from that and the upright to find the inboard pickup points.

Kudos to you for asking though, you'll get it worked out. Any of Staniforth's books are excellent.

Whittlebeast said he would give some computer generated control arm lengths and mounting locations that give good roll center control if I just provide him with the miata details. I'm currently getting all the measurements gathered up to send to him. It may be worthwhile to wait a bit to see what he comes up with before fabbing something.

Unless of course you just want to do it your way. I think you could probably fabricate what you have designed and it will do fine for your purposes. I'm sure you have given it more thought than many do. The car won't fly into peices or explode if the geometry isn't ideal, it may just not be as good as it could be otherwise. As was stated elsewhere here, proper suspension geometry is the one thing that improves your car's performance that doesn't cost a dime extra.

Those are some nice looking drawings. If you draw in your wheel centerline, tire outline, vertical chassis center line, and horizontal ground level line you can use them to see what your static roll center height is with some string.

I was trying to avoid all the maths and engineering - I do that all day. But now the bug has bitten.
I got the idea of extending the arms and doing the drawing from the race and rally car source book - that is one book I have had for many years. Now with the responses coming back I will have to do some calcs to see how far off my geometry is. By my understanding I will have the same roll centre as the Miata, with a lower C of G in the Locost - which I think is good. The longer arms should give less camber change - which I think is good .

I will have to read the source book again and convert my drawing into a string computer. Hopefully by the time I have finished Whittlebeast will have something to compare with.

I have been playing around with a demo version of Performance Trends "Suspension Analyzer v2.0", and reading the race and rally car source book and chassis engineering.

Initially I measured all the Miata hardpoints and put them into the computer prgramme. This gave me a baseline. Then I did the same with the locost dimensions as per the drawings above.

I then tried moving teh top chassis mounts up and down.

This gave me a whole bunch of results.

(Admin - this is my first attempt at HTML I copied the code from Excel can you allow it please or let me know how to show a nice table thanks)

**Admin edit** Adam, I couldn't figure it out, here is your table:
http://www.locostusa.com/adam.html


I think the 1/4" higher is the best compromise.
All the set-ups have similar camber changes
The 1" higher setup has a lot of lateral roll centre movement with the lowest roll centre height
The 1/4" higher has virtualy no lateral roll centre movement and very reasonable values for everything else.

Are my conclusions accurate?

I have created a lot of data over the last few days - I think I now have accurate dimensions for the Miata which I will be more than happy to share with anyone interested. Just let me know.[/html]

Is the unit of measure for that table inches? If so, 8" is pretty high for a roll center.

The units are actually centimetres.

Thanks for the table