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Learning how to build Lotus Seven replicas...together!
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PostPosted: November 27, 2010, 8:45 pm 
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Joined: November 27, 2010, 8:33 pm
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Forgiveness for what I'm about to do - stuff a Honda ST 1300 V4 into my 1959 Nash Metropolitan. The chassis plans call for Miata suspension components and, if things go as planned, a full carbon fiber body. Yes, I'm aware this sounds like a colossal waste of time and money but that's never stopped me before. Witness my "Suzuki SV/MV 650":
http://www.bayarearidersforum.com/forums/showthread.php?t=173852
I got pretty good at working with composites as I built the tank and bodywork for the Suzuki. I've always wanted to build a carbon fiber car body and have always had a 'thing' for the little Nash. But the 'stuff a V8/v6/late model 4 cyl automotive engine in one' has been done to death. I wanted something a little different and when I saw a pic of the ST 1300 motor and thought "Now, that looks interesting...". So I bought one! Just arrived yesterday. Oh, and I've had the Metropolitan (from here on, called simply 'the Met') for a few months. Lots of rust so that's why it looks like it does at this point.
Any way, let's get into this project, shall we? Wish me luck, I'm headed in!
Here's the car and the ST 1300. A 2006 model with only 6318 miles.
I've got the compete Miata front and rear suspension but it looks like the near 12" difference in width will require more than just narrowing everything and using the Miata parts. Some final measuring still TBD but it all looks like it'll work. I'm excited - a 100+ HP with a sequential '5 on the tree' shifter (!) and some other, ahem, interesting goodies.
In any case, I guarantee I'll have a bunch of questions about suspension geometry so hopefully you'll have patience with me as I work through this build.


Attachments:
PB130002 (Medium).JPG
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Last edited by tygaboy on November 27, 2010, 11:45 pm, edited 3 times in total.
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PostPosted: November 27, 2010, 10:01 pm 
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Just trying to get a feel for front/aft engine placement. I should end up with enough room to run a push rod front suspension and mount the shocks in front of or along the side of the engine.


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PostPosted: November 27, 2010, 11:49 pm 
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Here's the ST engine. It's really a pretty engine. 5 speed, shaft drive. Though the shaft is offset to clear the rear tire so I'll be looking to address that.


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PB200014 (Medium).JPG
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ST1300_Engine.jpg
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PostPosted: November 28, 2010, 12:31 am 
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Gosh, that looks like a really good fit!

If it weren't for the gearing, I'd say that the stock rear axle might be OK for 100hp but I'd check with Sprite/Midget racing folk to see what they put through them. It's the same as used on the original Lotus Seven.

Seems like most BEC guys go for a numerically lower rear axle to keep the revs down for cruising. Ford Pinto/Mustang II had a 3.08 axle for automatic transmissions, typically with 4 bolt hub and disc brakes. Upgrades to 5 bolt discs are available, and seeing that you will likely have axle work done that would be something to keep in mind. The Pinto front spindles are very amenable to a wishbone setup and aftermarket performance parts are everywhere, especially 5-bolt. Also, the original Pinto had small wheels that might fit your bodywork better, so I think Pinto running gear would work out better for you. I suggest you look into it further and see what you think. The decision should be driven by the rear axle ratio.

That's a very nice-looking project you have there. Best of luck with it.

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PostPosted: November 28, 2010, 1:38 am 
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My cousin has a Nash Metro in his garage with a 400 buick motor in it. I'll have to point him at this thread. They have been talking about restoring it.

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PostPosted: November 28, 2010, 8:04 am 
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To minimize the driveshaft angle, assuming that it turns the correct direction as-is, after narrowing the miata subframe as necessary, offset the diff housing to use the stock axle on the left side and make a new shorter axle for the right side. You may need to limit suspension travel to keep the tripod in the cup on the right side. Roll a 6' x 12" x .050" sheet to a 3" radius, then trim the floor as necessary to fit the new tunnel.

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PostPosted: November 28, 2010, 11:25 am 
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Miatav8,MstrASE,A&P,F wrote:
To minimize the driveshaft angle, assuming that it turns the correct direction as-is, after narrowing the miata subframe as necessary, offset the diff housing to use the stock axle on the left side and make a new shorter axle for the right side. You may need to limit suspension travel to keep the tripod in the cup on the right side. Roll a 6' x 12" x .050" sheet to a 3" radius, then trim the floor as necessary to fit the new tunnel.


Thanks for the input. Yes, the motor does turn the correct direction. As to the driveshaft angle, it's not only offset, it looks like it's going to be too high - I have the "double whammy compound angle". In talking with Jesse at High Angle Driveline, he says that there's a high probability that unless I get one plane lined up, I'm in for vibrations at highway speeds.
And in keeping with what is likely the going to be the most repeated phrase during this project - "Nothing is ever easy" - the ST has a 2.833 final drive so if I go with an offset diff, I need something with an appropriate ratio. Currently leaning toward a 3 series BMW that has a 2.93 - see pic.
And here's my first question: If I have different axle lengths, is there really nothing to worry about beyond binding points of the related suspension and axle pivots? No other negative impacts to consider?

If I opt to go non offset diff, the other option is to develop a gear/chain offset unit. This would open the door to 'correcting' the primary gearing of the engine and I could spin the output of the offset unit up to whatever I wanted. Now I can run whatever rear end I want. (I like the Subaru rear ends as they are quite narrow which is key for this build.) Now, I have yet to research what this approach does in terms of impacts to torque numbers and all that, but I'm kinda leaning this way.

All comments and help welcome!


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drivetrain_e46_diff.jpg
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PostPosted: November 28, 2010, 12:28 pm 
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Quote:
If I have different axle lengths, is there really nothing to worry about beyond binding points of the related suspension and axle pivots? No other negative impacts to consider?


One thing to consider is that different axle lengths will "load up" differently. This is a major causes of early FWD cars having torque steer. One side twists more that the other causing it to lag the applied forces. I have no first hand knowledge of how it would effect a RWD car. But I can imagine it should be researched a bit. If you find it to be a problem, it might be overcome by using custom half-shafts keeping equal length reduced shaft diameters. Alternatively you could add a spacer on the longer side , requiring the use of 2 equal length shorter shafts. Does this make any sense?

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PostPosted: November 28, 2010, 2:14 pm 
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How about a chain drive from the driveshaft flange that goes down and to the centre of the car to the driveshaft? basically a pair of pillowblock bearings with a driven sprocket between them and a u-joint flange at the back. That way you get the driveshaft down and in the middle of the car and can get any gearing you want with different sprocket sizes.
Kristian

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PostPosted: November 28, 2010, 2:37 pm 
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It is common for rwd vehicles to have unequal length axles. An extreme example is a legends car using a toyota axle. However, if the rear toe changes with suspension movement, the car can yaw one way under load and the other when lifting to shift.

As for the driveshaft angle, use CV joints instead of U-joints after doing what you can to minimize the angles.

Avoid chain when possible. As a chain wears, the pitch along the length varies, resulting in tighter and looser sections.

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PostPosted: November 29, 2010, 8:11 pm 
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tygaboy,

Before you go dropping the final drive ratio, remember, that the high stock final drive ratio is based on the diameter of the motorcycle wheel/tire. Check the diameter of the wheels you plan to use. If they are smaller diameter than the motorcycle wheel/tire, you have reduced the real final drive ratio.

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PostPosted: December 2, 2010, 12:32 pm 
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tygaboy wrote:
If I have different axle lengths, is there really nothing to worry about beyond binding points of the related suspension and axle pivots? No other negative impacts to consider?

I like the Subaru rear ends as they are quite narrow which is key for this build.


No issues at all, not sure where you got the worry from in the first instance.

Subaru would be ideal, early 4WD stuff which they are plentiful. The diff is actually a Datsun/Nissan R160, will be bullet proof for your build and even an LSD available if required later. Alternate ratios used to be available for racing and rallying not sure about the availability these days.

Read this ...

http://www.legacycentral.org/library/li ... ential.htm


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PostPosted: December 4, 2010, 8:27 pm 
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Joined: November 27, 2010, 8:33 pm
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Built a cheapy engine stand and laid out a 'what's this gonna look like?' mock placement of the Miata suspension and engine. Wheelbase is a whopping 85". The rear set up is an initial design I'm considering to accommodate the narrowness of the Met body. Note the overlap of the Miata inner CV joint. Should give you an idea of how skinny this thing has to be. The stock Met rear is about 51" from outside of tire to outside of tire. I may be able to up that by an inch or so, but not much more than that.
I'm getting my wheels/tires on Monday so once they're here, I can start the official work. I'm using a 'build in from the outside of the tires' approach. We shall see...


Attachments:
File comment: Now, THAT is a narrow track!
PC040012 (Medium).JPG
PC040012 (Medium).JPG [ 59.27 KiB | Viewed 13634 times ]
PC040014 (Medium).JPG
PC040014 (Medium).JPG [ 72.92 KiB | Viewed 13634 times ]
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PostPosted: December 4, 2010, 8:35 pm 
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Made significant progress on some rust repair today. This is just mock up as the Met has enough rust in some of the trickier places that I'm going to have it dipped, put on a frame rack to ensure it's straight (it's not right now) then get to welding in the new panels. So far, so good.


Attachments:
File comment: Inner and outer rockers removed. I hate rust...
PC040008 (Medium).JPG
PC040008 (Medium).JPG [ 67.16 KiB | Viewed 13634 times ]
File comment: Inner rocker in mock up.
PC040006 (Medium).JPG
PC040006 (Medium).JPG [ 63.65 KiB | Viewed 13634 times ]
File comment: Outer rocker in mock up. Looks good to me!
PC040003 (Medium).JPG
PC040003 (Medium).JPG [ 67.73 KiB | Viewed 13634 times ]
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PostPosted: December 4, 2010, 9:05 pm 
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Joined: July 28, 2009, 9:05 am
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Location: Buffalo, NY USA
reverse gear provisions/intentions?


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