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Learning how to build Lotus Seven replicas...together!
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PostPosted: January 26, 2020, 1:11 pm 
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First post after being a long time lurker. I thought about building a locost several years ago but opted for resurrecting a 1966 MG Midget with a toyota 3tc/5 speed swap instead of a locost. That project will hopefully be done and on the road this summer.

Back to thinking about doing a locost build after I complete my spridget. Lately I have been researching different types of rear suspension and came across this post by Stranger7 viewtopic.php?f=39&t=12142&start=75 that shows the original rear suspension in a Lotus 7. I understand that there were lots of problems with this setup. In researching about the problems I stumbled onto the Lotus Cortina A frame rear suspension. http://www.lotuscortinainfo.com/?page_id=1799. I have no idea if it would work in a locost.

Anybody have any experience with this type of rear suspension?


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PostPosted: January 29, 2020, 12:14 pm 
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It looks like the Cortina method is similar to a Fox body Mustang and probably other cars too. Transverse location of rear axles is a big subject. One issue is providing adequate structure for the forces involved and not having issues with binding or placing the axle in torsion etc.. The other issue is the way the geometry resists the roll forces in cornering. Choices you make here will or should affect your front suspension too.

Panhard bars can offer easy adjustability for height, which adjusts the roll center height, which adjusts the handling.

Can you draw something or tell us more?

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PostPosted: January 29, 2020, 12:31 pm 
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mikri,
The photo's in Stangers link are for a Series 1 Seven, which differs from the Series 2 & 3. The series 2 & 3 have a set up similar to the Cortina set-up in the other link. The issue with the Series 2/3 and Cortina set up is applying loads to the diff housing in a spot that has a sealing surface right above it. The casing will eventually distort and cause an oil leak, which then usually eats the rubber bushings. Using poly bushings sounds like a good idea, but they don't provide enough flex, and will speed up the distortion of the casing. A spherical bearing is a better solution. Here's a pic of my old Seven with the wide A-frame:

Image

One thing to note with the a-frame set-up, if you choose to go this route, is to make sure the bracket welded to the axle casing is sturdy. Dear old dads Caterham had a bracket failure (luckily at the top of our street, and going about 5 mph) which allowed the axle to move rearward, pulling the driveshaft out of the gearbox (Ford 2000E). Had this happened at higher speeds, things could have gone very badly.


The Series 1 set-up was different in that the RH lower trailing arm was triangulated. It works well, but typically does require the use of rubber bushings to allow for some misalignment. Here's another pic, albeit from my old Eleven that used basically the same set up as the Series 1 Seven:

Image

If I were to do it again, and not care about building a duplicate of a real Lotus, I would use a panhard rod. It's much simpler.

Rod


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PostPosted: January 30, 2020, 1:33 am 
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Thanks for the info. The chances of me every doing a complete Locost build are pretty slim. More likely would buy someone's abandoned project.

I do like to explore all possible options before I start on a project. Everything about a Locost seems fairly straightforward except for the suspension. Lots of variables to juggle there.

My take on the Cortina A frame was that it looked to have a more solid mounting than a pan hard rod mounted to what seems to me to be a very flexible framework behind the axle. To eliminate the flex problems you mention the Cortina A frame allows the rear axle to rotate both up and down and to rotate in a circular motion as the car rolls during a turn.

I have been working on two generic models of the Locost frame in Sketchup to get an idea of how everything goes together. One is basically a MacSorley 442. The other is a MacSorely +4 that I continually change to visualize different ideas that I come up with. It is not complete enough to post at this time and I keep changing things as I learn more. If I did build a car from scratch I would buy all of the parts and pieces and then design and make the frame to fit.

Again thanks for the info and sharing your experiences with this type of rear suspension.


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PostPosted: January 30, 2020, 8:17 pm 
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The 90s suzuki sidekick/ geo tracker has a similar system with a balljoint on the diff end of an A arm but it is on top. However, I think most any steel diff could be modified to accept a balljoint on the bottom with either bolt on or welded reinforcement.

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PostPosted: January 30, 2020, 8:28 pm 
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Quote:
To eliminate the flex problems you mention the Cortina A frame allows the rear axle to rotate both up and down and to rotate in a circular motion as the car rolls during a turn.


The flex issue is feeding loads into the bottom of the diff housing where the third member bolts in. The Cortina set up is nearly identical to the Seven, only the upper trailing arms are mounted in a slightly different spot. The 'tina will suffer the same issues as the Seven set up. As MiataV8 mentioned, the Tracker uses it at the top, which will alleviate the sealing issues due to flex/distortion as the oil level is nowhere near the top of the diff housing, but does result in a higher roll center.


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PostPosted: January 30, 2020, 11:07 pm 
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So in order for this to work on the bottom you would have to spread the loads out to the tubes and not rely solely on a connection on the bottom center section of the differential? Something like this. The vertical link is the coilover shock.


Attachments:
400 with A frame.jpg
400 with A frame.jpg [ 366.95 KiB | Viewed 1173 times ]
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PostPosted: January 31, 2020, 6:47 am 
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That would help some but this would be more effective at preventing flex at the sealing flange from the trunion without moving the pivot point. Red 1/8-1/4 base strip, light blue gussets 1/8-1/4 x 1 or 1.5 up the front as wide as needed, welded outboard of the sealing area. Dark blue up the back to the corner in the cover.

Placement of the pivot and the diff used can make a big difference. Just moving the pivot aft a couple inches would likely eliminate any sealing issue with no additional gussets but that alters the geo.


Attachments:
cortina.JPG
cortina.JPG [ 107.03 KiB | Viewed 1159 times ]

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PostPosted: February 1, 2020, 11:20 am 
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Thanks for all the info. I have this stuff saved in my Locost folder for future reference.


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PostPosted: February 2, 2020, 5:04 am 
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The Alfa Giuliettas had a very similar setup and it worked great. Trailing arms located under the axle and the triangle fastened to the top of the dif housing.

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PostPosted: February 2, 2020, 1:01 pm 
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Those Alfa's have an interesting rear axle/suspension. There is a center section which the tubes bolt to. The A frame bolts to a stud coming off of the upper rear corner of the center section on the driver's side. From looking at images online the bushing appears to be concave to allow the axle to twist while also allowing it to move up and down. I wonder if instead of being on the top of the center you could put it on top of the tube next to the center. That would lower the roll center.

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alfa suspension 02.jpg
alfa suspension 02.jpg [ 196.47 KiB | Viewed 1026 times ]


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