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Learning how to build Lotus Seven replicas...together!
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PostPosted: December 27, 2017, 4:27 pm 
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The next step in my chassis development is to test its strength. If I had unlimited funds, I would buy a high end 3D modeling CAD software licence and use the Finite Element Analysis (FEA) function to find my chassis weak spots. However, my funds are limited so I used the LoCost old school equivalent called the Finite Element Analysis using Balsa, Leverage, and Elmers glue (FEABLE).

FEABLE testing my chassis revealed several weak areas that were easily changed in the design. Doing FEABLE testing now will save many rework hours during the metal fabrication phase.

Here is a short video on the FEA and FEABLE stress testing methods -> Click Here


Attachments:
FEABLE balsa model.png
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FEABLE Solidworks model.png
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PostPosted: February 19, 2018, 4:14 pm 
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I've finally started the construction phase...Yay!

The first step is to build the chassis table. The Tiger 700 is a small car that will weigh about 270kg (600lbs), so the chassis table doesn't need to be as strong as other projects on this forum. I chose 3/4" plywood because its easy to work with and its less expensive. I was able to build the table with just 1 sheet of plywood and 3 sticks of 2"x2"x8' wood. I used some screws that were left from an earlier project so my total cost was about $70.

I plan to use the table throughout the project to raise the car to a level that is comfortable to work at. That means that the car will go on and off the table many times during construction. Therefore, I decided not to build table legs and just use the jack stands that I already had in my garage. The jack stands also allow me to change the table height as needed. I built the end of the table so a floor jack will fit under and allow the table to be lifted up to the jack stand height.

The hardware store had a cutting jig that makes straighter cuts than I can at home, so I had them cut 3 strips of 4"x96" from the plywood. These strips are for a system spars that support the table to keep it straight. The table is big enough for the chassis construction, but I will build wings for the front suspension and for body overhangs an the nose and tail. I want to be able to sit with my legs under the table, so I inset the supporting spars 6" from the edge and used a router to round over the bottom edge of the table to keep from banging my knee ...Ouch :rofl: :crying: :oops:

I finished the table top surface with 1 coat of clear mat finish. Then I marked out center lines in red and 50mm segments in black & blue. Then I put 2 more coats of the clear finish to protect the marks. I used water based clear finish to keep the Sharpe permanent marker for running.

To see the video of the table -> Click Here


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PostPosted: March 13, 2018, 1:50 pm 
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I'm on to the nest step in jigging up the front suspension hard points. I built wings for the chassis table that fix the front hubs at the wheelbase, track width, and height they will be with the tires mounted. There is a red center down the middle of each hub jig with a sight hole drilled to ensure they are precisely aligned on the chassis table. The hubs are mounted so they swivel where the King Pin angle intersects the ground. The swivel will let me simulate a typical 30 degree turn angle and set the proper tire to body clearance.

The A-arms shown in the photos are the stock units from the Raptor 700. I will use the calculations from the SUSP suspension modeling tool to shorten them for proper on-road front end geometry.

Once I had the hubs mounted, it looked like the track width was a little too wide. So I shortened the the track width from 1374mm to 1274mm and updated the SUSP model.

To see the video -> Click here


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PostPosted: March 13, 2018, 2:18 pm 
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As an aside, though it's slight, the vertical upright support is bending due to the weight of the unit. Cantilevered out to the outboard pickup points, the deflection can amount in a not-insignificant offset. If you're going for the last millimeter of precision during suspension design and fab, you might consider adding stiffening plates.

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PostPosted: March 14, 2018, 10:13 pm 
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KB58, you are correct.

The top portion of my hub jig has bent due to the cantilevered weight of the A-arms. I first noticed it in the photos and then I measured it. The top is bent over to about 1 degree of camber. The final camber spec is about 1/2 degree, so I'm glad I caught it before it got any worse. I'm now supporting the A-arms so there is no more deviation.

I may flip the hub and A-arms to the outside of the jig and let the weight bend the jig back into place.


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PostPosted: March 20, 2018, 10:34 pm 
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I chose a Mustang II Rack & Pinion steering unit and received it this past week. The Mustang II steering rack is a favorite among hot rodders, so you see them on many custom hot rods. This means that there are plenty of them around and they are cheap. I bought a new manual rack for $75 including shipping (no tax). The pivot points on the Mustang II rack are 22" apart, which is exactly what I needed, according to the VSUSP suspension modeling tool.

I made a very simple jig to hold the rack at the proper position while building the front sub-frame

To see the video -> Click here


Attachments:
Steering rack rear small.jpg
Steering rack rear small.jpg [ 842.61 KiB | Viewed 1505 times ]
Steering rack front small.jpg
Steering rack front small.jpg [ 746.61 KiB | Viewed 1505 times ]
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PostPosted: March 26, 2018, 6:29 pm 
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I finished the front lower A-arms.

I used the measurements from the VSUSP suspension modeling tool to create a jig to ensure the A arms precise. I started by drilling a hole in the board to lock in the ball join stud. Then I locked in position of the 2 legs between 2 screws drilled thru the board. I then drew the center line of where the base pivot tube should go. The original A arms could then be cut to length. To lock in the pivot base tube, I added another screw thru the board with room for a shim to ensure a tight fit during welding.

My welding skills are "OK" on steel, but I don't trust it on aluminum, especially a critical suspension part. So I took it to a professional welder who specializes in aluminum. When he saw the close fit of the joints and the jig, he replied "That's just too damn easy!" I guess it was easy because he finished it about an hour later and only charged me $60.

In the photos you can see how much I cut down the upper A arms, as well as the completed part.

To see the video -> Click Here


Attachments:
A arm lower 1 small.jpg
A arm lower 1 small.jpg [ 265.49 KiB | Viewed 1354 times ]
A arm lower 2 small.jpg
A arm lower 2 small.jpg [ 634.29 KiB | Viewed 1354 times ]
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PostPosted: April 7, 2018, 8:17 pm 
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Today I went all medieval, and did my best impression of "Brave Heart with Power Tools"

Image

I have crossed over the point of no return. I butchered my Raptor 700 chassis without remorse. I chopped the front off and continued the frenzy by chopping it into even smaller chunks.

To see the video -> Click Here


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Cutting the Raptor 700 Chassis.png
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PostPosted: May 10, 2018, 1:33 pm 
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The rear subframe is now finished. I cleaned up from the wild cutting frenzy, where the Raptor 700 donor lost its nose. Although it looked like a random chop-chop here and a chop-chop there, all the cuts were well planned. The cuts were optimized so my seat will be set back toward the engine as far as possible and to cut at points on the frame that had cross bracing to ensure strength.

Click here to see the clean up video -> Youtube

Once the nose was cutoff the frame, I was able to harvest quite a number of component mounts and tubes from it. The 2 forward most vertical tubes happen to bridge the gap just ahead of the engine. They also curved in the correct way, so once welded, they blend into the remaining frame and look like they were always there.
Attachment:
Rear subframe prep.jpg
Rear subframe prep.jpg [ 75.59 KiB | Viewed 1097 times ]


Since the transplanted tubes directly join at cross braces at the top and bottom of the frame, the result is stronger than the original configuration.
Attachment:
Rear subframe prep2.jpg
Rear subframe prep2.jpg [ 176.9 KiB | Viewed 1097 times ]


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PostPosted: May 13, 2018, 8:29 pm 
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Hi there. Just wanted to say I like your build - and the chassis layout is very neat.
Looking forward to seeing more.
MangPong.


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PostPosted: May 16, 2018, 9:22 pm 
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I finished the front suspension A-arms...Well, for the second time. To see the video -> Click Here

I tried to modify and reuse the Raptor's A-arms, but my measurements and calculations were wrong for how far the ball joints would articulate. The resulting A-arms had very little movement in the compression part of the cycle. That would have surly caused major damage the first time I hit a big bump. So back to the drawing board to redesign and re-fabricate all 4 A-arms.
Attachment:
First A-arm ball joints small.jpg
First A-arm ball joints small.jpg [ 490.47 KiB | Viewed 943 times ]

I found a set of Raptor ball joints on eBay that are for building custom A-arms. This time I set my ride height at nominal of 125mm (5") and rotated the ball joint in their neutral position. Then I built the custom A-arms with a curve to match the ball joint position. That ensured maximum articulation in both compression and drupe.
Attachment:
Rebuilt A-arms small.jpg
Rebuilt A-arms small.jpg [ 377.24 KiB | Viewed 943 times ]

I was able to reuse the Raptors suspension bushings/bearings, brackets, and grease in the top A-arm. The upper A-arms will have about the same force on them so the bushings/bearings should be plenty strong.

The lower A-arm will carry the spring load, so they need to be much stronger. I used 4-link bushings/bearings from Energy Suspension (PN 9.9105) to carry the load.
Attachment:
Upper A-arm bushings small.jpg
Upper A-arm bushings small.jpg [ 204.05 KiB | Viewed 943 times ]

I got plenty of use from the A-arm jigs I made :BH:

I now have about 90mm (3.5") of suspension travel in both compression and drupe.


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PostPosted: May 20, 2018, 11:38 am 
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I finished the front subframe. Click here to see the -> Video

Beware: This video is all about the accuracy and details needed to build a front subframe. It may be mind-numbing & boring to some viewers.

The subframe and A-arms took much longer than I anticipated. That's because it needs to be very accurate and I ran into some fabrication issues/improvements along the way.

The original design for the front subframe was overly complicated and a bit heavy. So I muddled through several redesign iterations along the way:
  • Once I had the wheel jigs mounted on the table, they looked a bit to wide, so I reduced the front track from 1374mm to 1274mm (-4").
  • The bottom tubes on the original design angled downward in the center. This was very difficult to jig and construct, so I made it a single straight horizontal tube.
  • Now that the front track was narrower, it was possible to mount the coil-over shock directly to the opposing side of he subframe. That simplified the fabrication and better aligned the spring forces on the frame's bottom tubes to reduce racking the subframe geometry.
  • The bracket for the lower A-arm pivot points was heavy and ugly. So I tossed the brackets and integrated them into the junction of the vertical/horizontal tubes.

Needless to say, this was a learning experience. :oops:

To ensure the car will handle well, the geometry of the front subframe needs to be very accurate. That meant building several wood jigs to suspend parts during fabrication. For example:
  • The wheel jigs held each spindle at the correct track-width and at the height to simulate the center of the tire diameter. The wheel jigs also set the spindles to the correct caster, camber and toe. This is important to ensure the front end will be in the correct alignment when completed.
  • The steering rack jig held the rack & pinion at the correct vertical and horizontal position. This is critical to ensure there will be no bump steer.
  • A 2 level scaffolding was built to hold the subframe horizontal tubes. The upper and lower horizontal tubes were each just a rectangle. The simple rectangle shape allowed precise measurements to ensure they were flat and square within 1mm. Once the tube rectangles were on the scaffolding, I used carpenters shims to precisely adjust the horizontal tubes to be level within 0.2 degrees. Then I used plumb bob on the string to align the top and bottom tube rectangles to each other and then centered them to the red center line on my chassis table. The last step was fitting the vertical tubes to create a box.

The jigs and shims were very helpful and only cost a total of $25.


Attachments:
Front Subframe and scaffolding.jpg
Front Subframe and scaffolding.jpg [ 951.06 KiB | Viewed 872 times ]
Steering rack front small.jpg
Steering rack front small.jpg [ 746.61 KiB | Viewed 872 times ]
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PostPosted: May 20, 2018, 3:40 pm 
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Interesting build. Have you looked into what it will take in your area to get this registered for use on the road? It seems nearly every state has their own hurdles/walls.

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PostPosted: May 21, 2018, 1:31 am 
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Hey Rumbles
I find this interesting with the rack perched high.
I aren't no mathematician, so I have simple rules of thumb when designing my front ends for minimal bump steer.
First - the UCA, LCA, and ITR pivots must all line up in plane on the PS and DS.
Second - the LCA and ITE, adjusting tube if used, and OTE of the rack assembly must be parallel to each other
Third - go have fun, no bump steer

Please explain your steering geometry with the rack perched high, I'm interested. Is it parallel to the UCA? :cheers: :cheers:

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PostPosted: May 21, 2018, 12:30 pm 
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His steering arms are that high already... when you put the rack coplanar with the upper or lower A-arm pivots, you eliminate rack height as a source of bump steer.


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