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Learning how to build Lotus Seven replicas...together!
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PostPosted: May 10, 2018, 1:40 am 
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Hello, I recently got the book "How to Build Motorcycle-Engined Racing Cars" on Google Play, at $15 I couldn't resist.

My plan is to have it be a streetable car, autocross and trackday car. Not sure how to register it, but I assume that can be done. The best handling car I ever drove was a Formula Ford, long ago, and now I have time and much more experience in manufacturing/fabrication. I'd like to replicate that kind of handling, rotation of the car just seemed to be directly connected to my brain.

I've been collecting a few parts and doing some design in Proe. I used to do CNC mold work and there is a local hacker space with CNCs, so I hope to do that stuff myself. I scanned the engine/airbox with a MS Kinect and Reconstruct me software to get an idea of engine space needed.

At this point I have a used Torsen diff, 27 spline, I think. I plan to "seal" it up using some method as the book says. While the book suggests mounting the diff offset to match the chain drive of the motor, I don't like having different axle lengths, so I have a layshaft to have the engine centered and the diff centered. Might be silly, but I like the symmetry. I think rubber mounting the engine/diff assembly would be a good idea for comfort.

I also have urethane bushing on the inner wishbones for comfort.

I want to use CBR1000RR brakes (310mm), it will be a very tight fit inside a 16" wheel. Can I relocate the bleeder if need be? I think I can use some stock brake hats and a custom adapter plate to the disc.

I'm most familiar with RC racing cars which have very adjustable suspension, so I'm trying to keep all that in mind during design.

Some renderings. Hopefully this won't be a vaporware project.

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PostPosted: May 10, 2018, 11:14 am 
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Welcome, John! Yes, Formula Fords are the cat's meow. I was so glad I got into those as opposed to modifying sports cars for the track. It feels like your spinal cord plugs into them when you start driving one.

You're doing beautiful work in CAD there. Have you been able to apply forces to your front uprights and see the stress and strain?

Start learning about geometric and elastic forces on your suspension. With your inner pivot points at an angle to the car centerline your wheels won't move vertically, they will also move forward and rearward slightly as they rise and fall. That will introduce anti-dive / pro dive forces. I think you are OK, but understand that that is there.

I think you may be looking at brake problems around lack of mass in the rotors. Your car will weigh more than a bike and the braking distances will be shorter, so a lot of heat will go into those rotors. They will get less air too being shrouded inside a wheel. You can get good parts from places like Wilwood, so no need to use crummy street car stuff if that's what is bothering you.

Are you thinking making your own axle pins? You can get some standard pins both for welding and bolting on, I think. I'm interested in upright design and am leaning towards that right now for my car. I think it's a space that needs some attention because the kit car parts based on street cars are expensive now and out of date too.

Keep us posted!

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PostPosted: May 11, 2018, 12:04 am 
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I haven't really done much for stress calculations yet accept for the wishbones like the book suggests. I'm not sure if I can do FEA with my proe license.

The angled inner wishbone mounts are just kind of visually placed. "The Book" says that taper will help anti-dive and squat. I do have anti-dive and anti-squat designed in from the side view. I will look at the fore-aft movement of the wheel when the suspension moves.

I was thinking the bike brakes would be fine as on a motorcycle you are really just using the front 2 discs during full braking and in the car a good portion will be done by the 2 rears also. I would think that would be roughly in proportion to the increased weight, but you make a good point that overall, it may be capable of decelerating quicker than a bike. I was just planning on DOT tires, I suppose if slicks were used that could be an issue.

I want to make a test wood model upright to see how things fit and if the CBR calipers will even fit. That is a bad thing about bike calipers (at least the CBR) is they are shaped wrong for using inside a rim.

I would consider car brakes, a little heavier on the unsprung weight, but if it simplifies things, it would be an option.

I think I want to just use off the shelf bearing hubs that have internal splines (for the front also, to keep each corner the same). I'm just trying to learn nomenclature for axles, cv, splines, etc.

Thanks for any input!


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PostPosted: May 15, 2018, 1:15 am 
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Here's a mock up of the upright and brake. If I can move the bleeder there's about 1/8" clearance to the rim.

I also tried to do some FEA in Proe, not really sure what I was doing. But I fixed the center portion of the upright and applied 500lbs outward at the bottom surface and 300 pounds the opposite direction at the top surface. Depending on exactly how it was constrained, I got stress values from 2,000 psi to 6,000 psi. If that's right, I would think that should be fine.

After looking more at brakes, it seems the thinner discs say for "low or moderate" use. Is it the intense heat cycle that is the problem? I would think regardless of brake thickness the same amount of energy would be dissipated. A lower mass disc would get to a higher temperature quicker, but would also cool down quicker. Maybe air ducts would help. If the disc stays below a certain temp, would that make it "ok"? Is there a "fatigue factor" with a more extreme heat cycle?


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PostPosted: May 15, 2018, 7:22 am 
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I am not sure if it is the right analogy, but ....

My late but not lamented Dodge Dakota was 'underbraked' and had the annoying habit of warping brake discs after panic stops. I wonder if these 'light to moderate use' discs would suffer the same fate in autocross or track day use?

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PostPosted: May 15, 2018, 8:31 am 
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The thinnest formula ford disks that I could find are a hair over 7mm thick. Looks like the CBR1000RR disks are 4.5mm or 5mm depending on year, or about 30% thinner. Formula Fords are also ruining 13" wheels with light weight slicks. A 16" wheel with DOT tires designed for much heavy cars has a significantly higher rotational inertia then what you find on a formula ford. Then compare that to the rotational inertia of a motorcycle tire that has two disks to do the stopping power, and I have my doubts that you will have an adequate heat sink to absorb the heat from braking without overheating the rotors and pads. Having good ducking will help get rid of the heat between corners but I wouldn't think it would do much for the rapid heating under hard breaking.


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PostPosted: May 15, 2018, 10:00 pm 
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Welcome :cheers:

I can't comment on BEC or FF's. I have no knowledge in that arena. If being streetable is one of your goals, I would suggest you check into the requirements of your state before you get too far along. Different states have different and unique requirements. At least one state, I can't remember which, does not allow bike engines to be put into a 4-wheeled build. At least one state, Texas, is rapidly cracking down on track-looking vehicles and has revoked previously registered A-r-i-e-l A-t-o-m-s and will not issue a license to a DF Goblin. Not all states follow the SEMA registration which makes it easier to register cars that appear similar to cars of a certain age. Build requirements change as the state you are in and type of vehicle you build.

Recently one builder found that he couldn't get the insurance he wanted due to having a roll bar!

I hope your state will accept your chosen build path. Best to know before you have too much time and $$ invested.

Keep us informed on your build.

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PostPosted: May 16, 2018, 12:31 am 
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Yeah, I suppose I should look a little harder into it's "streetability" if I really want to drive it on the road. It's Wisconsin, I did look through the online info a bit, I think it would qualify as a "homemade" vehicle. I did see that "homemade" vehicles cannot be driven in January, well there goes the mid-winter touring plans!

Insurance too, good point.


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PostPosted: May 18, 2018, 8:57 am 
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I presume you've checked out Tundra 7's build? viewtopic.php?f=35&t=17160 Not only is it an exemplary project, but he's also in the Green Bay area so is going through registering it in Wisco. You will want to call your local State Patrol office to talk to the inspector to get the low down. If you happen to be in the general Madison area, the guy to talk to is Officer Schultz in the De Forest field office. Pleasant fellow, I might add. Good luck!

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PostPosted: May 21, 2018, 5:57 am 
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Some wishbone and chassis work. I decided to make the inner side of the wishbones all 18" wide on center.

The brackets that hold the wishbone are bolted to the frame through a threaded frame tube insert (welded). There can be different versions of the bracket to adjust anti-dive/anti-squat and some roll center adjustment too.

1-1/2" square 12 gauge steel tube for the main frame. What you see in the pics is 560 lbs so far.

Thanks for the tip on registering.


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PostPosted: May 21, 2018, 12:46 pm 
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1-1/2" square 12 gauge steel tube for the main frame. What you see in the pics is 560 lbs so far.


That's too much, basically. Start with 16 gauge x 1.25" for the main rails and 1" for the rest. You'll need cross tubes at each of the bends in the main rails. Early on you need to draw in the steering rack and coilover mounts. If you want inboard suspension that's an early area you need to flesh out too. Are you going to use trailing arms for the rear suspension? Those details will affect the frame a lot.

If you are going to register this for street use, I think you will be happier to go for a two seater. Unless you are planning for a solitary life without friends or lovers. :( Being unable to even hold a grocery bag will limit your use of the car and they are a lot of work to make. The extra seat will not really slow you down....Changing those tubes in the drawing from 12 gauge to 16 gauge will save more than enough weight to make it a two seater.

I used "Grape" to do my tube frame FEA. It's not that hard to learn to use. Grape does not mesh the frame, it uses simple equations that describe the stress and strain of tubing to do it's work.

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PostPosted: May 21, 2018, 6:56 pm 
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How about 1.5 16 gauge? That knocks 40% off the 80lbs of frame I have so far. It's also slightly cheaper than 1.25 and of course 17% heavier. But is 1.25 pretty standard for this kind of thing? And I suppose try to keep the same gauge for other tubes for welding purposes?

I'll check out Grape.

As far as 2 seater, I've considered it, but people already don't like how I drive my 300C. I want to have at least a couple cup holders for the drive through. :lol: Maybe 2 seats on my second build!

Both front and rear suspensions are angled slightly, not really trailing arms. "the book" suggest this taper actually helps with suspension action. With my adjustable brackets I can change anti-dive/anti-squat easily. The rear will have a toe link, so that will be able to adjust "bump steer" on the rear.


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PostPosted: May 22, 2018, 6:00 pm 
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Whoa, chicanes are gonna be a problem! :lol: Just futzing with FEA in Proe a bit. I think it's about 250ft-lb/degree right now.


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PostPosted: June 5, 2018, 5:37 pm 
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A little more design work on the frame. "cranked" the front lower wishbone for some more steering lock and using an actual spherical bearing on the lower joint. I now have Wilwood calipers and rotors on it. I was looking for the "floating buttons" for the motorcycle brake discs and found some for $10....EACH. That would have been $200 for the fronts plus custom parts. So the Wilwood route is looking more attractive and I shouldn't have to worry whether they'll be enough.

The uprights now incorporate the brake mount and are still the same part left and right.


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PostPosted: September 6, 2018, 8:11 am 
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any progress here ?


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