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    Default V8 springer front end

    I'm in the process of building a v8 trike, wheel base=122", 350 sbc, 700r4 tranny, alum radiator. I want a 45 deg rake springer, what should I use for fork legs? I was thinking using Dom! Any thoughts? Thanks

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    How long are the legs? Is there a brake system? What is the weight balance?

    You can certainly make something strong enough with DOM, but that's like saying you can make a building with I beams. Are we talking a pole barn, or a skyscraper?

  3. #3

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    Understood, dual caliper, 38" from top of top tree to bottom of back leg, 45*, approx 46" from center front wheel to center
    Of motor approx 600# when complete, 700r4 tranny, 200#, wheel base 120". Thanks 53Bash

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    It's probably high, but lets guess it has about 500 lbs of a 1000lb gross weight on the front wheel. Good to be conservative, right? (Gross includes rider, fuel, etc, so 1K may actually be low, but 50% of the weight on the front seems high unless you intentionally front load it.)

    You want it to be able to support about 2,5000 lbs vertical load. That's double (for safety) what kicking the front end up at 2.5g as you go over a bump would put through the fork. (For my bike I went to 2 x 3g, but its meant for thrashing over dirt roads and such, and is barely light enough for such use.)

    Its also needs to tolerate about 2,000 lbs braking force; that's double the force from pulling 1g through the front brake.

    These numbers roughly line up with tests used by various transport agencies, or so I'm told.

    With a simple tube at 45 degrees, you really only need to consider the 2,500 lbs - if it can take one, it can take the other. So having brakes or not doesn't really matter.

    I'm looking at http://www.meracalculator.com/engine...tube-beams.php

    For a 38 inch long steel tube, 1 inc diamter, .1" wall thickness, 2500 lbs load gives 1,638,993.994 lbs bending stress. That's WELL beyond yield. That needs to be down around 55,000 psi.

    Going 1.5 and .125 wall gets down to 553,773.813 psi - still much to high.

    Going to 2 inch and .1875 wall gets down to 214,390.468 psi

    Going to 2.5 and .25" wall ... still 104,895.616


    Soooo... either my calculation assumptions are bad (they are VERY conservative, and I might be looking at the wrong sort of bending) or you will need some massive structural steel sized forks.

    Assuming my calculations make sense, I'd say the solution is a fork where the supporting legs are not round tubes. You could do a girder, or a springer can have the back legs be girder shaped with a springer, or you could use rectangular tube. An 8x2 x.125" rectangular tube gets down to tolerable stress levels (under 50k) but would be ugly as fuck and has extra weight down by the wheel. A similar depth girder should also work and could even be lighter material; a girder made of 1.25 x .125 tube with an 8 inch depth is (ideally) just as strong if not stronger.

    The rake of the fork maybe reduces the vertical load a bit. but cutting the load to (say) 1500 lbs still only gives a 40% reduction the the stress numbers, not the 10 fold reduction you would need to use reasonable sized round tube.

    Of course, if you actually have 500 lbs on the front tire, you will have trouble finding a motorcycle wheel & tire that holds up. So maybe the numbers above could be cut in half, and the fattest tube I listed would (probably) work. Thicker wall would up the margin of safety a bit - going to half inch walls gets you another 30% reduction in strain, for example. In general fat tubes (and girders - anything where the bend has to go deep) > thick walls.

    Crazy as these numbers sound, they don't seem out of line when you look at tube bender capacities - most small shop benders can bend any of the tubes I listed, and you are talking about bend loading your tube through a 38" lever arm.

    Take this all with a big grain of salt as I'm not even related to a structural engineer, I just hack my shit up based on info I find on the net. Even if the numbers are good, you should probably find some way to actually test your construction, say by making an extra leg, welding it to something immovable, and trying to bend the leg (measuring the force used to do so is the tricky thing - I managed to find a crane scale with a 2,000 lbs capacity and used a turnbuckle for the force.
    Last edited by 53Bash; 01-09-2018 at 8:22 PM.

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    Doh, just found one simple mistake. You have 2 fork legs, so the load is half what is on the wheel ...

    So, drop that load to 1000 lbs (less front wheel weight, 2 fork legs) and you almost surely could do 2.5 x .25" round tube legs. A bit big to be looking right, but not crazy big.

    2" x .5" maybe also works (about 10% more stress). Anything smaller looks sketchy, even if solid stock.

    Again, a girder could be a lot lighter. Plus, you can make a girder from solid stock without giving up much strength; A girder made form 1/2" rod is nearly as strong as one made from 1"x.125" tube. Again, assuming it is stable enough not to buckle, the strength of the girder comes as much from its depth as from the material, so can be MUCH lighter. If you used a girder made of 3/4" rod separated by 4"+ at the top, I bet you'd be fine.

    The concern you get into with a girder, though, is this thing is a TRIKE. Which means, unlike a motorcycle, side loading when going around a corner will be significant. I don't know how high the forces are, but suspect you'd not be cornering hard enough for it to matter. Still, worth keeping in mind in case you plan on drag racing or something where wonky handling due to the fork not holding a line while you try to steer might bight you in the ass.
    Last edited by 53Bash; 01-09-2018 at 9:19 PM.

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    Also, some portion of the vertical load goes through the plunger, so the rear leg may not need to support 100% of the vertical load - some of it (obviously) gets passed to the springs. If you aren't running brakes, this would allow for marginally thinner rear legs. Still, that's probably more a matter of allowing thinner wall tube than smaller diameter (which greatly increases bending stress).
    Last edited by 53Bash; 01-09-2018 at 9:22 PM.

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    Quote Originally Posted by okidims1 View Post
    I'm in the process of building a v8 trike, wheel base=122", 350 sbc, 700r4 tranny, alum radiator. I want a 45 deg rake springer, what should I use for fork legs? I was thinking using Dom! Any thoughts? Thanks
    There are a couple of posts from this Miller Welding forum site which may be helpful: http://www.millerwelds.com/resources...v8-trike-frame

    including this post by member Collie132:

    V8 Trike frame.
    Hi, I don't know if you have what you want yet, but if not I'm happy to help if I can.
    I built a V8 trike using a Leyland P76 engine and Trans. Jaguar XJ6 Diff.
    I built the frame, front end and body.
    I was told when I built it that the best rake for the front end was 47 deg.
    I used this and it's fine. I made a springer front end with the front set of forks bent to shape from inch and threequarter shaft, the rear set of forks are made from two inch x three eights wall thickness tube. Tripple trees are made from inch and a half med tensile plates. Main chassis is made from 2inch med black pipe. I'm happy to sketch any parts you may still require.
    Please email me at colchi@xtra.co.nz. Then I can send you a photo, As I don't know how to send thru this forum.
    My trike has been in many bike shows and won many trophys. It's not like any other trikes you may have seen.
    Thanks


    and these links: Welcome to Brothers of the Third Wheel; https://btw-trikers.org/
    and
    daytonabosshoss.com

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    53bash, you did some homework last night. I thank you very much. I'm gonna toss a few things around.

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    Thanks guys, that's a lot of info to let sink in this old head.

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    Quote Originally Posted by TriNortchopz View Post
    There are a couple of posts from this Miller Welding forum site which may be helpful: http://www.millerwelds.com/resources...v8-trike-frame
    Great sanity check. Looks like he went with 2" tube at 3/8" - a bit under what I was thinking, but again, I was being VERY conservative. For a realistic load of maybe 400 lbs or less on the front wheel, no reason it would not make sense.

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    Quote Originally Posted by okidims1 View Post
    I'm in the process of building a v8 trike, wheel base=122", 350 sbc, 700r4 tranny, alum radiator. I want a 45 deg rake springer, what should I use for fork legs? I was thinking using Dom! Any thoughts? Thanks
    You can use DOM tubing but the diameter will be very important.
    I would look at other V8 trikes to see what has been done.

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