custom cromoly DH FS Frame

Hi

after a lot of reading and planing I will present my project here. My goal is to build a FS frame with Cromoly tubing and Hub Motor first maybe convert it to Mid drive later.
i have designed a frame in CAD and started building a frame JiG for it. I'm going to use a Ht 3525 motor with a lyen 18 FET Controller and 20S4P Lipo from turnigy 4s Hardcase packs including methteks HVC LVC System.

Here's the major things I figured out building my frame
The headtube needs to be made pretty thick and machined prior to welding. Just make it a straight tube. ID is 33.7mm I think. Just check your headset cups.
The bottom bracket shell needs to be really thick or it turns oval after welding and you can't get the bb in. The ones they sell from Henry James and cycle-frames.com are meant for brazing so they are too thin for welding. Gotta make your own with thick wall tubing.
The suspension bearings need bushings on both sides, so 4 bushings total. they are really cheap fastenal variety from the auto parts store. I used 5/8id 1.375od bearings which are pretty easy to come by
Make the dropouts clamping style and cut them just over 10mm slot, certainly not under
make the disk brake adapter bolt on and adjustable !
The head tube gusset goes on the down tube if you want a gusset.
Make multiple holes for the suspension so your travel and geometry is adjustable

Remember your bike will be somewhere between a bicycle and motorcycle, so build it stronger than a bike with as wide at the pivot as possible. In the interest of going mid drive later leave extra space, wider than now necessary between the swingarms, etc.

@Iamsofunny
Thanks a lot for your experiences I will definately consider what you said.
What material did you use and how thick was the housing?
I wanted to buy a BB shell but your right I think i get one made. I will get the Head Tube maschined after welded in place.

@John in Cr
Yes I tried to have as much space at the Jackshaft as possibel It is however only 112mm.
the Jackshaft for a mid drive will also be the Bolt for the Swingarm.

I will upload pictues of my CAD Drawings tomorrow.

I'm almost done building a frame myself. Post some pics of your cad rendering!

You're going to have the same problem with the seat tube, too. Everything is going to want to warp and oval as you weld. I tried to plan for that and got thicker tube, but the seat tube is turning out to be an issue. its going to take ages to sand it out round by hand, and its not something easily machined.

And the bottom bracket, they make special heat sinks that keep them round. I should have done that, But I didn't and may have to cut mine off and try again. Lesson learned. It would also be better to have the threads cut after the BB is welded and machined.

Get a 100mm wide bottom bracket off a sand or snow bike, then you can go with a much wider pivot point.

here are some pictures.
The Design of the mainframe and swingarm is complete. missing ist the design of the dropouts especially the mounting on the swingarm.
And I have to adjust the Bottom Bracket position and design the mounting of the seat tube.
However this Cad modell is far from perfect since I redesigned the main frame and did not work on the assembly much since then.

Can anyone please tell me how to integrate the pictures as an image in the post?

More specs of the frame.
material 25crmo4 tubing 1.5mm or 1.2 mm thickness (the one i get easier, I would prefer 1.2mm) similiar to 4130 cromoly but less carbon
swingarm length 500mm weight 2kg (1.5mm)
Wheelbase with 200mm Front fork and no sag 1275mm with % sag ca 1300mm Head angle 66 degree(unloaded) to 64 degree when in full compression
estimated main frame weight 7kg. (1.5mm)
rear travel with 89mm stroke shock 266mm.

The main frame will get motorcycle style gussets from sheet metal on the uper and under tubes and at the end between the tubes that are welded on the swingarm mount.

Here are some pictures of the parts i will use.

I rode the dirtbike on the weekend and it does feel a bit like riding my bmx bike in my childhood, since it only 15" and I am 1.94 m tall now
But It has a strong frame I got it from my brother and I think it should wheelie pretty easy :-D

Any suggestion for a clamping removable dropout design and especially the mounting would be appreciated.
I want to fix the dropout by form and force (screw it on) but I am running out of ideas in the moment. :?

Hey guys

I just tried to whire up my controller in my test bike and see if the motor spins.
I tried two combinations of Phase and Hall wires. The second one already worked smooth and silent with good response to the throttle, freewheeling current 1.1A at 30.5V turning in the wright direction.

Do you think thats the wright one or schould i look further to be sure?

Thanks

That looks about right. IIRC the false positive combo that can sometimes happen draws like 4 amps.

Have you got the frame done yet?

Thanks
No I didn´t make any progress on the frame yet.
But I do have the tubes now and soon a shaft for the machined parts so I hope I can post some pics soon.

I took the testbike for a spin today . A lots of rain and duck tape but it worked nice.
I did run a really tame setting becase the batteries haven´t been on a bike and I ran only 20S2P instead of 4P.

I programmed 15 A in the Controller but the CA showed 20A when ever I gave full throttle does anyone know why?
The Shunt resistance ist 1mohm and programmed into the CA.

I also noticed the CA was on while I was rolling down a hill even when I unplugged the battery.
I guess this means regen works but when and why does it kick in?
I think I even disconnected the Connector for Regen.

Last question the thread for the freewheel sheared off on the motor. Does this happen often ?
I was thinking of getting a cover machined to prevent this.What do you think?

If I were building a FS e-bike, I would try to incorporate a floating brake for the rear. If you plan to take it off-road or travel at high speed, it would be good to avoid brake jack as much as possible. Also, when it comes time to cut/miter tubes, remember that closer fit BEFORE welding means tighter tolerances AFTER welding. A good fixture for machining and cutting is every bit as critical as the fixture for welding (if not MORE important.)

Hi sigimem

Would you mind sharing your CAD drawings? (DWG,DXF, SKP ?)

I really like the frame setup.

(John in Cr's comment - Remember your bike will be somewhere between a bicycle and motorcycle, so build it stronger than a bike with as wide at the pivot as possible. In the interest of going mid drive later leave extra space, wider than now necessary between the swingarms, etc.)

Very valid statement -GO BIG on the pivot, GO WIDE on rear stays (min 150 mm inside rear stay or bigger, you can always shim to fit the biggest badass frocker if you don't convert to Mid Drive later.)

As for the rear dropouts - go MX style REAR HORIZONTAL (Nice, thick and juicy! also a lot easier to adjust chain tension issues.)

If you are planning to go mid drive later - where are you considering your motor mount and reduction ratio if planned?

I might be wrong but I like the idea of keeping the weight (battery, motor setup) between the seat tube and down tube. Lower CG?

As for your question on the CA (Cycle Analyst) - I have the direct plugin vs. 2.25 - Just the bomb piece of kit! Coupled this to my current ride and have the utmost confidence when riding - (Got a shite LifeP04 battery pack 2C 10Ah) - On the CA I just dialled in LVC (Low Voltage Cut-off) and 9 AMPS max and often see MAX Current draw of 19-21 amps on pull off (Yeah, I get lazy and WOT the twist grip on pull off but everything settles back down when I hit max 25 km/h . I don't get the best Wh/Km ratio but I do abuse that twist grip!

My readings off the CA -

Wh/Km - 6.5 (Embarrassing)
WATT-HRS - 416
VM (Volts MIN) - 31.1
REGEN -0 (Geared Bafung).
A MIN - -0.01
A MAX - +20.22
MAX SPEED 55.4 Km/h (Down the "Mineshaft -AKA death bend" hill - Always a laugh on the way home - Would love to hit that on a flat when I get a real E-BIKE!).
AVG SPEED 14.7K m/h

Like your approach and bike building JIG - Subscribed.

Cheers Mark

Sigimem, don't mean to rain on your parade, but that swinging arm layout is going to have HUGE chain length variations as the rear suspension moves through its travel. You will hav trouble keeping the chain tension reasonable. Going to a mid drive layout will mkae it even worse when the motor power will introduce huge anti-squat forces on the suspension through chain pull. The swinging arm pivot should ideally be concentric with the bottom bracket. There is lots of very good information out there on frame design.

I hate to say it but the only way this will ever be a decent handling bike is if you replace the rear shock with a solid rod.

Also, when you're using CrMo tube, you need to have the frame heat treated after welding, as it the welds don't regain the strength without it, and the welds WILL break eventually.

2moto said:

Sigimem, don't mean to rain on your parade, but that swinging arm layout is going to have HUGE chain length variations as the rear suspension moves through its travel. You will hav trouble keeping the chain tension reasonable. Going to a mid drive layout will mkae it even worse when the motor power will introduce huge anti-squat forces on the suspension through chain pull. The swinging arm pivot should ideally be concentric with the bottom bracket. There is lots of very good information out there on frame design.

I hate to say it but the only way this will ever be a decent handling bike is if you replace the rear shock with a solid rod.

Also, when you're using CrMo tube, you need to have the frame heat treated after welding, as it the welds don't regain the strength without it, and the welds WILL break eventually.

Click to expand...

2moto brings up some very good points. You could probably use a jackshaft system and put a drive/idler cog at the pivot point. That would solve that problem. It might be good to have when you switch to mid-drive, too. And take heed to the advice regarding heat-treatment. The heat-affected zone on chromoly is pretty weak. The weld might hold, but the HAZ will be MUCH weaker. The same goes for just about any alloy, though. Brazing actually produces a more distributed HAZ and can sometimes be a better method than TIG (if you don't have access to good heat-treating facilities.)

I second the jack shaft location Idea.

For joining, you can stagger the welding by welding only a small section of something like 1 or 2 cm at a time, and move to the opposite side of the frame joint. Repeat until finished. This will help minimize the warping. Works with brazing also. Also, when welding I always found it best to use copper (brass a close second) plugs or clamps to hold critical shapes. Copper also transfers gobs of heat and will reduce the HAZ.

The HAZ and its effects will vary greatly with the specific alloy as well as what the existing heat treatment or work hardening is of the tubes. Also important is if they are seamed or seamless tubes. If using drawn butted (thicker at the ends) tubes, I would braze as you will loose all the advantage of the drawn /work hardened steel if you re heat treat the frame. Doing this this would be a major downgrade to the original tubes. Post heat treating a welded frame will have no such issues if you start with soft tubes, but be ready for possibly big changes to the frame in heat treating unfixtred.

Best luck with the build.

I guess i did not point that out. The bolt that holds the swingarm will also be the jackshaft. For the first version maybe a simpler version and later a tube with sprockets on it .
So there should not be any lengthening of the chain when the rear swingarm is moved.
@ 2moto do you see another reason why the bike should have a bad handling?

For the floating brake I want that on my bike I just haven´t found the right way to get it in my dropouts and swingarm nicely.
The bike layout was much inspired by the superco silencer and a homebuild that looks similar http://www.littermag.com/2009/wp-content/themes/mimbo/images/superco-silencer-downhill-bike2.jpg
It also has a floating brake and because i ride motorcycles I like this idea a lot.

I will make a test weld with ambient temperature and 200 degree with the tubes and fortunately I have the chance to analyse the welding at my university. So I can actually know how my material behaves after the welding. I hope I can avoid heat treatment after the welding.

However at the moment I don´t have much time so the bike build will not progress much until february .
Even my test bike does not run anymore probably a Hall sensor.

Thanks for your input I didn´t look in the thread for some day, as I thought not much people answer nice to see some do

The jackshaft will solve the chain tension issues that would otherwise arise from the suspension movement. Due to initial angle of the swinging arm, you'll still have a substantial amount of wheelbase change, though. Why not engineer it to co-locate the swinging arm pivot with the bottom bracket? That way you'll save weight (elimination of the jackshaft, chain, bearings, etc.) and solve most of the other problems associated with that design.

It just seems that your design (and that of one in your picture) introduces unnessary and unwelcomed problem. Maybe I am missing it but what problem that your design solve?

The point in the hole frame is that the rear swingarm has the same direction of movement as the fork.
Therefore ist should react sensibel on small bumbs etc.
Another thing is that the frame is designed to have a stiff "bridge" between the Steering tube and the swingarm pivot point.
The wheelbase varies within 0-20mm if the compression of the suspension in front and rear is equal.
There is a weight distrubution to the rear wheel especially when fully compressed but i didn´t see that as a negative point. Another point is the jackshaft is needed for a change to mid drive if it ever happens and the frame is going to be really wide at that point.

sigimem said:

The point in the hole frame is that the rear swingarm has the same direction of movement as the fork.
Therefore ist should react sensibel on small bumbs etc.

Click to expand...

Except the front and rear never compress at the same time.

Yes but a bumb will cause the wheel to move up and back and with that pivot point and swingarm length the rear wheel will move in this direction, at least more than with a low pivot point.