Hobbit engine+bike build/photo dump

Joe Schuitema /

A quick background:

Was into mopeds. Had a maxi and ups magnum for a few years. Moved into 2 stroke motorcycles and sold puchs. Sold motorcycle and rode a loaner hobbit at the ghost riders birthday ride. I now own a red hobbit.

My plan for the engine:

- Treats Metrakit "Airsal" 70cc kit

- Aluminum stuffed race crank with good bearings/seals

- Vforce reed block with angled block adapter

- 26mm carb/intake

- My own pipe, based on the destroyer dimensions

- Pa50ii ramp plate, stiffer clutch springs, notched variator

I thought about modifying the rest of the bike, but decided it would never be what I would want it to be. So, I'll be building a new bike that'll use the hobbit engine/subframe. Some points of the bike:

- Hydraulic forks off of a pw80(26mm fork tubes)

- Double leading front brake hub off of motorcycle

- 48" wheelbase

- Long tank, clip ons

- Steel tube construction, 1" diameter, 1/16" wall thickness

- Light bike, lots of fast, lots of brake

My plan is to have the bike built and ready for the ghost ride next year. If that fails, I'll put the engine on the hobbit frame and have that to ride. I've started the build about a month ago so I've got lots of pics and information to post. I'll break it into a few different posts for ease of reading.

Re: Hobbit engine+bike build/photo dump

Joe Schuitema /

I first notched the variator and turned the surface down on the lathe to get rid of the worn in bump.

Next is the reed block. I bought the vforce reeds off ebay for super cheap. A buddy of mine has the trickmetric spacer/adapter but doesn't like how it's parallel to the case, rather than angled to promote better airflow. Here's my solution:

Re: Hobbit engine+bike build/photo dump

Joe Schuitema /

Next is the cylinder. There's a couple other writeups on this kit so I won't get too in depth. I added material to the hobbit cases and bored it out to fit the cylinder skirt. It's not necessary to add material to the transfer ports. However, I thought it would be a good idea down the road if I get a bigger kit, I've already got the work done.

I took a sanding drum to the exhaust port to smooth out the cast aluminum. I also case matched the cylinder to the case. I really opened up the boost port on the base so it'll match the case well. The piston got a window. I wasn't sure how big I wanted to go. I think I'll make it more square on the bottom with large fillets like the top. The bridge on the cylinder will be removed.

I also powdercoated the cases after I was finished with the dremel work. I found its easier to keep them clean and they look sharp. the varnish look on the machined surfaces is just residue from the tape I haven't cleaned off.

Re: Hobbit engine+bike build/photo dump

Joe Schuitema /

The head. I've got a pullstart I made for the bike so no decomp for me. I machined a threaded plug to fill the hole. I thought about welding around the finished plug but I fear I'll warp the head. I may put a set screw further up to keep it from possibly unscrewing.

Another guy posted a base spacer of 1.5mm with two 0.8mm gaskets. I've got some 1.5mm gasket paper that I'll double up and will give me the correct height. I tested a small piece in a vice to see how compressed it would get. It went from 1.5mm(0.060") to 1.37mm(0.054"). Please keep in mind that is a lot of pressure in a vice, not 7-9 ft/lbs it'll be seeing on the bike. I wanted to make sure it wasn't soft and I'd loose torque on the head bolts.

This is what I've got so far on the new bike. I've been studying vintage 50cc race bikes and looking at how the frames are built. I've only got it in 2d so it's hard to imagine where the tubes are running.

Re: Hobbit engine+bike build/photo dump

Nicely done!....keep the pics coming...love watching the process.

Re: Hobbit engine+bike build/photo dump

I really want to see the intake/carb placement now... Are you building an intake or buying one?

Re: Hobbit engine+bike build/photo dump

Looks great!

What if you got rid of the tube in the middle of your frame?

2014-12-03_0758.png

Re: Hobbit engine+bike build/photo dump

Uneven distributed load... This is roughly what the frame is doing if you remove that member.

loaddistribution.png

Re: Hobbit engine+bike build/photo dump

What size tubes?

Re: Hobbit engine+bike build/photo dump

1/16 tubing seems awfully thin for such a beefy bike.

Unless you're running a shit ton of it, with gusseting and boxing everything in.

Re: Hobbit engine+bike build/photo dump

Joe Schuitema /

I'm planning on using 1" diameter 1010-1026 type steel tubing. Either .060" or 0.083" wall thickness. I'm leaning more towards the 0.083", most of the mopeds run that thickness and I should have some safety margin over weight savings. Definately gussets at the headtube and the shock mount area. This is a poor and nonproportioned rendering of what the frame would look like at an angle.

Re: Hobbit engine+bike build/photo dump

One other thing, I would use an aluminum spacer instead of 3mm of gasketing.

It might just compress "X" amount when cold, but with heat, n heat, n heat it very well might loosen up to an undesirable amount.

If it were me, I'd run a 2mm metal spacer (copper or alum) and then some thin paper or none at all with a thicker spacer. I've never had an issue with cyl base leaks just using yamabond on my spacers. Many will disagree, but it's worked for me. If it's prepped right, it's solid...

Re: Hobbit engine+bike build/photo dump

JBOT: Fear is the Mind Killer Wrote:

-------------------------------------------------------

> One other thing, I would use an aluminum spacer

> instead of 3mm of gasketing.

> It might just compress "X" amount when cold, but

> with heat, n heat, n heat it very well might

> loosen up to an undesirable amount.

> If it were me, I'd run a 2mm metal spacer (copper

> or alum) and then some thin paper or none at all

> with a thicker spacer. I've never had an issue

> with cyl base leaks just using yamabond on my

> spacers. Many will disagree, but it's worked for

> me. If it's prepped right, it's solid...

Yup

Re: Hobbit engine+bike build/photo dump

Great write-up so far!

Re: Hobbit engine+bike build/photo dump

Yea jbot is back. !!!!!

Re: Hobbit engine+bike build/photo dump

ahhh, that frame is really cool in 3d. Makes sense now!

Re: Hobbit engine+bike build/photo dump

Looking good Joe! Quality craftsmanship as usual!

Re: Hobbit engine+bike build/photo dump

JBOT: Fear is the Mind Killer Wrote:

-------------------------------------------------------

> One other thing, I would use an aluminum spacer

> instead of 3mm of gasketing.

> It might just compress "X" amount when cold, but

> with heat, n heat, n heat it very well might

> loosen up to an undesirable amount.

> If it were me, I'd run a 2mm metal spacer (copper

> or alum) and then some thin paper or none at all

> with a thicker spacer. I've never had an issue

> with cyl base leaks just using yamabond on my

> spacers. Many will disagree, but it's worked for

> me. If it's prepped right, it's solid...

yup, 2 mm aluminum is good.

Re: Hobbit engine+bike build/photo dump

Joe Schuitema /

I purchased some .75mm gasket material and made a 1.5mm aluminum spacer. I played around with the different thicknesses. I'm not sure what I was seeing yesterday, but I don't think I need all that base thickness. If I used the original 3mm base spacer, the exhaust duration would be 185 degrees and transfers at 138 degrees. Using only the aluminum spacer(1.5mm) the piston sits at or just under the transfers and sticks about .25mm above the bottom of the exhaust port. The port timing at this height drops the durations to 177 for the exhaust and 126 for the transfers. Those numbers to me seem like a winner. The small amount of piston in the exhaust port is negligible and can be dealt with with some sanding. What I didn't want was the piston to drop below the port and have a lip the gasses would ram into.

The piston pin needs some spacers on either side of the conrod to keep the bearing centered. I drilled out some 5/16 thru-hard washers out to 10mm so they don't wobble about when running. At 1/16" thick, two of them gives me 0.5mm of side play. I do like the big oiling hole in the conrod.

A question regarding the base spacer. Should I run the aluminum spacer with goop or use one of my earlier made 1.5mm thick gaskets with goop? I'm leaning towards the aluminum, both for removal in the future and keeping the piston height and squish consistent.

Re: Hobbit engine+bike build/photo dump

if you use aluminum for the base gasket you can anneal it at 400 degrees in a conventional oven for like 30 minutes... turn the oven off and let it cool slowly. a thin smear of goop or spray copper to seal it.

Re: Hobbit engine+bike build/photo dump

I don't know why I didn't even think about cutting the bridge on the skirt port, I knife edged it like a dummy.

Re: Hobbit engine+bike build/photo dump

Cut every bridge off... I cut them off pistons and errything...

Re: Hobbit engine+bike build/photo dump

Benjamin LaPlante /

The powder looks nice! Did you preheat the cases before applying?

Re: Hobbit engine+bike build/photo dump

Marc Friedman Wrote:

-------------------------------------------------------

> Yea jbot is back. !!!!!

^ this, and great work!

Re: Hobbit engine+bike build/photo dump

Joe Schuitema /

Good advice on annealing the aluminum. My toaster oven/powdercoat oven goes up to 500 degrees so that'll work. I did not preheat the cases. Just a good connection between the cases and the powder gun.

I took 0.75mm off the head and 1.25mm off the top of the cylinder. With the 0.5mm head gasket, It'll give me 1.14mm of squish. I didn't measure the head volume before I decked it. The volume with the material removed is 7.4cc. I haven't measured the squish band area percentage but would like to keep it around 40-45%. This one seems a bit thick and I like to keep the msv on the low side to avoid preignition at high rpm.

Re: Hobbit engine+bike build/photo dump

inspirational

right about now...

Re: Hobbit engine+bike build/photo dump

If I can offer my $0.02 on the frame construction... the design looks great, but I think you're overkill on the tubing spec. I would be going with something like 3/4" x .049" 4130, much higher strength to weight ratio, but not insanely expensive either. It's more expensive than mild steel, but if you're going to put all that TIG time into it anyways, I feel like it's worth the extra dollars. Wick's Aircraft Supply (a web store) will do small orders and has pretty good pricing.

I've got a custom frame built from (mostly) 3/4" x .035" 4130 that isn't anywhere near as rigidly trussed as your drawing, and it's, uh, quite stiff.

see here

Obviously there are design differences but I'm just offering a reference point from my personal experiences.

Also, we have a Hossfeld bender with 3/4" and 7/8" dies at my shop if you need someone to bend some frame components for you. (I'm looking at the sideview with the parts that mate to the front motor mounts)

Either way, looking good.

Re: Hobbit engine+bike build/photo dump

Garth your flickr is just as awesome like this thread!

Re: Hobbit engine+bike build/photo dump

Joe Schuitema /

I briefly though about using chromoly early on, but was worried about cost and welding/machining. I'll need about 20 feet of tubing to complete the frame. If I use mild steel, it'll be about $40. Chromoly in your size is about $85 for the 0.49" wall thickness. However, I save about 9lbs on the frame. I did some reading on the steel and I don't think I'll have any issues with cutting or welding it. Chromoly it is. I modified the frame to the new 3/4" tubing and extended the lower tubing up to the top of the head tube. I've seen many frames with it like this. Once the plans are finalized, I'll start ordering materials.

Re: Hobbit engine+bike build/photo dump

Nice, 9lb is a big change! I don't know what your plan for mitering and welding is, but we usually just use Starrett hole saws in a milling machine. Indexing miters on opposite ends of tubes gets a lot easier with the aid of a pair of tube blocks.

Not sure what your plan for bending the back of the seat-area of the frame is, but you might want to get in contact with a tube bending place to see about them doing those bends, and might have to design around the specific centerline radii of their bending dies. Then again I don't know what you've got in your shop :)

Want to reply to this thread?

We'd love to have you join the discussion, but first you'll need to login (or create an account).