Mounting Issue

[Anhevius]

I am in the process of getting everything needed for my first build (and holy crap is it a lot for what I'm planning on).

I have hit a bit of a conundrum regarding my bike though.

I have a good solid bike, a Malvern Star. There are two concerns I have. The first is that since it has 20" wheels, how will that affect mounting the sprocket clamshell?

The big issue though is the bottom 'tube' of the frame. It's a rather unique shape, which serves no real purpose in my eyes other than a great place to stamp the company name on the bike. Since it is most certainly not round, I had the thought of shaping a new bracket with a simple square design. Inside of it would be two rubber blocks that go around the frame to make the mount solid. I'm talking about a high-density hard rubber here, not something soft and gummy.

Has anyone done anything like this before, and what were the results given? I really don't have the money to be going out and buying a brand-new bike, especially with the cost for them down here. This bike is in good mechanical condition, frame is solid, and it already has thumb shifters on it instead of twist.

So, will my idea work, or should I just work on saving up for a different bike? I do have access to all of the tools needed for a simple fab job like this, so that's not a concern at all.

Here's a pic of the bike. If more are needed, just let me know. https://www.dropbox.com/s/wah31twasydalrz/2014-03-07 16.08.28.jpg

 

[HeadSmess]

i stand by my guns....scrapyards are the source of good frames, on the cheap... surely you southerners have scrap yards? other than chains, i havent set foot in a real bike shop... they dont want to help, they just want you to get that $3000 FS scott off the rack and cough up for it, never mind why you actually need 8 inches of travel front and back...

only issue with 20" wheels is needing a 30-36 tooth sprocket. but they arent 20" wheels in the pic? 24" possibly? count the spokes. if theres 32, the clamshell wont fit. designed for 36 spokes, they are.

id be simply welding mounts to it, as long as its steel and not alloy...

 

[butre]

20 inch wheels might be a problem, depending on the spoke pattern. how many spokes are there?

For the bottom tube, you're on the right track, but ditch the rubber unless you just really don't want to scratch the paint. if the engine can move at all it becomes an unreliable mess, and ideally the mounting should be as solid as possible.

 

[Anhevius]

They're eighteen spoke wheels (or seventeen, I was kinda tired when I counted them) and the spare tubes I got with the bike are 20" tubes. (Got it from someone on Freecycle)

As for a bike shop, that ain't happening. I'll get one from Big W if I really need to, it only needs to last about a year anyways.

The scrap yard idea isn't a bad one, except that I don't have a car or any way to get to one, nor do I know where they are (I've moved here from the US, still learning where everything is). Plus I really don't look forward to assembling a bike piecemeal, though if I have to go that route I'm sure I can find a decent scrap yard.

Butre: I couldn't care less about the paint. I'm looking at how feasible crafting the support blocks to hold the mount steady will be. Plus I just don't have the money to be buying large blocks of metal I'm trying to find the type I've used before, it came in blocks, and was almost as hard as some metals. Though honestly, I'm tempted to just cut the frame apart and replace the bottom tube with a different one. It'd be less trouble, though that depends on the welding style used to make the bike frame.

It's all not a big issue yet, since I'm buying all the little stuff first, then the motor and everything to get it going right, and THEN the bike last. So if worst comes to worse and my best option is buying one at Big W or something similar, then I can just save up and wait for it.

 

[HeadSmess]

you counted on one side only they are 36 spoke wheels, so the standard sprocket fits.

if you got 17, you cant count. double 17 is 34 and 34 is NOT divisible by 4! ie, theres NO wheel out there with 34 spokes. has to be divisible by 4. 28, 32, 36, 48, etc... radial spokes might not obey that rule but i avoid radial spokes like i avoid dog poo...

i assure you, that if that picture is the bike you have, that it does NOT have 20" wheels. look on the sidewall of the tyre, it will have something like "26x1.75" on it. and the rim should have the same sort of number stamped on it somewhere, normally next to the valve hole. as long as the 26 bit matches, its right



if youre already thinking of welding...just WELD THE BRACKETS ONTO THE FRAME. space em out on some tube so you can get the bolts in, something like that so much quicker and easier then...


hardest bit about the scrappers is finding the one that lets the general public wander around... i love my local scrap yard probably half his income, the people wandering...scavenging....looting

 

[Anhevius]

The problem with direct welding the bracket to the frame is the shape of it. It's not easy to see in the picture, but it's a sharp-pointed oval, so there will need to be SOMETHING in there to disperse tension and vibration, as the top point is only about 5mm wide, and it's a steep drop angle from there.

You were right, I only counted one side, and it was eighteen. I double-checked it a while ago and re-counted. I just went out and double-checked, the wheels are Weimann 26x1.5/1.75 (Aftermarket maybe? The guy I got it from said he did a whole bunch of work on it, including new tubes (with Slime in them no less!) and tyres, and a new chain plus gave me two brand new butyl tubes. He was honestly very awesome about it, since he gave it to me through freecycle, even met me at a train station so I could pick it up easily.) I'm not sure why the guy I got it from said they were twenty-inch....maybe he meant the frame height and not the wheel size.

So, going by that, I think replacing the motor mount bolts with some MUCH longer ones, and welding in some support material to the bottom tube is probably the way to go, for the time being. I'm pretty sure the frame is steel and not aluminum, since it's supposed to be a heavy-duty mountain bike and not a lightweight street one.

This is what I get for trying to think when I'm sleepy.

And yea, I had a yard like that back when I was in Florida, you signed a waiver and paid $5 to get in, then whatever you wanted you paid for by weight, at the prices he paid people that brought it in. He didn't make anything off of selling the stuff, but got a bit extra from the entrance fees. I never paid because when I went looking for things I would sort the piles I was going through into the various grades so his guys just had to scoop it up instead of sorting then processing.

I admit, this is actually great, since it kills a large portion of the expenses I was looking at. Now all I have to do is figure out why the rear sprockets skip randomly when it's on the smallest gears. I'll prolly do that when I take the bike apart to repaint the frame (that horrid yellow color needs to go in the WORST ways!)

Thanks for the help!

 

[Anhevius]

Well crapsticks.

Went to go look at the wheel sticker again...and saw another on the frame. 20" frame, so my guess was right. The one under it though has me annoyed. It's an alloy frame. Guess I'll have to block the mount, instead of direct welding it. Though I suppose I could weld a steel plate along the bottom tube, and then weld the mount to it. Not sure how well a steel to alloy would hold though, even with the MIG set to about two or three. Now if I could get my hands on a stick welder, that wouldn't be a problem at all!

When they say alloy frame, exactly WHAT is it an alloy of? Cause if I need to, I could always get my hands on several Etch-A-Sketch's and flash-weld it on (yay thermite!) if need be, or even use a copper or tin plate buffer. Still cheaper than buying a new bike.

 

[HeadSmess]

usually 2021, if its a malvern star, from memory. cheaper rather than stronger, anyways.

well, weldings out of the question, that parts sorted dont even contemplate alloy to steel (it simply doesnt)... even if you got a tig with AC and hf start... exhaust clamps is the quickest option, fully fabricated mounts 2nd... im thinking something like some heavy wall square tube, cut diagonal, to make two "vees". lug them so you can clamp em together tight, and have the (separate) engine mount bolts attached to the top "vee" via some plate or whatever...

yellow? my frame is pink! and she must have been a big girl cus its a 24" frame! freaking HUGE! cromo shogun from my favourite source for $20

well, the few remaining scraps of paint are pink, at least... the bits the rust hasnt covered yet


as for the chain jumping...

stiff links. bent sprocket teeth. out of index. hi gear limit screw needs adjusting. thems the only issues i can think of currently.

 

[Anhevius]

I'm thinking stiff links, since I haven't ridden it much and it is a brand-new chain.

I honestly need a taller frame eventually (two meters all, and my boots rub against the front tyre when I turn it if they're forward).

Now, another crazy question. I'm unfamiliar with bicycle chains, my experience comes with motorcycle chains. For my motorcycles, I would use 140 weight gear oil (for a car's rear diff), with a tube of teflon powder and a tiny bit of petrol to thin it out so it'll go through a squirt nozzle easily. Would that work for a bicycle chain, or should I be using something else instead?

Though, with the 2021 alloy, wouldn't I be okay to weld on aluminum instead of steel to make a mounting point for the bracket? I admit fabbing a bolt-on mount plate would probably be a thousand times easier, but I like to eliminate as many ways for things to go wrong as possible. What I was thinking would be a simple double-layer collar with a flat plate on top, and a block of aluminum over that. Have the mounting bolts pass through the block and flat plate and bolt to that.

Or is 2021 just absolute crap for that idea? I will freely admit that my welding experience is with steel and iron only. Things like heavy machinery repairs, industrial fabrication work, and frame work on old cars.

 

[HeadSmess]

as long as you got facilities for alloy welding, it will weld problem is, do you trust it once it loses any heat treatment it may (or may not) have received in manufacturing?

general consensus is alloy frames snap due to fatigue. no point adding in the stress of welding. make steel brackets and spread the load out a bit, theyll be fine...


anything works on bike chains, i prefer letting them sit in a tub of molten wax and graphite, myself, but ill use anything on them...if i ever think of it. if the things just grumpy on the small sprockets, hard shifting...then its index issues, just adjust the cable for that one. could also be the wrong chain, check clearance between it and adjacent sprockets. a new chain shouldnt have stiff links. not stiff enough to be an issue, at least. not going into high gear or overshooting it, is the limit adjuster screw, and of course, could just be some gunk around the tensioner sprockets (the two dangling on the shifter) or the tensioner cage is bent. or its been put on incorrectly.