Follow Up - Diamondback crankset replace?

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8/16/2016 7:35 PM

Hi all,

My First Question:

To get ride of a rear coaster brake on a 20" bike, is it really as easy as buying a rim off Ebay and a freewheel sprocket and switching out the tire, or is it more complicated than that? I'm posting on this forum because I'm assuming someone can give me sound advice.

My Second Question:

I think the crank arms are too short and this is leading to the bike being hard to pedal. Can I swap out the current crankset with a new one that is has longer cranks? My goal is to make this bike smoother and easier to pedal for my son.

Story:

Frustrated Dad here. Got a 20" Dyno Blaze for my 7-year-old son (nothing special since it is from around the year 2000...not like the Dynos I grew up with!!!) off Craigslist for $20. He upgraded from a smaller bike to keep up with his older sister's on our bike rides. Anyway, the bike is very hard to pedal and my first thought was that is was due to the small length of the crank arms. While I still think this is true, I did notice there was some grumbling from the rear bearings. Being a tinkerer, I took apart the hub and then remembered it was the dreaded coaster brake...there is just too much friction in that hub for it to spin freely. Nevertheless, I took pictures as I disassembled it, cleaned all parts in paint thinner, repacked with grease and had a bunch of trouble getting it back together so it spun SOMEWHAT freely. Man, it was frustrating due to the amount of time and care taking it apart and putting it back together, only to be left in the same place: the bike is still hard to pedal and I think it is partly due to the friction of the rear coaster brake and the fact that the crank arms are too short.

Thank you for any input on how to make this bike easier to ride.

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8/16/2016 9:59 PM
Edited Date/Time: 8/16/2016 10:02 PM

Hello.

As for the rear wheel, the axle on your Dyno is a 3/8 size. On BMX bikes, 14mm axles, meaning you most likely won't be able to fit a cassette (freewheel) properly as this is the axle size that mostly all cassette hubs run. Keep in mind that is the hub itself that causes the bike to go when pedaling, not the sprocket.

As for the cranks, the Dyno uses a 1-piece crankset. If you wanted longer, you'd have to ensure that the next pair you get can accommodate 1-piece setups.

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Scooter kid trying to ride a bike.
@scootereyn

8/16/2016 10:48 PM

Could you post a couple of pics? I can't find any of a dyno blaze with a coaster brake.

You can usually disconnect a coaster brake. Does it have mounts for a v or u brake? Or a hole in the seat stay bridge for a calliper?
There are wheels on the market with 10mm axle for either a freewheel or cassette. But the cassette wheels are usually unsealed unless you pay a lot of money, and they're awkward as hell to work on if anything goes wrong.
You should also be able to find longer cranks, 1 piece, or cheap 3 piece cranks that can be used with an unsealed bb

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8/17/2016 10:57 AM

grumpySteve wrote:

Could you post a couple of pics? I can't find any of a dyno blaze with a coaster brake.

You can usually disconnect a coaster ...more

This, and even then you can get a replacement BB for 20-30 bucks pretty easily, and convert to sealed (have a shop install if you aren't confident in your mechanical abilities)

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"Hey anybody ever make that mistake like right when you wake up in the morning and you believe in yourself?" -Kyle Kinane

"BIKES!" -Tom Segura

8/17/2016 11:03 AM

A smaller front sprocket would be easier/cheaper if you want the bike to pedal easier. Your child has short legs (assuming, since he's seven years old), so longer cranks may not fit him as well.

But as stated, post a picture of the bike.

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8/17/2016 5:38 PM

You will spend more on parts, then what you paid for the bike.
I would take my kid new bike shopping.
Maybe invest in one for yourself
And ride together?

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F¬ Bom 5403692331

8/18/2016 7:49 AM

Photo

Photo

Question:

I have a 2003 Diamondback RM Jr and I'd like to install a smaller crankset. The a current crankset is: 1-piece 165mm, 42 teeth and the rear cog is 1 piece, 16 teeth. Can I purchase a smaller crankset for this bike that is 1-piece or will I need to replace it with a 3-piece set up? Do I need a "Diamondback" crankset or are there other brands that will fit? I am mechanically inclined (to a point), but I am a total novice when it comes to working on bikes.

Story/Follow-up:

I appreciate most of the responses on this thread. I ended up putting the Dyno Blaze up for sale (already have a buyer!) and just purchased a 2003 Diamondback RM Jr. for $40 (pics attached). This is what I wanted to do from the start: get a used, decent BMX bike for cheap. While I'm admittedly out of my league when it comes to BMX bikes, I can tell this bike is clearly not in the same realm as the Dyno Blaze bike I referenced in this post (just weight alone tells the story). For a used bike, this Diamondback is in good condition, but I have now come across the opposite problem I had with the Dyno: my 7 year old is probably too small for this bike as determined by the crank arm length. My thought is that if I decrease the length of the crank arm (set) and save the current set, he'd be able to have the bike for quite some time as he grows.

I appreciate your input.
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8/18/2016 9:46 AM

I think that crank is rather specific to this model and sprocket, so if you change cranks, you'll have to find an accommodating sprocket as well. Have you considered lowering the seat height? Maybe he'll be able to pedal like that, then, as time goes on and he grows, the seat adjustment will always be there to raise the seat.

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Scooter kid trying to ride a bike.
@scootereyn

8/18/2016 10:01 AM

I raised the seat so my daughter could try it as she's 9 years old. Even her knees were up way too high as she pedaled..

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8/18/2016 2:14 PM

Seats aren't really designed for sitting on on a bmx, especially whilst pedalling.
That looks like a 3 piece crank to me, with possibly a mid or Spanish bb? (could even be a euro, I can't see it well enough in the pic). Which would mean your only option would be a new 3 piece crank and a sprocket. If you just want to change the gearing your could replace the chainring.

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8/18/2016 3:44 PM
Edited Date/Time: 8/18/2016 3:46 PM

You need to get shorter crank arms. It's probably got like 165-170mm (estimate) and you need more like 145 I bet...

So first you gotta find out what size/type bottom bracket it has. Is that a Euro screw-in BB with 3-piece cranks/chainring or is it a 1-piece crank? Then you'll k ow what type of cranks you need to shop for.

As others said, hard for a kid to pedal sitting on a BMX withiut having too big an overall bike. If the reach from seat to bars is good, smaller cranks might be the only way he'll be able to extend legs AND only way he'll feel stable enough to start standing on pedals.

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Frmrly BmxBos
Ref: Robinson79, aaron.samuel.green

8/18/2016 4:34 PM

readybmxer wrote:

I think that crank is rather specific to this model and sprocket, so if you change cranks, you'll have to find an ...more

Sprockets are not part of the cranks, they can be removed and changed with 1 piece cranks pretty easily. You just have to take the cranks out and then remove the bearing race (which threads on) and the sprocket comes right off. They aren't brand or model specific UNLESS you want it to look exactly the same.

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"Hey anybody ever make that mistake like right when you wake up in the morning and you believe in yourself?" -Kyle Kinane

"BIKES!" -Tom Segura

8/18/2016 11:42 PM

readybmxer wrote:

I think that crank is rather specific to this model and sprocket, so if you change cranks, you'll have to find an ...more

dave lawrence wrote:

Sprockets are not part of the cranks, they can be removed and changed with 1 piece cranks pretty easily. You just have to take ...more

With the crank on the new one it has an integrated spider.

If the bb is square taper, you might be able to get a chainset for a kids bike that will fit it. Chances are it'll have a smaller chainring, which will make the gearing lower (easier to pedal). There's a cap over the crank bolt, take that off and take the crank bolt out, then take a pic of what's under the bolt. Also take a pic of bottom bracket from the non drive side. There'll probably be a really simple cheap fix if it's square taper.

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8/19/2016 9:38 AM

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Thanks for the replies. The guy at the local bike shop said I should sell it at the next BMX race at the local track. He said the mods I would do are not worth it since the bike is made soley for racing (small frame, skinny tires, basically no seat, etc.). He said I should invest in a 24" MTB that my son can grow into.

While I might take his advice at some point, based on the latest responses from the collective "you" (thank you!), I still might try the crank modification and then I bring my son down to the track to see if he has an interested in trying out bmx...

So, I have attached a picture of crankset with the BB dust cover off and the outer bolt removed (from the driver side). Can you tell if this BB is a square tapered version?

Thanks for all the input.
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8/19/2016 11:01 AM

grumpySteve wrote:

With the crank on the new one it has an integrated spider.

If the bb is square taper, you might be able to get a chainset for ...more

haha I actually didn't catch that. Probably a 110 MM spacing for that 5 bolt setup.

Chainrings are actually really easy to swap out-same with the cranks-you would need to pull the arms and put new ones on, no BB work at all (unless it's messed up of course).

You might even be able to find a set of cranks AT the local track. Lots of people sell parts at them on race and practice days.

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"Hey anybody ever make that mistake like right when you wake up in the morning and you believe in yourself?" -Kyle Kinane

"BIKES!" -Tom Segura

8/19/2016 11:46 AM

Photo

Photo

Sorry...just noticed you wanted a pic from the non-driver's side. Looks like I'll need to get a crank removal tool from the local shop (I'll bring the bike to make sure I get the correct type...) and if I do need to replace the BB, it looks like I'll need some sort of single or double toothed-type wrench (looking at that chrome ring...).

Once I get the crankset off, is there a way I can tell if "any ole" crankset will fit the existing BB? If I were to do a search on the internet at that point, what would I look for to match the BB? Or, should I bring the bike to the shop with the cranks removed so they can match a crankset?

Thanks in advance.
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8/19/2016 12:12 PM

That's a standard mtb style bottom bracket, with a square tapered axle. You can pick up a chainset for that for about £15 off ebay. Just measure the crank arm (centre of pedal boss to centre of crank bolt hole) and count the teeth on the chainring. Have a search on eBay for single speed chainset, then compare the crank arm length and number of teeth on the chainring. I'm sure you'll find one shorter, and the number of teeth will determine how hard/easy it is to pedal (more teeth harder, less teeth easier). It's a cheap fix for now.

Of course the guy in the bike shop will say it's expensive and you should buy a bike from him, that's his job haha. But a 24" mtb is neither a proper mtb, or a bmx. If you'd rather go the bmx route, just buy the chainset and it's done.
I mean, you could sell that probably quite easily, but any other bmx is going to be roughly the same size unless you get an 18" wheel, but your kid will outgrow it. Although an 18" would probably be perfect right now and will get your kid used to the feel of a bmx. It still won't be ideal for sitting down to pedal on though

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8/19/2016 12:16 PM
Edited Date/Time: 8/19/2016 12:17 PM

Agreed re forgetting the 24" mtb idea. Guy at shop said that cause that's what he has in stock.
Plenty of good bikes for kids that don't have massive knobby tires, a hundred gears, suspension, heavy parts, etc.
Why not a 24" BMX cruiser then? It's just cause that mtb style has become the DEFAULT, and it drives me bonkers

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Frmrly BmxBos
Ref: Robinson79, aaron.samuel.green

8/19/2016 1:12 PM

dave lawrence wrote:

haha I actually didn't catch that. Probably a 110 MM spacing for that 5 bolt setup.

Chainrings are actually really easy to ...more


Great. Thanks so much for your input and I agree with what was mentioned about the popularity of MTB (gears, heavy, etc).

After searching on Ebay, it appears that finding the combination crankset with sprocket/chainring as one unit is proving difficult because of the crank arm length I'm after (somewhere between 135mm and 150 mm). However, if I buy the sprocket/chainring separate from the crank arms, I may have better luck. Excuse my ignorance, but is this possible when I remove the current crankset and expose the BB? Will the separate chainring somehow mate up with the BB and the separate crank arms that I buy?

Thanks again.
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8/19/2016 1:18 PM

After searching on Ebay for separate chainrings, it appears that most have a 4-5 bolt pattern that requires and accompanying star-shaped (web-like) crank arm. This is opposed to the multitude of crank arms on Ebay that are just that...an arm (no star-shaped attachment).

So if I'm looking to purchase the chainring separate from the crankarm, does that mean I am only looking for a "star-patterened" crank arm.

Excuse my ignorance in advance.

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8/19/2016 1:37 PM
Edited Date/Time: 8/19/2016 1:45 PM

You need to be searching for a single speed chainset rather than 3 piece crank, maybe try youth chainset, or something. Try a couple of variations of those words. Although the search thing on ebay is pretty terrible when you want something specific haha. Most kids mtb's will have this type of chainset on them, like the 20" bikes that are single speed on the front. I'm in the UK so can't give an exact supplier, but here's an image I grabbed from one of the suppliers of the shop I work in
Photo

At 9 years old, up to about 160 should be OK, 135 would probably be too short. This particular one has a 40t sprocket which will make it a bit easier to pedal. The only other thing you'll need is a crank arm extractor, but they're not that expensive.

You MAY have an issue with the length of the bottom bracket axle, but chances are it'll be fine. And again, if the chainline really is out so much that it would need replacing, a new sealed bb is cheap and easy to find. Establishing the correct length will depend on how far out the chainline is. But as I say, it should be fine.

Edit: If you can find the chainset, a shop would probably swap them over for close to the cost of the tool, they'd also be able to check the chainline, and if need be, replace the bb to get the chainline sorted. The total cost of all parts and labour should be no more than £50. It may seem a fair bit to spend to change a couple of bits on a kids bike, but that's a chainset, new sealed bb, and fitting (meaning they'll be liable if anything goes wrong with it, like the chainline being out, play in the bb etc. Depending on their guarantee), and the bike will be dialled for your kid. When they grow enough, swap the old bb and chainset back onto the bike, and they'll be standing up to pedal, and pumping around the track with no problem, leaving all the kids in their age group in the dust smile

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8/19/2016 4:09 PM

grumpySteve, your input is SO appreciated!

After an extremely exhaustive search, I don't think I'm going to find a reasonable priced crankset that will suit the need. I found just a few 140mm cranksets/chainsets but nothing with less than 42T.

Just when I was ready to give up, I was so happy to have found one on Amazon. It was perfect! 140mm and 36T. When I went to check out, there was a $22 shipping fee as it was coming from the Czech Republic. I'm not paying that shipping on top of the cost of the item. In my research, I have determined that there are many more opportunities for bike parts in Europe that what we have available to us in the USA.

Deflated...

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