# BMX drivetrain help (Eastern Thunderbird)



## Slurry (Dec 23, 2008)

I have an Eastern Thunderbird 26" dirt jump bike that I'm having problems with.

The drivetrain is spec'd with:

Eastern Tribe 48spline BMX cranks
Eastern Medusa 25t sprocket
Eastern "9'er" Loose bearing rear hub, 14x135mm, one-piece 12t driver. I have read the instructions for servicing this hub and I am quite familiar with how to perform it properly.

Quite frequently, I experience a popping/slipping sensation that I have never been able to resolve. It happens only when riding, and happens quickly enough that I cannot stop the bike mid "pop" and see whats going on. when thrown in the stand, everything appears fine. 

My two thoughts were that a: The chain was trying to pop off the chainring or driver, or b: the driver itself was slipping (pawls failing to engage).

My chainline is as straight as I feel is acceptable, I've used the park chainline checker several times and its near perfectly straight. While I wont dismiss it as an option, I feel like the popping/slipping is coming from the driver.

Unfortunately, I cant find anything wrong with the hub. It uses one set of bearings in the hub shell and another set in the driver itself. The six pawls all share one circular spring clip, which seems like an inferior design, but all the pawls seem to be moving smoothly and none of them stick. I've tried lightly greasing the pawls as well as leaving them totally dry, but i still get the slipping. My only other though was that grease from the hub bearings was working its way into the driver and causing issues, but dont really find any extra grease in/around the pawls when I pull the hub apart.

I have ran the chain with what I considered "normal tension" (think SS MTB), but have also tried running it loose as well as "BMX tight". I had no idea BMXers ran their chains so tight, but I tried it anyways. regardless, the popping noise persisted in each setup. The Medusa BMX sprocket has a little side-side wobble, but I don't know if its enough to cause any issues. Personally I hate the idea of the BMX micro-drivetrain, but it seems so few people have issues with it that I figured it would be fine...

The proprietary 14x135 axle means I have few options for alternative rear hubs. Eastern sent me their nice "Bi-Directional" rear wheel (sealed bearings, reversible driver), which turned out to me a total POS and ended up blowing out in about a week. I can get into if anyone would like to know, but the point is I dont really have another wheel to throw on the bike to see if it is in fact the driver. 

Eastern has been helpful replacing parts that sucked/broke right off the bat, but they have played innocent the whole time, claiming they've never see someone with so many problems on "an award winning bike".

Any ideas?


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## 907guitaristbikerguy (Jul 21, 2010)

my eastern night train does the same thing.
it DOES feel like it's the driver slipping momentarily.
i haven't tried to pull the thing apart, and don't know how to... maybe i should?

the bike only has maybe 15 hours of ride time? just a rough guess.
it's pretty new.
but i don't think it did this when i first got it, though.


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## clewttu (May 16, 2007)

have had two nighttrains now (one stolen) and havent had an issue with this
i had a similar feeling issue on another bike (i think by the description), and it was a damaged link in the chain that would get hung up on the sprocket/driver, it was fairly obvious what the problem was so id imagine isnt the case here


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## 907guitaristbikerguy (Jul 21, 2010)

it doesn't happen very often, so i doubt that's the case.


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## cmc4130 (Jan 30, 2008)

I know about 5 people with Eastern Thunderbirds or Nighttrains (all '09 models) and none of them have talked about this as an issue. ( Although maybe they just didn't notice or talk about it. )

My first impressions were the chain or chainline. Some chains simply do not fit well onto certain sprockets and cogs. For example the stock chain on the We The People Avenue is noisy and seems to 'pop' etc. etc. Replaced with a KMC Z chain http://empirebmx.com/store/comersus_viewItem.asp?idProduct=6806 and it fixed the issue instantly.

Does your chain have consistent tightness during a full rotation of the cranks? It's possible the sprocket is not seated completely straight (check the little adapter washer between spindle and sprocket), is bent, or possibly just not machined perfect.

Even small differences in alignment can make a difference. Try putting a very slim washer on the inside of one dropout, then the other, to see if moving the wheel over slightly helps--or see if you can move the sprocket in or out by removing or adding crank spacers.

If it's the hub internals, I don't know . . . I'd have to look at it.

by the way, if you do end up getting a different rear hub, remember you can easily put in 3/8" to 14mm adapter washers. Any good BMX store will have them for riders who want to run a 3/8" axle in a 14mm dropout. I realize that mtb hubs are 10mm not 3/8" (which is like 9.5 mm) but you could probably drill em out.


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## Slurry (Dec 23, 2008)

i have upgraded the chain from the stock chain to both a KMC half link chain and then a KMC bmx chain, both eighth inch. Oddly enough i just now noticed that the stock driver is 3/32...

Im sure everything on the crankset is aligned and seated. There are no stiff links in my chain. 
I've got most of the issue to somehow go away by reverting to a normal chain tension. I think i have a buyer for the bike too 

SO i guess it must have something to do with the exterior drivetrain.


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## Guest (Oct 19, 2010)

I have the same hub and the same issue. Anyone know of other hubs out there? Also, I am looking at a bi-directional hub, any suggestions other than Eastern?


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## TheRed06 (Sep 24, 2010)

I noticed my brand new thunderbird doing that as well... Didnt really think anything of it, im gonna take it back to the bike shop and see if they can do anything about it.


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## tobyone67 (Oct 3, 2005)

I've got '09 T-Bird and it did the same thing, although it turned out that the rear spocket wasn't totally tight on the hub (slight wobble). A couple-a quick turns with my cog tool and all was well again.


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