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Pics of external BB conversion

zdubyadubya

Turbo Monkey
Apr 13, 2008
1,273
96
Ellicott City, MD
Hey guys,

Does anyone have any pictures contrasting the two set-ups for a 150mm rear-end with a 73mm BB? I know that one involves using the RF adapter or some other like spacer kit for an external BB and the other uses a 73mmx128mm ISIS BB. I am faced with this dilemma and am curious how the chainguide mounts up in both scenarios. Which method have you tried and how is it working out. Most importantly, can I see some pics!

Thanks
 

davet

Monkey
Jun 24, 2004
551
3
Is it supposed to be complicated?

My Orange 224 has a 73mm bb/150mm rear, and my Shimano SLX crankset mounted up just like any other bike, and the Gamut P30 chainguide did as well.
 

zdubyadubya

Turbo Monkey
Apr 13, 2008
1,273
96
Ellicott City, MD
I was more wondering if the chainguide is mounted against the frame (ISCG tabs) and the cranks are spaced out to maintain the chainline, would it affect the placement of the chainring on the rollers. but it sounds like it doesnt. so good news!
 

jcook90

Turbo Monkey
Mar 3, 2006
1,211
1
Connecticut
I'm running an LG1 and gravity lite cranks on a morewood izimu so it has the same specs as what you are describing. I have the LG1 bolted on with 1 set of spacers underneath, and then the bb is on there with no spacers. I needed 2 real thin ones on the crank axle and everything lines up now.
 

IH8Rice

I'm Mr. Negative! I Fail!
Aug 2, 2008
24,524
494
Im over here now
i have the MRP Mini-G on my Revolt.

theres 2 spacers on the ISCG '05 tabs, two spacers on the BB (as per RF's recommendations) and my chainline is pretty good
 

PhilipW

Monkey
Mar 13, 2007
311
0
Leominster, MA
Hey Zdub,
The issue is mismatched chainlines as a result of a 50mm chainline up front and a 57.5mm chainline in the rear. The reason for the 128mm ISIS spindle OR 83mm Raceface with stepdown kit is so that the cranks can be pushed out to match the rear spacing. As it is, you would get excessive rubbing in most the gears on your bike if you ran a normal 50mm chainline crankset.

We're a bunch of sticklers around here in regards to a "proper chainline", but in a pinch you can USE a 50mm crankset...but it will be noisier and give you a less than perfect chainline.

As far as a chainguide setup...with 50mm it would be the same as usual...right to the tabs.

The 57.5mm chainline would either need super long ISCG bolts, or you bypass the tabs and bottom bracket mount the guide with the adapter flipped outward. This allows you to use shorter bolts and less spacers (stronger).

Cheers,
philip @ e*thirteen

P.S. the Revolt uses a proper 83mm BB shell with 150mm rear end. This thread is about mismatched chainline bikes.
 

davet

Monkey
Jun 24, 2004
551
3
how critical is proper chainline on a single chainring DH bike with a chainguide?

I run he 73mm bb/150mm rear on my Orange, previous on the Norco Team DH it was 68mm bb/150mm rear. It may break all the rules but I've never broken a chain, never had premature drivetrain wear and never had shifting issues.
 

davep

Turbo Monkey
Jan 7, 2005
3,276
0
seattle
Define critical....

and premature compared to what?

I guarantee there is/was/will be more friction and drivetrain wear with your current setting. Your front chainring is ~7mm too far inboard, meaning your average as well as maximum chain cross is 7mm more than what a 'correct' single ring bike would see, as well as 2mm more than what is considered 'cross geared' on a three ring crank (big no no). No chain retention device or deraileur is designed to deal with this kind of chain angle.

On a dedicated DH bike, you spend most of the time in the smaller cogs (further outboard by a 4.34mm per cog). In your smallest cog, your chain is 24.36 mm out of line.

It certianly your call, but to claim there are no downsides is not exactly truthful.
 

DirtyMike

Turbo Fluffer
Aug 8, 2005
14,437
1,017
My own world inside my head
So her eis my input...... You are going to wear parts faster when your chainline is incorrect, Maybe you wont snap chains, but you will wear cassettes chainrings and retention devices too fast.

Probably the easiest way to get these setup properly.


Old style BB's. Longer spindles to match up the chainline with the rear of the bike if an easy way to set these properly.

As far as the guide... Phill pretty much layed out how to line it all up properly, follow those and life is good.

Personally I like using the Howitzer system for these applications, its as simple as ordering the Proper BB, of coarse making sure you face the shell properly as you should with any BB install, and seting the guide proper.

Reason I like that system over the old style BB.....You get an Outboard bearing.
 

zdubyadubya

Turbo Monkey
Apr 13, 2008
1,273
96
Ellicott City, MD
All,

Thanks. Philip, as always you had the one key that I was kinda gunnin for. I need to flip the adapter over so I'm not running a ton of spacers. Everyone else, thanks for your input.
 

Udi

RM Chief Ornithologist
Mar 14, 2005
4,915
1,200
davet - I agree with those who suggest excess wear. It's the same story on the specialized 135mm offset rearend bikes, chainline is out, and I found I'd burn through front chainrings (e13 7075-T6 ones) quite fast. Switched to a proper 83x150mm bike and the problems disappeared.

If I had a 73x150mm bike, I'd run an 83mm (external BB - eg. saint) crankset on a 73mm BB with spindle spacers to give me the same chainline as an 83x150mm bike. I think that's the best solution. But if you're happy with your setup then by all means keep running it.