Basic chainline question



S

Sscarich

Guest
Hi

I have been doing my winter training on a late 80's mountain bike with a triple
chainring. I have had sore knees; I suspect it is either the 181mm length or
the high Q-factor. It is nearly 3/4" wider than my usual road setup. I have
decided to intstall a road double, to replace the current MTB triple. My
question is:

Is there going to be an inherent chainline problem, due to the new, narrower
BB/Crank setup? It seems obvious that this will be a problem, but I just
wanted some quick feedback, before I take on the project. Also, any
suggestions would be appreciated.

Thanks,

Steve
 
On Mon, 17 Jan 2005 02:55:02 +0000, Sscarich wrote:

> Hi
>
> I have been doing my winter training on a late 80's mountain bike with a triple
> chainring. I have had sore knees; I suspect it is either the 181mm length or
> the high Q-factor.


181 is an odd length for cranks. Are you measuring them in order to get
that number, or reading it off the cranks themselves? If the former, make
sure you measure center-to-center. 180mm is a long crank arm, even for
mountain bikes. It all depends on your preferences and size, of course.
175mm is more or less standard for mountain bikes, and for "average" size
men, 170mm is standard for road bikes. But your purpose is winter
training for road riding; probably the same length as your road bike is
preferable.

I am not convinced that q-factor is all that important. I prefer the
newer "compact" cranks that have a bit of bend outward at the pedal, or,
perhaps, are displaced inward at the spindle. They tend to have wider
q-factor, but for me that is no big deal, and getting the spindle end out
of the way of my bony ankles is a big deal.

> I have
> decided to intstall a road double, to replace the current MTB triple. My
> question is:
>
> Is there going to be an inherent chainline problem, due to the new,
> narrower BB/Crank setup? It seems obvious that this will be a problem,
> but I just wanted some quick feedback, before I take on the project.


Should not be that big a deal. You may want to get a different bottom
bracket to improve things, but whether or not you want longer or shorter
depends on the type of cranks involved. You might want to go 2 to
5mm wider than the recommended size, since the cassette is further out
from the centerline on a mountain bike. That will increase the
q-factor, of course. But it should work OK in any event. Especially with
the long chainstays of your old mountain bike, the shifting should be fine.

Oh, check to see how the front derailleur works with the road cranks.
Mountain-bike derailleurs are made for smaller rings than road bikes have.
It's usually OK to use a road derailleur with mountain cranks, since the
larger circle of the derailleur will not lead to interference (I use
this), but the other way around may cause trouble. If you use Shimano
indexing, prepare for hassles there if you swap derailleurs, because most
Shimano mountain bike derailleurs require more cable pull per shift than
their road versions. Rear is OK, but someone decided otherwise for the
front.

--

David L. Johnson

__o | "What am I on? I'm on my bike, six hours a day, busting my ass.
_`\(,_ | What are you on?" --Lance Armstrong
(_)/ (_) |
 
Sscarich wrote:
> I have been doing my winter training on a late 80's mountain bike with a triple
> chainring. I have had sore knees; I suspect it is either the 181mm length or
> the high Q-factor. It is nearly 3/4" wider than my usual road setup. I have
> decided to intstall a road double, to replace the current MTB triple. My
> question is:
>
> Is there going to be an inherent chainline problem, due to the new, narrower
> BB/Crank setup? It seems obvious that this will be a problem, but I just
> wanted some quick feedback, before I take on the project. Also, any
> suggestions would be appreciated.


Steve, a different issue: there may be clearance issues if the
chainstays angle outward or flare outward greatly going from BB to
dropouts. Shortening the BB spindle and putting on bigger chainrings
could perhaps (surprisingly) cause the *larger* rings to hit the stays
where earlier they cleared.

I would guess this to be easy to predict by looking at the stays before
your start swapping cranks.

Mark Janeba
 
"Sscarich" <[email protected]> wrote in message
news:[email protected]...
> Hi
>
> I have been doing my winter training on a late 80's mountain bike with a

triple
> chainring. I have had sore knees; I suspect it is either the 181mm length

or
> the high Q-factor. It is nearly 3/4" wider than my usual road setup. I

have
> decided to intstall a road double, to replace the current MTB triple. My
> question is:
>
> Is there going to be an inherent chainline problem, due to the new,

narrower
> BB/Crank setup? It seems obvious that this will be a problem, but I just
> wanted some quick feedback, before I take on the project. Also, any
> suggestions would be appreciated.
>
> Thanks,
>
> Steve
>
>


The standard mountain bike crank is usually longer than the standard road
crank.
Check the lengths of the cranks. Maybe you need to shorter. Also mountain
bike seats are set lower, maybe you need to up the height a bit on the
mountain bike. I reduced a problem on my knee by adjusting the cleat on my
right shoe so that my foot was closer to the crank. It's not the width
that's the problem but the fact that it's assymetrical. Your right foot is
out wider because of the chainrings.

Marty