BB and cranks

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BB and cranks

Postby tuco » Thu Mar 01, 2007 10:32 am

I'm thinking about replacing the double chain ring on a bike with a triple.

Most of you know the story :
The reasoning is the bike is for a U15 with a roll out of 6m.
With a roll out of 6m the large chain ring on the double is unused.
Going to a triple will allow me to lock out the large ring giving 6m roll out on the middle ring and the smallest ring will allow for easier hill climbing.
The shifter won't have to be changed because only two of the chain rings (smallest and middle) will be used.

No doubt the answer will depend on the brand/type of the bottom bracket but I know little about bottom brackets other than they exist.
The cranks are 165mm Truvativ touro.

Now the question.
Will the bottom bracket need to be changed going from a double to a triple?
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by BNA » Thu Mar 01, 2007 11:09 am

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Postby europa » Thu Mar 01, 2007 11:09 am

Ask yourself this - what's the cost of changing the BB. The cost might be such that it's a non-issue. Of course, that is more likely if buying second hand.

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Postby tuco » Thu Mar 01, 2007 11:28 am

europa wrote:Ask yourself this - what's the cost of changing the BB. The cost might be such that it's a non-issue. Of course, that is more likely if buying second hand.

Richard


Exactly why I want to know if I have to change the the BB except I'm working the other way round.
Do I have to change then then if so, check price.

If I have to then it will probably not be cost effective to change it (the bike was only $450 second hand) unless I can pick up a used one cheaply. We have a large club here and the chances of a used one being available is better than average.
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Postby Mulger bill » Thu Mar 01, 2007 11:36 am

Pretty sure you'll need a longer BB spindle for a triple chainset, i'll check it out.
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Postby timbo » Thu Mar 01, 2007 12:26 pm

From memory (or whats left of it), the axle is wider to accommodate the extra chain-ring. Whether this will fit into the old bottom bracket, I don't know. Check to see it the front and rear derailleur need to be replaced or not as well, as the added small chain-ring may find the chain rubbing on the front derailleur and a longer cage rear derailleur needed to take up the chain tension of the smaller chain-ring also. Chain length will need to be altered, but a chain-breaker and joining pin can fix that. Hopefully with minimal modifications it will all run smoothly. I'm tempted to say "see your local bike shop" but often they will try and sell you a whole new emsemble.
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Postby mikesbytes » Thu Mar 01, 2007 12:46 pm

Tuco, you mention top gear limitation based on U15 rules, which restricts top gear.

BUT, what does you daughter require in the way of a bottom gear? Is the existing bottom gear not low enough?

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Postby sogood » Thu Mar 01, 2007 1:04 pm

Yep, need a longer axle for the triple.
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Postby tuco » Thu Mar 01, 2007 1:45 pm

mikesbytes wrote:Tuco, you mention top gear limitation based on U15 rules, which restricts top gear.

BUT, what does you daughter require in the way of a bottom gear? Is the existing bottom gear not low enough?



It could be lower. We have some nasty hills in half of the seasons races and one race is a time trial up a hill which I conquered once and really don't want to tackle again.

A lot of the other riders her age have the lower gears so thought it would make things fair and easier for her.

It's beginning to look like a big job.

As Timbo mentioned, I'd like to stay clear of the lbs and talk to the club guys. I'm sure the lbs guys just see dollar signs walking into their stores.
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Postby Mulger bill » Thu Mar 01, 2007 8:30 pm

sogood wrote:Yep, need a longer axle for the triple.


Thanks Sogood, been thru 4 books and can't find the ref :oops:

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Postby mikesbytes » Thu Mar 01, 2007 9:22 pm

sogood wrote:Yep, need a longer axle for the triple.


Correct. I think the difference is small 6-10mm. It will depend on the frame.

But that leads me to another question, is it more efficient to have the cranks narrower or wider apart. What is the opitum width for a crank ?

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Postby tuco » Thu Mar 01, 2007 9:41 pm

mikesbytes wrote:
sogood wrote:Yep, need a longer axle for the triple.


Correct. I think the difference is small 6-10mm. It will depend on the frame.



Okay, that would probably explain the 113mm and 118mm sizes I saw for the BB.
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Postby sogood » Thu Mar 01, 2007 9:45 pm

mikesbytes wrote:Correct. I think the difference is small 6-10mm. It will depend on the frame.

But that leads me to another question, is it more efficient to have the cranks narrower or wider apart. What is the opitum width for a crank ?

That's the Q factor isn't it? Some people are obsessed about keeping that value down while others say it makes bugger all difference.

Without doing a thorough scientific reading on the subject, I would have thought it makes sense to try to keep your hips working in their neutral position. With increasing Q factor, the hip will need to be more abducted. So if this hypothesis is true, then maybe the Q factor's relevence should be similar to the choice of drop bar width ie. Q factor should match the width of one's hip. So if you have a wide hip, then a higher Q is desirable, while for those with narrow hips, a smaller Q is better. Anyway, it's a hypothesis that makes sense to me. Will see what scientific data is out there to back it up or debunk it.

Personally, with my recent pedal change and resetting of cleat position etc, I clearly was not comfortable with the initial new cleat's position. The slightly wider Q was noted. After further adjustment and reducing the Q factor, I am now much more comfortable. So yes, one can be sensitive to the Q factor one is used to.
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Postby Mulger bill » Thu Mar 01, 2007 10:00 pm

Setting the Q factor to match the rider sounds logical, bike fitters will not want to be hearing of another variable.

From a fat tyre POV, a larger Q factor will improve cornering as the outside leg can be weighted more effectively, not that it helps here of course, sorry.

Sogood, how did you reduce the Q on your bike? What BB and crankset?

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Postby sogood » Thu Mar 01, 2007 10:12 pm

The theory sounds plausible, but I get a feeling that all commercial setups have a Q factor greater than our hip width. So no one can get a true match.

I guess I cheated a bit by claiming that I reduced the Q factor through cleat adjustments ie. I moved my cleats toward the outer side of my shoes (two bolt Premium Crank Bros cleats), thereby bringing my feet closer together. I may be wrong, but I understand that that the real Q-factor is defined as the outer to outer measurement of the two opposing cranks.

Your point on the benefit of a wider Q in MTB'ing is interesting. Maybe that's why CB's regular egg beaters pedals have a Q factor that is that much larger than their Quattro road pedal.
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Postby Mulger bill » Thu Mar 01, 2007 10:24 pm

So I'm led to believe, but not according to Sheldon Brown. A 'bent site said something about it. :?
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Postby sogood » Thu Mar 01, 2007 10:28 pm

Does Sheldon even MTB? Gosh, it's a sacrileage to question Sheldon's words!!! :shock:

Ok, he has the proper definition on Q-factor:

Tread (a.k.a. "Q factor")

This is the horizontal width of the crankset from pedal mount to pedal mount. It is equal to the sum of the crank clearances above, plus the thickness of each crank. I can do the math if you provide the measurements.
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Postby Mulger bill » Thu Mar 01, 2007 10:38 pm

Serves me right for glancing :oops: Thanks for that.

I'd never knowingly question any of Sheldons statements.
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