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 Post subject: BB choice help?
PostPosted: Wed Jun 08, 2005 8:13 am 
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Location: Tas, Aus
Ive narrowed my choice of bottom brackets for my new bike down to the FSA Platinum Pro MegaQuad and FSA Platinum Ti MegaQuad. Just hoping someone could tell me if one was more reliable or stiffer than the other. I'd rather sacrifice a bit of weight for strength and durability in this area as I'm only trying for a uci legal bike. Thanks. I'm open to other suggestions within the price range of these two.


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 Post subject:
PostPosted: Wed Jun 08, 2005 1:06 pm 
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I only know from several tests I've read that Ti equivalents of steel BB axles are always less stiff. It's by quite a considerable amount as well. I think you only can get around that limitation through a dedicated design, i.e., over-sized axle, etc.


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Posted: Wed Jun 08, 2005 1:06 pm 


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 Post subject:
PostPosted: Wed Jun 08, 2005 2:38 pm 
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The Pro and the Pro Ti are not identical. The Ti version has a thicker cross section profile on the spindle to try and beef up the stiffness. I don't have first-hand experience with both, so I won't try to make a riding comparison. However, unless you're 200lbs and can pound the bike into the ground, you're not going to notice a difference in the stiffness. Don't let anyone here tell you any different.

They both use the same bearings in the same configuration, so in terms of durability I would imagine they would be similar.


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PostPosted: Wed Jun 08, 2005 3:50 pm 
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@ stumpytrunks: You are posting this in the Road forum, so I assume you are looking for a road double bb, with a spindle of 108mm. If so, I would wager the stiffness difference between the two is negligible, given that, as nexusheli points out, the Pro Ti is an M14 versus the CrMo M15; it may be more of a issue on longer spindle bb's.

I just think that in many of these cases the stiffness thing is over-rated (stiffness being the latest buzz-word fad in cycling) and that even if differences are apparent in certain isolated component bench tests the differences are not something that would really be specifically noticed and/or affect an everyday rider. Just my .02.

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 Post subject:
PostPosted: Wed Jun 08, 2005 4:13 pm 
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Cadence is right IMO. There are a lot of people running around touting how stiff something is and how much deflection for a given amount of force this and that. Well when you're actually riding your bike do you care? Obviously you don't want a noodley bike but by the same token who cares if one component is 1% stiffer than the next.


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 Post subject:
PostPosted: Wed Jun 08, 2005 5:26 pm 
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The decrease in stiffness can range between 30-50%...I certainly wouldn't want a Campy Ti BB that's less stiff by that amount. That type of range isn't some nebulous theoretical value anymore. Depending on the actual amount of flex it's not that good for the bearings either.


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 Post subject:
PostPosted: Wed Jun 08, 2005 6:06 pm 
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I think there's more then one issue here - one of performance and the other of durability/longevity and lastly of feel/handling. Certainly a stiffer crank/bb setup should deliver some performance benefit but it will mostly be for a sprinter, and I'm willing to bet it's a significant measurable difference in the case of a sprinter. For me I'm not going to really be a faster a rider with a stiffer crank.. the reason I like a stiff crankset (for me it's the DA10) is the solid feel and handling first. IMO the more solid the better in a crankset - that's what attaches you to the bike. I also think it's a more durable setup and will require less adjustment over time - being stiffer in design and with the larger bearings, it will be under less stress then otherwise. An added plus is it's very easy to remove and put back on.


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 Post subject:
PostPosted: Wed Jun 08, 2005 7:27 pm 
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Location: Belgium
Hi,

Quote:
The decrease in stiffness can range between 30-50%...


Certainly.
Yet when you have a given BB would changing the steel axle for a Ti one cause that kind of degradation in stiffness?
I seriously doubt it.

It would be good engineering practice to optimise a design in such a way that any lack of torsional stiffness would not cause any problems bar perhaps for the most extreme circumstances.
For those circumstances you'd rather add a little more weight to the design if that's what would solve the problem, right?

IOW, it's not because something's lighter that it's necessarily more flexy within a design context. Or vice versa for that matter.....

Ciao, :wink:

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Last edited by fdegrove on Wed Jun 08, 2005 11:28 pm, edited 2 times in total.

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 Post subject:
PostPosted: Wed Jun 08, 2005 7:36 pm 
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The problem is the space limitation with-in the current BB shell dimensions. It's one of the most difficult hurdles ISIS has to deal with and that's just using steel. Sure, you can increase the wall thickness of the spindle as well. That sort of defeats the purpose of using a lighter material however. In the end the product has to offer enough weight reduction in order to justify the price differential.


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 Post subject:
PostPosted: Wed Jun 08, 2005 11:00 pm 
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Hi,

Quote:
The problem is the space limitation with-in the current BB shell dimensions.


Putting the BB bearings on the outside is just one engineering solution.
One that Shimano adapted and one that's likely the most economically sound.
Being one of the biggest manufacturers naturally helps if you want to set standards....
No idea whether they actually patented it but if so then there are still a few alternatives available.

Mass production costs will likely sky-rocket which is probably what has kept others from implementing them.
Either way, there's little point increasing BB stiffness if it's going to be put in a flexy shell housing so those two need to go hand in hand or any benefit either way will be lost.

Ciao, :wink:

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 Post subject:
PostPosted: Wed Jun 08, 2005 11:24 pm 
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They can't patent it, because the original design comes from Roger Durham. Instead, Shimano, the great innovator, simply waited until the patent expired and stole it.

.....you're probably not originally a mountainbiker so I forgive you for not knowing this :wink:


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 Post subject:
PostPosted: Thu Jun 09, 2005 12:26 am 
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Hi,

Quote:
.....you're probably not originally a mountainbiker so I forgive you for not knowing this


LOL....How generous of you.... :P
Indeed I'm not an MTBer.
Just a biker with some engineering juices flowing,....occasionally.

Quote:
They can't patent it, because the original design comes from Roger Durham.


O.k....I could quote Marx here but I'll wisely refrain from doing so. :wink:

Either way, bracketshells seem longer to me than strictly needed; there's at least 5mm of wasteland either side.
Still if Shimano can see fit to use outboard bearings with axle lengths ranging from 103mm to well over 113mm, I can't see the problem for the ISIS folk to do the same thing using mostly 102mm, 108mm for duplo chainsets and what I think would be 111.5mm to perhaps 120mm for triples.
It's the shorter length posing the possibility of a fitting problem, not the longer ones.

Going from the assumptions above and adding to this that (I assume once again) triple systems aren't used for pro racing (I take it they use CT cranks instead) leaves us with a 102mm axle length and the accompanying Q-factors, useable chainline scenarios and maybe a few other details I'm overlooking....

Given that there's space available on the outside of the BB shell allowing to go inwards, towards to seatpost tube side of the frame if you like, and that we can freely adopt Shimano's reasoning that the bulk of the forces are put on the drivetrain side (is this really a fact?) we can easily increase the useful diameter of at least one end of the bearing which would increase stiffness more or less linearly as the bearing diameter increases.....

So, assuming there's no major flaw in my reasoning, what's keeping them from oudoing Shimano and going one better, I wonder?

Ciao, :wink:

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 Post subject:
PostPosted: Thu Jun 09, 2005 12:42 am 
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Quote:
So, assuming there's no major flaw in my reasoning, what's keeping them from oudoing Shimano and going one better, I wonder?


Your reasoning is pretty solid. There's one small item you overlooked: When you increase the size of the axle (spindle) and need to maintain the same outer diameter of the BB, you leave little room for bearings. The biggest problem hovering over Isis is the lack of reliable bearing configurations.

The other problem you run into with variations on BB's is that you either have to redesign the entire bike around your new BB (and convince other manufacturers to do the same), take a look at what Pinarello and Cannondale have done. Or you have to do something like outboard bearings where you use the existing system but try to work around it. When you do this however, you run into the problem of added weight. It's been worked around already, but it's not cheap.

Keith Bontrager should have his quote turned into a law, like Moore's law of transistors: Weight, Cost, Performance, pick two.


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 Post subject:
PostPosted: Thu Jun 09, 2005 1:42 am 
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Hi,

Quote:
When you increase the size of the axle (spindle) and need to maintain the same outer diameter of the BB, you leave little room for bearings.


Actually, I wasn't planning on changing spindle diameters.
No need to do this over the entire length of the spindle anyway, maybe just at the outer ends where the outboard bearing(s) would be fitted.
Rather the diameter of the ballbearings and their accompanying housings.

Bigger bearings could, in theory, be housing bigger, hence less balls.
Since a ball bearing and it's race make for a point contact when motionless and a line once going into a circular motion the larger the diameter they describe the less revolutions they'll make at the same revolutionary velocity (ouch).

This in turn reduces heat through friction and the bigger bearing assy would in theory be stiffer to begin with and it can theoretically be attached closer to crank and crankarm fixings.
In fact, I see no particular reason why you'd have to stick with the classic cartridge type of ball races.
You could just as well use ball/cup types or it's cousin the cone/cup type I suppose.
Even sleeve type bearings are not unthinkable given modern high-tech materials but, IMHO, might turn out too expensive to manufacture in view of the tolerances required for them to be stiff enough at the right places for this application.
Besides that you need to prevent the sleeve for moving sideways so that may well put the out of the range of options.

Quote:
It's been worked around already, but it's not cheap.


Sure.
Still it's fun the explore all possibile options regardless of what the market is willing to cough up for the end product.
It keeps on surprising me that no matter how far fetched the idea, if it works markedly better than what we have currently available, someone always seems to want to foot the bill.
Regardless of Moore's law of diminuishing returns, that is.... :wink:

And of course this one too:

Quote:
like Moore's law of transistors: Weight, Cost, Performance, pick two.


Ciao, :wink:

P.S. Totally off the wall idea: pressurized airbearings for TT races by 2015 perhaps....
All that running on airless tyres, tyres being made of some space age cellular foam formulated just right for the job....Ah......

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 Post subject:
PostPosted: Thu Jun 09, 2005 1:45 am 
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Quote:
Given that there's space available on the outside of the BB shell allowing to go inwards, towards to seatpost tube side of the frame if you like, and that we can freely adopt Shimano's reasoning that the bulk of the forces are put on the drivetrain side (is this really a fact?) we can easily increase the useful diameter of at least one end of the bearing which would increase stiffness more or less linearly as the bearing diameter increases.....

So, assuming there's no major flaw in my reasoning, what's keeping them from oudoing Shimano and going one better, I wonder?

Yes, there is a flaw: by moving the bb shell inboard towards the seatpost as you suggest, there would be a greater and perhaps impossible restriction on chainstay design/diameter/etc.; not a minor consideration.

As far as the "decrease in stiffness can range between 30-50%" mentioned above...that is absurd. A 50% less stiff spindle? What spindle compared to what spindle?

Finally, to repeat: this is where these stiffness issues get ridiculous: if component A) is stiff enough that there is no discernable flex and negative effect on performance who gives a d#%n if component B) is 1%, 10%, or 10,000% stiffer, really? Campa/Phil/etc. bb's are still going strong, absolutely fine for most purposes, after how many years now?

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"Gimondi è un eroe umano, che viene sconfitto ma che continua la sua corsa fino a tornare a vincere." - Enrico Ruggeri


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Posted: Thu Jun 09, 2005 1:45 am 


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