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Idiotic bike hacks
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mmiloou
Feb 25, 19 13:29
Post #1 of 11 (2022 views)
Slap some sense in me if needed:

-Running a bb30/gxp adapter on my tt bike, I have a gxp quarq and everything is fine, on a scale ranging from "stupid" to "might be worth a try" would getting a eccentric BB and have it in the lowest position be worth a crack? I've heard from a couple reliable sources that 8-10mm has a 10watt advantage. I'm ok shooting $20-$30 for a Chinese BB and seeing where that goes.

-Just as silly: I'm getting a 175mm SRM in the mail, I run a quarq-riken on rival 165mm arms, seems that options to trying something shorter with power is hard to find (only rotor/p2m for crank based?). The SRM is on hollowgram arms, these seem to have a straight flat section, again on a scale to "idiot" to "make us laugh" could I have a machinist cut the arms and weld them back to 155-160mm?
I'm not super attached to the 175mm arms, pretty confident there is space to loose 15-25mm on a 175mm arm, just need to be 100% that the machinist can weld it straight and that both arms are the same length (I know aluminum is harder to weld).

Curious to hear comments and thoughts (or hope you had a good laugh)
Last edited by: mmiloou: Feb 25, 19 14:30
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Re: Idiotic bike hacks [mmiloou] [ In reply to ]
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SharkFM
Feb 25, 19 14:23
Post #2 of 11 (1913 views)
How much is your shoe and pedal stack? By removing unnecessary bulk there I saved around 10mm, which is the same thing. Shoe sole, then pedal hardware add up to a lot. I have Bonts basically sitting directly on the spindle.
Dropping the cranks is great, which lowers your saddle by same. Just take it easy in the corners!

Welding crank arms I would not do under any circumstance. Unless you can get the welds x-rayed and verify the material properties of the crank arm have not changed, still carries risk of sudden failure. Nice surprise if you can put up enough watts.
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Re: Idiotic bike hacks [SharkFM] [ In reply to ]
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Tom_hampton
Feb 25, 19 15:05
Post #3 of 11 (1823 views)
SharkFM wrote:
How much is your shoe and pedal stack? By removing unnecessary bulk there I saved around 10mm, which is the same thing. Shoe sole, then pedal hardware add up to a lot. I have Bonts basically sitting directly on the spindle.
Dropping the cranks is great, which lowers your saddle by same. Just take it easy in the corners!

Changing stack is NOT the same as using shorter cranks. pedal stack offsets the entire pedal circle upwards or downwards---more or less like raising or lowering the seat. If you decrease the pedal stack and then lower the seat you will have exactly the same knee/hip geometry as you did before before changing the pedal stack.

Changing crank length lowers the top of the stroke, and raises the bottom. Its like having MORE stack at the bottom, and LESS stack at the top. Having shorter cranks decreases the hip/knee flexion at the top of the stroke for the same knee extension at the bottom.
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Re: Idiotic bike hacks [Tom_hampton] [ In reply to ]
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MattyK
Feb 25, 19 17:50
Post #4 of 11 (1653 views)
Tom_hampton wrote:
SharkFM wrote:
How much is your shoe and pedal stack? By removing unnecessary bulk there I saved around 10mm, which is the same thing. Shoe sole, then pedal hardware add up to a lot. I have Bonts basically sitting directly on the spindle.
Dropping the cranks is great, which lowers your saddle by same. Just take it easy in the corners!


Changing stack is NOT the same as using shorter cranks. pedal stack offsets the entire pedal circle upwards or downwards---more or less like raising or lowering the seat. If you decrease the pedal stack and then lower the seat you will have exactly the same knee/hip geometry as you did before before changing the pedal stack.

Changing crank length lowers the top of the stroke, and raises the bottom. Its like having MORE stack at the bottom, and LESS stack at the top. Having shorter cranks decreases the hip/knee flexion at the top of the stroke for the same knee extension at the bottom.

I think he means it's the same as using the eccentric BB.

Can't comment on that one.

Would not do the crank cut-and-shut. Aluminium strength is highly dependent on its heat treatment, which welding would not conform to, unless you then post-processed the whole crank.
Plus you'd likely introduce a stress riser on a material already known for fatigue cracking.

<edit> yeah looking further at the hollowtech design. Nope. nope. no f-ing way. Plenty of images of the standard cranks delaminating and or failing otherwise, can't imagine how bad/risky a modified set would be. Might be feasible to billet machine some new arms, but almost certainly not worth the expense.
Last edited by: MattyK: Feb 25, 19 18:01
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Re: Idiotic bike hacks [mmiloou] [ In reply to ]
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mike s
Feb 25, 19 17:54
Post #5 of 11 (1645 views)
Seems like a lot can go wrong, and if so would be catastrophic. You can get an SRM Origin with crankarms in 160mm. Worth the investment
Last edited by: mike s: Feb 25, 19 19:34
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Re: Idiotic bike hacks [MattyK] [ In reply to ]
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SharkFM
Feb 25, 19 18:28
Post #6 of 11 (1567 views)
MattyK wrote:
Tom_hampton wrote:
SharkFM wrote:
How much is your shoe and pedal stack? By removing unnecessary bulk there I saved around 10mm, which is the same thing. Shoe sole, then pedal hardware add up to a lot. I have Bonts basically sitting directly on the spindle.
Dropping the cranks is great, which lowers your saddle by same. Just take it easy in the corners!


Changing stack is NOT the same as using shorter cranks. pedal stack offsets the entire pedal circle upwards or downwards---more or less like raising or lowering the seat. If you decrease the pedal stack and then lower the seat you will have exactly the same knee/hip geometry as you did before before changing the pedal stack.

Changing crank length lowers the top of the stroke, and raises the bottom. Its like having MORE stack at the bottom, and LESS stack at the top. Having shorter cranks decreases the hip/knee flexion at the top of the stroke for the same knee extension at the bottom.

I think he means it's the same as using the eccentric BB.

Can't comment on that one.

Yes/correct that is what I was referring too!

I saw more than a few reports and video/images of failed Ultegra 6800 cranks. Looks like some sort of cast alloy ass'y.

As kids we cut off forks then inserted the cut fork into long pipes with flattened ends, holes drilled, tack welded. This was to make choppers. Moved the weight bias back and front wheel could bounce. Easy to pop wheelies. A tri bike is the other way, much harder to get the front -end up and bust along on the rear wheel :).
Training Tweets: https://twitter.com/Jagersport_com
FM Sports: http://fluidmotionsports.com
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Re: Idiotic bike hacks [mmiloou] [ In reply to ]
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Karl.n
Feb 25, 19 19:28
Post #7 of 11 (1476 views)
mmiloou wrote:
I've heard from a couple reliable sources that 8-10mm has a 10watt advantage. I'm ok shooting $20-$30 for a Chinese BB and seeing where that goes.

Don’t know how you’ll get to 8 mm. A bb30 shell is 46 mm? A gxp bearing od is 37? Half the difference is 4.5 mm, give an extra 1mm for clearances brings you to 3.5 mm. If 10 mm gets you 10 watts at 30mph/50kph what does 3.5 mm get you at 25 mph?
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Re: Idiotic bike hacks [Karl.n] [ In reply to ]
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nonobovey
Feb 25, 19 19:45
Post #8 of 11 (1437 views)
Karl.n wrote:
mmiloou wrote:
I've heard from a couple reliable sources that 8-10mm has a 10watt advantage. I'm ok shooting $20-$30 for a Chinese BB and seeing where that goes.


Don’t know how you’ll get to 8 mm. A bb30 shell is 46 mm? A gxp bearing od is 37? Half the difference is 4.5 mm, give an extra 1mm for clearances brings you to 3.5 mm. If 10 mm gets you 10 watts at 30mph/50kph what does 3.5 mm get you at 25 mph?


The eccentric design functionally drops the GXP interface to the bottom of the BB30, so you'd get most of that 9mm (I'm guessing 8mm is reported drop).
Last edited by: Nonojohn: Feb 25, 19 19:45
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Re: Idiotic bike hacks [mmiloou] [ In reply to ]
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bluntandy
Feb 25, 19 22:45
Post #9 of 11 (1317 views)
Depending on the Srm cranks. You can have them cut and rethreaded.
175mm would give enough room for 20mm shortening. I have had a pair of the 3 bolt octalink cranks shortened
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Re: Idiotic bike hacks [Nonojohn] [ In reply to ]
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DeanV
Feb 25, 19 23:55
Post #10 of 11 (1276 views)
Nonojohn wrote:
Karl.n wrote:
mmiloou wrote:
I've heard from a couple reliable sources that 8-10mm has a 10watt advantage. I'm ok shooting $20-$30 for a Chinese BB and seeing where that goes.


Don’t know how you’ll get to 8 mm. A bb30 shell is 46 mm? A gxp bearing od is 37? Half the difference is 4.5 mm, give an extra 1mm for clearances brings you to 3.5 mm. If 10 mm gets you 10 watts at 30mph/50kph what does 3.5 mm get you at 25 mph?


The eccentric design functionally drops the GXP interface to the bottom of the BB30, so you'd get most of that 9mm (I'm guessing 8mm is reported drop).
No, you will get less than 1/2 of the 9mm as was pointed out correctly above.
You are working from the centre so it is the radius that counts not the diameter.
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Re: Idiotic bike hacks [mmiloou] [ In reply to ]
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gary p
Feb 26, 19 10:37
Post #11 of 11 (976 views)
Somebody please explain how +/-9mm of additional bottom bracket drop saves ~10 watts. Unless you simultaneously change your posture, which you could presumably do without lowering the BB, are you not punching, more or less, the same size hole in the air, just at a slightly different elevation? Is 9mm of seatpost and 9mm of cockpit stack really worth that much?
"They're made of latex, not nitroglycerin"
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