Tuning a Dura Ace SRM for lower weight

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jamesbass
Posts: 114
Joined: Wed Jun 16, 2010 1:19 pm

by jamesbass

Just pulled apart my SRM system to clean it and saved 10g. 8)

This got me wondering about shaving off more weight. Those Dura Ace metal cranks (circa 2008) seem super heavy but the shape & size of them indicates they may be able to be dremeled a bit. The guy I bought the unit off had over-tightened the left crank to the point where the crank bolts were rounded out. We had to drill the thing off and it ended up destroying the crank, or so I thought. It took us ages to get it off and there's a massive hole through it but he says he rides it on his commuting bike fine and he's been shaving a bit more weight off it as an experiment.

Before I reach for the dremel on my replacement crank, does anyone make an alternative left-hand crank that would be Dura Ace compatible and lighter? Mine is 250.5g.

Next up, the chainrings. These are unknown FSA 34/50T rings at 35.6g/108.5g respectively. What sort of savings are available here?

Finally, the chainring bolts. The system has a 130BCD outer and 110BCD inner so there's 10 torx bolts totaling 9.7g. Don't think I can save weight here but they need replacing so does anyone know what type of bolt I would need to buy to at least match this weight?

There are delicate electronics inside so I don't want to go crazy, but if anyone has any tips for reducing the weight of this thing, it'll help me.

Weights for those interested:

FSA 34T - 35.6g
FSA 50T - 108.5g
Left Crank with bolts (Dura Ace) - 250.5g
Right Crank (Dura Ace) + SRM Spider - 481.1g
Bolts - 9.7g
SRM PCV Head Unit - 77.7g
SRM Cables - 15g

TOTAL: 978.1g

cajer
Posts: 673
Joined: Sun Jul 14, 2013 1:26 am

by cajer

I wouldn't touch the spider itself as by drilling or dermeling you would change the stiffness of the system throwing off strain to stress ratios. Making your srm give bad power values

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jamesbass
Posts: 114
Joined: Wed Jun 16, 2010 1:19 pm

by jamesbass

Sure. That's a good point. I was going to send it off for calibration post-tuning but I think it's probably a bad idea.

WeightySteve
Posts: 291
Joined: Wed Jun 09, 2010 10:44 am

by WeightySteve

Are the new crank arms shimano make specifically for SRM any lighter or are they just re-badged (and rounded off a bit) 7800's?

As far as I know they use the same bolt pattern to the spider so should be swappable out from the older ones. Been thinking about this for my 7900 SRM's for a while, but this post has re-triggered my interest in it.

Turns out they are not compatible.... http://www.scientific-coaching.com/repl ... oduct_info

Dalai
Posts: 1491
Joined: Wed Dec 15, 2004 10:54 am
Location: Melbourne, Australia

by Dalai

Before you touch the right 7800 SRM crank arm with a dremel, make sure you have a spare lined up in case something goes wrong. As these are not available new from SRM anymore...

11.4
Posts: 1095
Joined: Tue May 23, 2006 4:33 am

by 11.4

Just a bad idea overall. As pointed out, it can alter your powermeter accuracy, and recalibrating won't solve the problem. It can be an error that varies with force applied to the pedals. Further, that crankarm has never been amenable to customization; even SRM had problems incorporating the powermeter unit into it originally. If you are that sensitive to weight (did you clean out dirt or did you leave parts out when you overhauled your SRM), take off the powermeter or switch to a lighter one. This is primed to be a horror story in a thread six months from now.

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Calnago
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by Calnago

^ Those were my thoughts when I saw the first post initially. Bad idea. I thought it best just to steer clear... next.
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boots2000
Posts: 1393
Joined: Mon Oct 15, 2007 9:28 pm

by boots2000

Bad idea. I have cracked a Dura-Ace SRM crankarm and I weigh 64-65 kg.
Can't imagine a dremeled out arm.

sadisticnoob
Posts: 48
Joined: Fri Feb 22, 2013 7:30 am

by sadisticnoob

Mine left crank arm (7800) weights in at 183grams without bolts. (175mm)

Image

not too sure if yours is a hollowtech 2 or the octalink version

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