Shimano launches carbon crankset

grimpeur

There is something a little off here...

I've reviewed your calculations and figures from that FSA pdf and everything makes sense...but, I recall the FSA advertisement in the magazines was a little more revealing and quoted the actual stiffness figures along with the stiffness / weight ratio. I recall that Record was top and Dura Ace was second for stiffness, with the FSA middle of the ground for stiffness. Why I recall this is that I was about to buy some K-Force Light cranks but was put off by this! I am a pretty big rider at 76Kg so did not want something flexy!

I'll have a look through my old magazines and try to find the ad to convince myself I'm not actually going mad.

overthehill

Dorian Gray wrote:

Carbon fibre has the potential to be significantly stiffer than aluminium. As always, the key is in the design. If stiffness were automatically proportional to weight then any old Joe in his garage could come up with a great crankset. The truth is, some designs are better than others.

Here's a PDF file from FSA about their K-Force Light crankset, which has a hollow carbon fibre design. It contains the following relevant graphic:



The numbers come from a test by independent German laboratory EFBe, though I couldn't find them on EFBe's website. From these stiffness-to-weight ratios and the published weights of the cranksets, we can work backwards to find the relative stiffness of the cranksets, from stiffest to most flexible:

Campagnolo Record (carbon) - 692 grams - stiffness value 1.000
SRAM Force (carbon) - 791 grams - stiffness value 0.965
FSA K-Force Light (carbon) - 633 grams - stiffness value 0.955
Shimano Dura-Ace (alloy) - 740 grams - stiffness value 0.838
Stronglight Compact Pulsion (carbon) - 600 grams - stiffness value 0.776

Published weights may vary slightly from actual weights as tested by EFBe, but they'll certainly be close enough to make the above comparison valid.

So what do we see? The stiffest crankset, Campy Record, is made of carbon. The crankset with the best stiffness-to-weight ratio, the FSA K-Force Light, is made of carbon. The crankset with the lightest weight, the Stronglight Compact Pulsion, is also made of carbon. The only aluminium alloy crankset, Shimano Dura-Ace, is among the heaviest and also among the most flexible. (Looking at these numbers, SRAM may have over-engineered their Force crankset to eliminate the risk of getting an early reputation for broken cranks.)

Now, if Shimano's claims of 10% greater stiffness (over current Dura-Ace) and weight of 709 grams for the new half-carbon crankset are accurate, the stiffness-to-weight chart would look like this:

FSA K-Force Light (carbon) - 633 grams - stiffness-to-weight ratio 0.188
Campagnolo Record (carbon) - 692 grams - stiffness-to-weight ratio 0.180
Stronglight Compact Pulsion (carbon) - 600 grams - stiffness-to-weight ratio 0.161
Shimano Dura-Ace (carbon) - 709 grams - stiffness-to-weight ratio 0.161 (equal to Stronglight)
SRAM Force (carbon) - 791 grams - stiffness-to-weight ratio 0.152
Shimano Dura-Ace (alloy) - 740 grams - stiffness-to-weight ratio 0.141

And the absolute stiffness chart would look like this:

Campagnolo Record (carbon) - 692 grams - stiffness value 1.000
SRAM Force (carbon) - 791 grams - stiffness value 0.965
FSA K-Force Light (carbon) - 633 grams - stiffness value 0.955
Shimano Dura-Ace (carbon) - 709 grams - stiffness value 0.922
Shimano Dura-Ace (alloy) - 740 grams - stiffness value 0.838
Stronglight Compact Pulsion (carbon) - 600 grams - stiffness value 0.776

I'd like to know the orientation of the test load applied by the EFBe lab before making a certain conclusion, but it looks like carbon is the way to go for both stiffness and low weight. Placebo effect of metal equating with stiffness notwithstanding.

All very interesting, but at the same time imho meaningless!! And what does the difference between 1.000 and 0.776 actually mean in real terms? As in when you are actually riding a bike. All things being equal is the "wrong" choice going to cost you a quantifiable amount of time in a TT or mean that you can't get up to speed quick enough to cover a break in a road race? I don't think so.

Lies, damn lies and stats and all that :roll:

aracer

I think it was just replying to drenkrom's assertion that cf cranks were flexy, and proving him wrong. Of course other research I've seen would suggest that crank flex makes little difference to power transfer.

Dorian Gray

grimpeur wrote:

'll have a look through my old magazines and try to find the ad to convince myself I'm not actually going mad.

Would love to see that if you do find something. As I said, I couldn't find the test results on the EFBe website, which would be useful.

overthehill wrote:

All very interesting, but at the same time imho meaningless!! And what does the difference between 1.000 and 0.776 actually mean in real terms? As in when you are actually riding a bike. All things being equal is the "wrong" choice going to cost you a quantifiable amount of time in a TT or mean that you can't get up to speed quick enough to cover a break in a road race? I don't think so.

Lies, damn lies and stats and all that :roll:

I arbitrarily assigned a value of 1 to the stiffness of the Record crankset, and the others are just relative to that. These figures don't correspond to any real life value; they're only useful for comparing the relative stiffness of the different cranksets.

My own feeling is that many people mistake frame flex in the bottom bracket area for crank flex.

grimpeur

Dorian Gray wrote:

My own feeling is that many people mistake frame flex in the bottom bracket area for crank flex.

I completely agree with you here. I recently switched from a LItespeed Tuscany titanium frame to a Cervélo R3-SL and Cervélo is noticeably stiffer in the BB area than the Litespeed. This is very very noticeable when climbing out of the saddle.