SRM PM7 diet

[Ferry]

I could use some input. I have bought a Specialized Tarmac SL8 frame and will transfer the components of my TCR (Dura Ace R9170). I took the opportunity to weigh the separate components. I am a bit shocked. The SRM Origin PM7 in my configuration weighs 854 grams:

• SRM composite crank arms 172,5 mm: 342 grams
  SRM spider 112 grams
  SRM 24 mm spindle 160 grams
  SRM aluminium chainrings 52 - 36 230 grams
  SRM chainring bolts 10 grams

First, there are some grams to be saved by chainging the spindle. I also have a spare 30 mm spindle of 116 grams (-44 grams). I am also considering the 24 mm titanium spindle of 94 grams (-66 grams). What would be your choice (the Tarmac SL8 has a BSA BB)?

Second, the chainrings. These shift well but I had no idea of the weight. Would Extralite rings be an option or would the quality of shifting reduce dramatically? Perhaps CarbonTi?

Lastly, I will keep an eye out for replacement crank arms (preferably THM (-124 grams), probably the Look trilobes (-82 grams)).

[OtterSpace]

I dont have a SRM so cant add much info regarding use, setup, or wear issues but I did look into SRM a bit recently compared to other options.

Here are some expected weights not including chainrings and chainring to spider bolts of a few spider PM & crankarm options:

Elilee X-Novanta (~275g) & Sigeyi cinch (~105g) ≈ 380g / $1,099
SRAM Red E1 (~305g + 8g bolts) & Sigeyi 8 bolt (~105g) ≈ 418g / $743
THM (~218g + 112g DUB spindle) & SRM (~115g) ≈ 445g / $2,749
Praxis Doon (sub 340g) & SRM 3 bolt (~115g) ≈ sub 455g / $1,614
Look (~252g + 112g DUB spindle) & SRM (~115g) ≈ 479g / $2,249
SRAM Red E1 (~305g + 8g bolts) & Quarq DFour (166g) ≈ 479g / $743
vs yours: SRM Composite (342g + 160g 24mm spindle) & SRM 112g = 614g

Depending on what bolt interface your SRM uses you might be fairly locked into using only a handlful of crankarms and axles but if you have the 3 bolt you can go with the Praxis Doon.

Personally I try to go 28-29mm diameter for weight reasons and only use 24mm on beater builds using stock 105 crankset which is great but heavy as a boat anchor.

Chainring wise I try to stick stock Shimano for the best shifting after having run all of the options you listed on various builds. Carbon Ti is just a tiny bit lighter for a lot more money and slightly worse shifting. Most Shimano drivetrain Carbon Ti chainring users who are aware of the weight deltas went that way for aesthetics or because they are on 5 bolt (like THM Clavicula). Extralight I only use on very light builds (sub 6kg) as they are less stiff and dont shift as well as other options and there generally is better low hanging fruit on bikes over 6kg when saving weight. However, your options will be driven by your bolt pattern on your SRM.

52/36 Chainring weights from R2-Bike:

9100 Dura-Ace (4 bolt 110bcd): 110g + 32g = 142g
9200 Dura-Ace (4 bolt 110bcd): 113g + 31g = 144g (+2g vs 9100)
Carbon Ti (4 bolt 110bcd): 104g + 33g = 137g (-5g vs 9100)
Extralite (5 bolt 110bcd): 103g (-39g vs 9100)
vs yours: "SRM aluminium chainrings 52 - 36 230 grams" (+88g vs 9100)

Could be worth considering selling your current SRM and moving on too depending on your priorities. The Praxis Doon + 3 bolt SRM is almost the same weight as the much more expensive THM SRM option so I'd go for that if you want to stick with SRM.

[xsnailx]

i would get the srm ti spindle + thm crankarms + carbon ti rings or kcnc rings, unfortunately from my experience extralite rings do not like 12 speed chains

[StefanBRNRD]

Did not have the best experience running the 24mm with Look cranks. Just came loose to often or noisy. Changed to DUB and Look, all good.

[Ferry]

Thanks a lot for your input. I would like to keep the PM7 as it just works well.

It is an Origin so no 3 bolt interface. Replacement cranks are limited to THM, Look trilobes and Cybrei (dub: 29 mm). I will look out for used THM cranks. There are also a couple of Look Trilobes for sale currently but I do not need the adjustability of the crank lenght.

Spindle wise: I have a 30 mm axle which is lighter but I also have an option on a 24 mm titanium axle. Any thoughts on bottom bracket / bearing life 24 mm vs. 30 mm. spindle? (I have not had any creaking with the current 24 mm steel spindle).

Lots of weight to be won by replacing the chainrings. I still have the Dura Ace rings but I do not like the aestethics combined with the SRM cranks. I am running Dura Ace R9170 with a 12 speed chain. As I understand that is not a good combination with Extralite?

[OtterSpace]

Spindle diameter & BB gets contentious fairly quickly as there are tons of factors, user priorities, and bike to bike variation.

However for me personally I'd avoid 24mm Ti as its kind of mittigation/bodge to reduce weight of an inherently heavier system at the expense of stiffness and the spindle's ability to resist wear from a steel bearing race.

24mm steel is heavy but can use cheap plastic BB and the bearings will basically last forever while working on even poorly manufactured frames.

30mm Al is light but more likely to need problem solver BB which are often a bit heavier reducing the weight advantage a bit and less set and forget.

28-29mm OD Al crank allows the use of ~30mm ID BB with pastic OD to ID shims to mittigate issues so is the better light weight solution but also less set and forget than 24mm.

The choice is up to you.

Personally for the situation you describe which retains your existing PM I would go Cybrei dub crankarm, new bb, Carbon Ti outer ring, 9100, 8100, or 9200 inner ring. Carbon Ti says their rings are matched but I'd put my money on the Shimano inner ring working better than the Carbon-Ti one for a lot less money and no major impact on asethetics. If the risk doesnt pay off buy the expensive Carbon-Ti inner ring.

[toxin]

Also bear in mind the diff between dura ace and carbon ti rings in the same configs is like 5 grams

[spdntrxi]

Carbon-Ti EVO rings are working surprisingly well for me so far. Just switched from extralite mostly due to stiffness . I'm on THM crankarms, so no option for DA rings. I actually prefer the extralite rings for aesthetics, because carbon-ti is kinda fugly because they are so thick.

[CampagYOLO]

If possible, stick with the Dura Ace rings. The better shifting far outweighs the minor weight gain.