Carbon rear triangles (MTB)

Curious if anyone has an answer.

Why do some companies, (Salsa for example) have a carbon bike with everything carbon except the chain stays?

Can they not spec a full carbon bike?

Hump

Probably fall under 2 reasons:

  1. Cost - aluminum is likely cheaper and may have a VERY small weight penalty compared to a similar carbon piece. Look at carbon stems - most are the same or heavier than a good lightweight aluminum one.

  2. Complexity of build/strength needed - certain bikes may require a complex shape that is better suited to the properties of aluminum. Either due to the strength needed or cost to build the shape out of carbon(see #1)

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Because they break. Iā€™ve had several carbon XC frames that have had chainstays break. I think aluminum is just a better material for the stresses put on chainstays. Like mentioned aboveā€¦they could probably make carbon stays that were better but they might be too heavy or expensive to serve the purpose.

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Off topicā€¦.I really wish more companies sold complete aluminum bikes with higher end component specs. Carbon frames are great but expensive and aluminum frames are really good these days. I see more benefit in higher end suspension, brakes, hubs, etc. on aluminum frames than low end components on a carbon frame.

I know I can build up my own bike but I prefer to buy completes.

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That would be nice, like pick between a similar spec, but aluminum or carbon.

Hump

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I couldnā€™t agree more. My current XC bike is a 2020 Giant Anthem 29 2ā€¦all aluminum frame. I swapped out everything to Shimano XT, swapped out all the aluminum components to carbonā€¦bars, stem, seat, seatpost, & wheels. Its a large and weighs 24.5 lbs, not too bad but I donā€™t have to constantly look for cracks on the rear triangle. My previous bike was a Felt Edict, broke the rear triangle three times, Felt replaced it the first time, the second time they said they only had a few left and had to save them for ā€œteamā€ riders, so I had it repaired and it failed again. Picture is of the stock Anthem.

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I think itā€™s weight/cost. The previous Spark, and several other brands over the years, had a Carbon main triangle, aluminium rear triangle option.

That was a super appealing option to me having had seat stay damage on my aluminium anthem that never caused issue.

I agree nice aluminium options would be cool. Iā€™m super into the 2022 Trek Top Fuel 8. Good spec, great colour, and an aluminium frame which should be recyclable and more hardy/less precious.

It is heavy though. Medium is 14.5kg so thatā€™s nearly 2kg more than carbon equivalent

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Thatā€™s a great weight for an aluminum framed bike.

Hump

@KonaSS is bang on the money.

An additional factor is that companies would like to offer cheap(er) bikes that they can advertise as carbon. Even though @mtbjones makes a good point about aluminum, I think in practice this puts many companies at a disadvantage. A lot of customers buy bikes according to specs, because you canā€™t tell how they ride from the stats. So if you have a $4k bike with very good components and an aluminum frame at a ā€œmediocreā€ weight, I donā€™t think itā€™ll sell all that well.

Even on the road bike end of things, the only two aluminum bikes with some cachet are Cannondaleā€™s CAAD-series and Specializedā€™s Allez. On the MTB side, I would like to have a BMC Twostroke AL frame to replace my current (old) carbon frame.

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Salsaā€™s tagline for years has been ā€œAdventure by bikeā€. They donā€™t try to compete on lowest build weight, theyā€˜ve always marketed towards bike packing, adventure touring, backcountry riding and similar.

Iā€™ve personally never had a problem with carbon rear triangles on a MTB, but if youā€™re not racing XC against weight weenies I can buy the idea of an aluminum rear triangle being a simpler, more durable and dependable option for those who want most of the advantages of a carbon frame but with less risk of being stranded in the backcountry.

For what itā€™s worth, I have a 2014 Spark 930 with an alloy rear triangle, and the rear triangle still broke and was replaced under warranty (by the previous owner).

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