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My gut is telling me that this is a typo, but here's what I'm seeing.

I'm looking at the 2017 Rocky Mountain Element which seems to feature a chainstay that can range from 439-437mm.

For comparison, the 2017 Rocky Mountain Instinct, seem to feature a fixed chainstay length of 452mm.

For the other parts of the geometry that have a range, I get why for these as the suspension compresses and changes the value. However, I can't see how the chainstay would change in length in this case.

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    As the rear suspension are goes through its arc the effective chainstay length will change. The rear wheel would have to travel perfectly vertical for this not to happen (depending on how you define the chain stay measurement). I suspect Rocky Mountain, simply reported the variation for the XC race bike, but didn't bother for their trail bike.
    – Rider_X
    Commented Sep 23, 2016 at 18:10
  • Notice where the chain stay pivots. It's above the bottom bracket, so the distance from bottom bracket to rear dropout changes as the stay pivots. If the pivot were centered on the BB this would not happen. (However, 2mm if change is negligible with most rear derailers.) Commented Sep 23, 2016 at 18:19
  • @DanielRHicks The Instinct's bigger suspension travel could introduce a bigger range than the Element?
    – TechFanDan
    Commented Sep 23, 2016 at 18:32
  • @TekiusFanatikus - I'm guessing that if the difference is less than 2mm they don't bother listing. The difference between the two could just be random variation. But there is also apparently some difference in the two bikes with regard to how the stay attaches to the rear dropout, and this might affect the number. In any event, it's negligible. Commented Sep 23, 2016 at 18:38
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    I put it down to the marketing guy for XC bikes is a details freak and the marketing guy for trail bikes is not. Don't read too much into the engineering information in marketing brochures, they are not put together for that purpose.
    – mattnz
    Commented Sep 23, 2016 at 20:02

2 Answers 2

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Apparently, there are differences not readily apparent on the frame graphics. Per Rocky Mountain:

The decision to include variable rear center information on the Element relates to the redesigned linkage, hence why the Instinct doesn’t show any change. The rearmost pivot is much closer to the rear axle on the Instinct, so the change is virtually nonexistent.

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  • It's your question and your answer, but for me this doesn't properly answer the question.
    – andy256
    Commented Sep 23, 2016 at 21:58
  • @andy256 pasted the full reply I was given. HTH.
    – TechFanDan
    Commented Sep 23, 2016 at 22:19
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    They actually are fairly apparent. If you look at the graphic for the Element, the dropout is much chunkier and the place (pivot) where the chainstay connects is some space away from the axle (let's make up 1.5"). On the Instinct, the dropout is less chunky, and the pivot where the chainstay connects is perhaps 1/3 of the distance (0.5", made up). Commented Sep 23, 2016 at 22:27
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Notice how the the chainstay length of the 2017 Rocky Mountain Element is written: 439-437mm

Don't you find it odd that the smaller number comes after the bigger number? Wouldn't it be more normal to write it 437-439mm?

This is because 439mm is the unloaded or normal/nominal chainstay length; and 437mm is when the suspension is fully loaded. It changes because the rear triangle suspension is not pivoting perfectly around the bottom bracket and thus the geometry is changing just slightly.

Most folks wouldn't be bothered by a 2mm change over the suspension's travel. Either they are really proud that it's only 2mm (older designs are much worse) or they're really anal. The omission of the range for the other unit is either because it really is < 1 mm or because they no longer feel it's an important detail to note.

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