[mtnfiend]

The 575 has a 27.2mm seat post. I noticed the seat post actually decreases in diameter as it gets near the seatpost. Why?

It seems that the smaller diameter tube would require Yeti to increase the wall thickness of the tubing, thus increasing weight. In addition the change in diameter requires additional manipulation of the tubing. It seems it would be easier and cheaper to just leave the tubing diameter the same throughout the entire tube. It also seems to me they would be able to spec out a larger diameter seatpost, which might have thinner walls thus weighing less.

 

[drumstix]

The 575 has a 27.2mm seat post. I noticed the seat post actually decreases in diameter as it gets near the seatpost. Why?

It seems that the smaller diameter tube would require Yeti to increase the wall thickness of the tubing, thus increasing weight. In addition the change in diameter requires additional manipulation of the tubing. It seems it would be easier and cheaper to just leave the tubing diameter the same throughout the entire tube. It also seems to me they would be able to spec out a larger diameter seatpost, which might have thinner walls thus weighing less.

Think of it as a good thing, some of the highest end frames are like that, Turner 5 Spot, 6 Pack, Ventana as well does this. I have had 4 frames now that I have used the same seatpost on.

 

[FoolCyclist]

27.2 is the standard

The 575 has a 27.2mm seat post. I noticed the seat post actually decreases in diameter as it gets near the seatpost. Why?
.

27.2 diameter post is the standard. Its kinda a hold over from road bikes. Just about every seatpost out there can be found in 27.2. It just gives you more choices. Most manufacturers have adopted this to allow their customers to be able to run their choice of posts. Now why does the 575 seattube diameter increase. Well a larger diameter tube with the same wall thickness is usually stonger. If you want to increase strength in a tube without adding much weight, you just increase diameter. Klein championed this idea when he started using large diameter aluminum tubes on his frames. The frames were some of the lightest of the time and didn't sacrifice any strength. On the 575 the diameter of the post increases right before the brace that holds the swing link attaches to the seattube. the increased diameter adds some strength to that weld area. The larger diameter tube extends down to the main pivot bracket also, again adding strength. Overall its a strength vs weight issue going on for the 575. Increasing the diameter of this tube gives the bike a little added strength with a minimum addition to weight.

 

[mtnfiend]

"27.2 diameter post is the standard. Its kinda a hold over from road bikes. Just about every seatpost out there can be found in 27.2. It just gives you more choices. Most manufacturers have adopted this to allow their customers to be able to run their choice of posts."

Two bike builders I just looked at, Specialized and Santa Cruz, both use 30.9mm. Granted according to drumstix Turner, Yeti, and Ventanna, both use 27.2mm. I would not call 27.2mm a "standard."

"Well a larger diameter tube with the same wall thickness is usually stonger. If you want to increase strength in a tube without adding much weight, you just increase diameter."

No, if you increase the diameter and keep the wall thickness the same you've created a heavier tube. If you increase the the diameter and decrease the wall thickness you get a lighter tube. I can speak with 100% confidence that you get a stronger tube as a result though. The large diameter/thin walled tubing tends to give the bike a very stiff felling. Which is great on a suspended bike, but can lead to a sore butt on a hardtail mountain bike or road bike on rough trail or streets. It's great of transmitting power to the ground however.

Another added bonus of a larger diameter tube is an increased welding surface.

 

[Dogboy]

"27.2 diameter post is the standard. Its kinda a hold over from road bikes. Just about every seatpost out there can be found in 27.2. It just gives you more choices. Most manufacturers have adopted this to allow their customers to be able to run their choice of posts."

Two bike builders I just looked at, Specialized and Santa Cruz, both use 30.9mm. Granted according to drumstix Turner, Yeti, and Ventanna, both use 27.2mm. I would not call 27.2mm a "standard."

"Well a larger diameter tube with the same wall thickness is usually stonger. If you want to increase strength in a tube without adding much weight, you just increase diameter."

No, if you increase the diameter and keep the wall thickness the same you've created a heavier tube. If you increase the the diameter and decrease the wall thickness you get a lighter tube. I can speak with 100% confidence that you get a stronger tube as a result though. The large diameter/thin walled tubing tends to give the bike a very stiff felling. Which is great on a suspended bike, but can lead to a sore butt on a hardtail mountain bike or road bike on rough trail or streets. It's great of transmitting power to the ground however.

Another added bonus of a larger diameter tube is an increased welding surface.

27.2 is as close to "standard" as it gets in the bicycle world-something any experienced cyclist would know. As far as the seat tube changing diameter as it goes down, you have the "standard" 27.2 post along with a larger diameter tube at the BB area. This increases strength and rigidity.