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Discussion Starter · #1 ·
I've been kinda wondering about this for years, and just now I saw this twisted and tortured seat tube pictured in an ad, and it finally pushed my curiosity all the way up to "gotta ask."

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The Giant is of course not unique in this regard. Most FR/DH bikes have similarly slack seat tubes.

Is it so the seat will be further forward when lowered?

Or is it so the seat will be further back when raised up? (Why would anyone run a high seat on a DH bike anyway?)

Edumacate me. I has teh curious.

kthxbye
 

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It's due to a few things. Arguable the largest driver among them is the slack geometry of the front end. Downhillers tend to like shorter feeling bikes, which means a shorter effective top tube length. But there is a limit to how short you should go. To illustrate this, imagine a bike frame as being a sort of parallelogram between the forks, the effective top tube, the seat tube, and from the bottom bracket to the front axle. It isn't a perfect parallelogram, but it's reasonably close for the sake of argument. One necessity is a slack front end. Another requirement is an effective top tube length that won't have your knees hitting the bars when you sit down. Since the front end has a very acute head tube angle, the seat tube angle is accordingly slackened to maintain that rough parallelogram shape. If you didn't slacken the seatpost angle, your effective top tube length would be scrunched down so much that you wouldn't be able to pedal it at all if you were sitting. The saddle would just be too far forward to be usable. XC riders like steep seat tubes, because it moves their center of mass further forward over the front end. This helps a lot with climbing, as their center of mass doesn't go behind the rear tire's contact point, making them wheelie and loop out. Since DH bikes aren't made for climbing this looping over backwards is a non-issue. But going over the bars is. If they move the seat (and therefore the rider's body position further aft, it is much easier to descend without that 'over the bars' feeling. There are plenty of other reasons, but many get a bit too physics-nerdy. Anyway, hope that helped.
 

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My naive opinion is that it is simpler to design the bikes like that since you do not have to optimize for a steep ST angle like for trail bikes. Get the ST out of the way to have more room for a well designed suspension linkage and its mounting points.
 

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Another part of it is that the seat tube is usually moved around a bit to accomidate the rear suspension, usually by moving it forward and making the angle more slack.
 

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Why would anyone run a high seat on a DH bike anyway?
Actually, it's a pretty common assumption for people to think that DH bikes' saddles should be bottomed out as low as possible, but most of the pros tend to have them a bit higher than the average Pinkbiker. Here is Greg Minnaar's Champs bike last year. The seat is higher than the bars at rest. Something rarely seen at my local DH areas
p5pb10033445.jpg

Granted, he's pretty tall, but unless the person is super short, their race bikes tend to have their seat somewhere between really low and really high. Like Rachel Atherton's bike here. She is 5'6", and her seat is around bar height at rest too:
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The seat, when high enough, can provide a very important control point for riding. It seems that freeriders tend to value seat-to-nuts clearance pretty highly, so their seats are often bottomed out.
 

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Actually, it's a pretty common assumption for people to think that DH bikes' saddles should be bottomed out as low as possible, but most of the pros tend to have them a bit higher than the average Pinkbiker. Here is Greg Minnaar's Champs bike last year. The seat is higher than the bars at rest. Something rarely seen at my local DH areas
View attachment 887796

Granted, he's pretty tall, but unless the person is super short, their race bikes tend to have their seat somewhere between really low and really high. Like Rachel Atherton's bike here. She is 5'6", and her seat is around bar height at rest too:
View attachment 887795

The seat, when high enough, can provide a very important control point for riding. It seems that freeriders tend to value seat-to-nuts clearance pretty highly, so their seats are often bottomed out.
Good point, but pro's also need to pedal every section of a course that they can to be competitive, and therefore don't want a seat height that is inefficient for pedaling. Normal folks don't need that..
 

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Good point, but pro's also need to pedal every section of a course that they can to be competitive, and therefore don't want a seat height that is inefficient for pedaling. Normal folks don't need that..
good point. But rarely are they sitting down while pedaling, since they're superhuman beasts.
 

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To the OP - since DH bikes point downhill for the most part of their usage - slack seat post makes sense like a slack head tube... Picture the bike on a downslope... The angles all of a sudden aren't as slack.
 

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Discussion Starter · #9 ·
To the OP - since DH bikes point downhill for the most part of their usage - slack seat post makes sense like a slack head tube... Picture the bike on a downslope... The angles all of a sudden aren't as slack.
The fork is a moving part, the seat isn't, so I don't see much insight in that analogy. Slack HTA is valuable because is changes the caster angle, which has a big impact on how the bike rides. But there's a range of seat tube angles that can put the seat where it needs to be, especially considering that so many DH bikes have such short seat tubes anyway. I don't see the benefit.

Of course, I don't see a drawback to it either... I just wonder what the designers are optimizing for when they choose low STAs.
 

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The seat, when high enough, can provide a very important control point for riding. It seems that freeriders tend to value seat-to-nuts clearance pretty highly, so their seats are often bottomed out.
All that goes out the window if you have an aggressive angle on your seat tube. Your position changes the higher you go, you move further away from the cockpit as you extend the seat.
I found this out recently with a dropper, with the seat all the way down I was straight above the cranks, with the seat all the way up i was further away, the crank was infront of me, the seat tube angle on my bike is so that it changes the distance from the cockpit as the seat is extended. The OP is on the money. With some bikes you will notice it more than other bikes, all depending on the angle of the seat tube.
 

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Discussion Starter · #11 ·
To the OP - since DH bikes point downhill for the most part of their usage - slack seat post makes sense like a slack head tube... Picture the bike on a downslope... The angles all of a sudden aren't as slack.
This is a good reason for having the seat located quite a ways aft, sure. That makes perfect sense.

So picture a DH bike, with the seat located where it needs to be... and then imagine a line from the seat to the crank where most non-DH bikes would have the seat tube.

And then help me understand why the actual seat tube comes down from the seat at a very forward angl,e and then the kinks to a steep angle on the way down to the crank. I totally understand why the seat needs to be in the right place, I'm just puzzled as to why the seat tube angle is so different from other types of bikes.
 

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Discussion Starter · #12 ·
Another part of it is that the seat tube is usually moved around a bit to accomidate the rear suspension, usually by moving it forward and making the angle more slack.
Winner. I should have read this first. It just hit me that on most DH bikes, the rear wheel moves forward as it moves up. Moving the middle of the seat tube forward (or the bottom of a shortened seat tube, as the case may be) creates a space for the wheel to move into.

It took me forever to see this because my bike's rear wheel path is almost parallel to the seat tube. Bit of a forehead-slapping moment there.

Thank you all for bearing with me, it makes sense now.
 

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If you use clipless when you DH, (as I do for all riding), then you need a seat to "pull against" when pedaling/spinning. Makes climbing waaaay easier.
 

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Think about what where the wheel/tire is going to be at the end of the travel, now where do you put the pivots, how long do you want the chain stays to be, what would the wheel base be, etc? Most designers will pick the key points of a design then find a way to connect them with out things colliding.
 
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