Seat Tube Angle

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7/20/2016 7:16 AM

I am curious about this dimension. I have looked up and read the other posts. Other than giving slightly more or less rider room, in your experience does this have any effect on say a basic hop or manual?


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7/20/2016 8:21 AM

It's relative to the chainstay and toptube length. A really slack seattube will give the feeling of a short chainstay and shorter toptube. Whereas a steep seat tube will give the feeling of a longer chainstay, and longer toptube by effectively shifting your standing position (this is the reason bikes outside of bmx will give an actual toptube length and an affective toptube length, as it changes the position of the seat relative to the bb). With a slack chainstay, it's effectively pushing the bb forward, so with short chainstay, the back wheel will be more underneath you (in comparison to a frame with a steeper seattube but same length chainstays)

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7/20/2016 7:56 PM

Interesting! I never thought about how it relates to chainstay. Thanks guruSteve.

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7/20/2016 10:07 PM
Edited Date/Time: 7/20/2016 10:09 PM

It does affect the top tube feel but not the chainstay feel at all. The reason for this is because it comes down to the center of the bottom bracket which is one of your end points of the chainstay so no matter what degree the tube above it is the distance between your wheel axle and your bottom bracket (CS length) will
Remain exactly the same. The top tube is different because it's on the other (un-fixed) end of the seat tube and like Steve said it will push your tt forward or back more. Pretty much a bike with a 21tt and 69• ST and a 9in STheight will feel like a 20.7 with a 71• ST. Also your ST height will make the changes more dramatic. a higher ST will move the tt further back or forward than a shorter ST.

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7/20/2016 10:14 PM

This article goes into most of BMX geometry's mysteries and covers your question directly. Check it out.

http://bmxunion.com/blog/insight-bmx-frame-geometry/

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7/20/2016 10:40 PM
Edited Date/Time: 7/20/2016 10:43 PM

Sean_Goff wrote:

It does affect the top tube feel but not the chainstay feel at all. The reason for this is because it comes down to the center ...more

Nope. Simple geometry. If the bb is further forward but the chainstay remains the same length, the rear wheel is pulled forward and is more underneath you. Compare the wheelbase to 2 frames with the same length tt, same bb height, same chainstay length, but one has a slacker seattube angle

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7/20/2016 10:55 PM

If you say so man but go and ask sandm or cult before you tell me I'm wrong. You've got it backwards. The difference is this.. No matter what your ST angle is your feet will remain the same distance From your axle which is what determines your rear end feel. So just like whether you have a long or top tube on your bike the cs will feel pretty much the same minus the difference in front end leverage from the longer tt. HOWEVER the "feel" of the TopTube is dictated by its distance from the bottom bracket and not the chainstay. What that means, is like you were saying if you move the sEatube at the bottom bracket forward it will not change the cs length but it will change the BBs position relative to the HT which is at the other end of the tt and hence make your front end feel shorter. I could be wrong but go ask the pros first who actually work in the industry and see what they have to say about it. wink

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7/20/2016 11:07 PM

Any bicycle designer will tell you that what determines the feel of the front end is the relative distance of the headtube from the bottom bracket. A more lax sEatube angle brings the be closer to the Headtube giving the bike a shorter front end feeling. What it doesn't change however, is the chainstay length. So your end up with the same chainstay length and a shorter front end. In other words, changing the ST angle will make the bike feel like it has a longer or shorter top tube but not longer or shorter chainstays. It's math, it always happens that way.

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7/20/2016 11:45 PM
Edited Date/Time: 7/21/2016 12:32 AM

I work in the industry. I'm a qualified mechanic with over 10 years experience, and I've been riding bikes over 20 years. I used to work in a shop that specialised in custom building road bikes, and we used to measure the customer's to build the bike for them. If you don't believe me, ride a bike with a slack seattube

If all it would affect is the feel of the toptube, why wouldn't brands just make a shorter toptube?

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7/21/2016 1:44 AM

In my eyes and without writing a paragraph , it does not feel different . 71 is the average angle and anything less will feel like you have more room I imagine , if you're going into that much detail of a frame ; don't . Pick a toptube , chainstay length , colour and dropouts , if they're good then you're good haha .

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7/21/2016 8:12 AM

grumpySteve wrote:

I work in the industry. I'm a qualified mechanic with over 10 years experience, and I've been riding bikes over 20 years. I ...more

I'm not saying you don't work in the industry but you really should already know this if you do. I don't know you and don't feel like arguing but it's true and you don't seem to want to accept it comin from my mouth which is why I keep saying to ask the pros who do it. SEatube angle changes the relative "effective" length of the tt or to break it down better it moves the headtube closer to or further from the BB which is the same thing lengthening or shortening a TopTube will do. It's just a fact.

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7/21/2016 8:15 AM
Edited Date/Time: 7/21/2016 8:26 AM

Get on the BSDsite and there's something all about that and how the Passenger, because it has a 69 STA has a different "effective" tt length than bikes with a standard 71STA.
Also, on the new Volume Warhorse frame they said they raised the ST height to make the tt feel shorter. I'd tell you how that works too but you won't believe me so go ask them. I ride a 21 in Villicus with a69 sta and the exact same rear end as I had before and you know what... The rear end feels just like it used to. I'm trying to end a stupid argument here by giving you reliable facts and sources you can check this info with since it's so hard to believe coming from me.

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7/21/2016 8:51 AM

Also race bikes are totally different because you race sitting on the seat and yes.. Then ST angle makes A HUUGE diff but in BMX it's nil because you never really sit down to perform tricks so where the seat is is kind of a moot point.

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7/21/2016 9:44 AM

Do a manual on a bike with a slack seattube and tell me the chainstay length has the same feel. Simply saying I'm wrong doesn't make you correct.

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7/21/2016 10:06 AM

I do dude. I just told you I ride a 21in 69er and it has the same drivetrain as my last frame that had a 71. The rear end feels exactly the same But the front end feels shorter than my friends 21in BTM with the same (13.25) rear end. It's overall different because shorter Tts make everything more twitchy but I'm telling you I know exactly what you're saying and it really doesn't affect cs feel like it does on a race bike.

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7/21/2016 1:11 PM

CS length is measured from the center of the BB to the center of the axle. I dont care if the frame has a 68 degree ST angle or a 72 degree seat tube angle, the CS length will stay the same.

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7/21/2016 1:54 PM

rolland bluntz wrote:

CS length is measured from the center of the BB to the center of the axle. I dont care if the frame has a 68 degree ST angle ...more

Thank you! I don't see how he doesn't see this. Moving the seat angle makes the ht closer to the BB making the front end feel different but it doesn't change the distance between the BB and the rear axle.

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7/22/2016 2:35 PM

I take it you're both completely ignoring the position it will put the rider in?

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7/22/2016 3:05 PM

grumpySteve wrote:

I take it you're both completely ignoring the position it will put the rider in?

I know what steve is getting at, while the CS LENGTH doesn't change, it effectively will place the wheel FURTHER UNDER the rider with a more slack seat tube. The rear will feel different, because the rider's position ON the bike will change slightly-when you stand straight up on the bike holding the bars, you will effectively be FURTHER FORWARD relative to your seat position on a slack ST angle VS a steep one because the seat position WILL CHANGE.

Your position on a bike IS A HUGE factor in how it feels. Don't believe me? get a layback post and your exact seat and ride for a week. Then go back to a straight post. You will notice a difference in your bikes feel, even when EVERYTHING ELSE IS STILL THE SAME.



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"Hey anybody ever make that mistake like right when you wake up in the morning and you believe in yourself?" -Kyle Kinane

"BIKES!" -Tom Segura

7/22/2016 4:35 PM

There is really no helping you guys ask a pro and explain your theory to them and they'll tell you why. You are still wrong and I will also tell you why.
1. The position the seat is in does not affect the way a bike rides. This is different on a road bike because your butt is fixed on the seat and that changes where the cranks are relative to you. If you were to sit on your BMX seat while driving then YES you will feel the difference. But you don't. Yes Dave, you and Steve are both right about the be moving forward but you have both overlooked the fact that when your bb moves forward so does your rear axle so no matter how far forward you move it, your rear axle will still be the same distance from your spindle this changing.....NOTHING.
2. When your bb moves forward it CHANGES the distance between the headtube and the bb making the front end feel shorter/longer. This is the EXACT SAME THING that happens when you make your tt longer or shorter. Let's focus again on how our rear axle also moved with our bb and that distance hasn't changed but look again at how the distance between the be and headtube changed. So now, where do you think you're gonna notice the difference?
3. If you were to be sitting on your seat, YES, your point that the rear end positioning under you changed and yes you would notice the difference in feel but that is ONLY if you're sitting on the seat. When you aren't, your body will automatically go back to its original balance center but with a more lax STA that whole balance point will just be closer to the headtube just like when the TT is shorter. And oh yeah, your rear axle will still be just as far from your balance point on the spindle as it was before. You're mixing up race bike dynamics with a totally different riding style and what determines that difference is the fact we don't sit down to ride BMX. I'm out. Believe what you want. Just do the research first. like, actually do it.. You probably know someone in BMX who can with authority (not just assumption) affirm this.

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7/22/2016 5:38 PM

Sean_Goff wrote:

There is really no helping you guys ask a pro and explain your theory to them and they'll tell you why. You are still wrong ...more

To respond to your first point, if you move the rear wheel further forward along with the BB to keep the CS length the same, and a more slack angle on the seat tube to keep the TT length, guess what? YOU MOVE THE RIDER CLOSER TO THE BARS because the BB MOVES FORWARD TOO (ESPECIALLY FOR SOMEONE STANDING). Feel INSTANTLY CHANGES, especially while standing, but when sitting it is more of a cranks forward/upright sitting position. (Think old fart bikes). The balance point and center of gravity becomes higher and closer to the bars due to the smaller wheelbase.

2. It is NOT the same as your TT length changing. Don't believe it? Ride two of the exact same frames, one with a 21" tt, and another with a 20.5 TT. The rear end typically won't change there, but the frame will NOT feel the same. When you change the TT length, you also change the DT length to keep the same angle, making the front end of the frame (everything in front of the Bcool LONGER.

I understand that you think that same distance = same feel but that is simply NOT true. CS length may not change, BUT BB POSITION, ST ANGLE, TT LENGTH, HT ANGLE, BB HEIGHT all do factor in how a frame feels. Honestly so does pedal thickness.

You might not notice in matching frames with a half degree difference on the ST angle ONLY, but if take a frame with a 13.5" CS and put the BB an inch forward (which moves the axle the same distance forward), and had a very slack, say 60 degree ST angle with a 21" TT, it would feel VERY different from the frame with a ST angle of 70 on a 21" TT.

Know why?

BECAUSE THE WHEEL WOULD BE AN INCH FURTHER FORWARD COMPARED TO THE "NORMAL SPEC'D" FRAME. The wheelbase would effectively and physically be an INCH SMALLER. When standing, you would be closer to the bars to keep that balance point you mentioned, otherwise you would be leaning further back if you kept the same distance from the bars, which could cause accidental loop outs and a terrible ride.

Guess what? THAT IS A DIFFERENT FEEL AND YOU WOULD NOTICE.

In all technicality with my example, you would have to adjust to a MUCH slacker HT angle to give room between pedals and front wheel or you would have toe drag really bad when turning or barring something.

Yes this is an extreme example, but your BB position factors in SO MUCH, and you are clearly ignoring that for someone who is "so well educated and well versed" with this information.

By the way I have a degree in Metal Fabrication and Welding, and 17 years experience with bikes.

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"Hey anybody ever make that mistake like right when you wake up in the morning and you believe in yourself?" -Kyle Kinane

"BIKES!" -Tom Segura

7/22/2016 7:14 PM

Wow Dude. You just confirmed everything I just said. But you're too dense like the other guy to realize it. Also welding and fabrication is totally different from design geo and trig which is what is being discussed here. Also my whole point was that the BB position changes the feel so thanks again for missing that too lol. Good luck with metal fabrication and all. You're a great guy I'm sure but really, like I said before. Ask the people who actually deal with this subject to tell you how wrong or right you are.. Lol it's getting embarrassing now. You literally just confirmed my point and got on to an unrelated one about ht angles. Yeah, wheelbase changes... Just like it does when you shorten the tt hahahaha you probably still don't get it.

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7/22/2016 8:56 PM

This thread is fucking ridiculous and everyone involved should be banned.

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7/22/2016 9:11 PM

Xxohioanxx wrote:

This thread is fucking ridiculous and everyone involved should be banned.

Hahahaha right? I really feel dumb about it lol but I can't let it go! Haha Dave and Steve, you guys are great, I promise to buy you a beer if we ever meet!

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7/22/2016 10:39 PM

If the tt length is shorter, it shifts your whole body weight back, further over the back wheel compared to a bike with a longer tt. Hence it will put your body weight further over the back wheel, giving the feeling of a shorter cs. Just standing on the bike it will feel like just a shorter tt, but when you do anything that involves leaning back the front end will be easier to lift.
Think about it. Use a manual as an example. Leaning back with straight arms, your body weight will be further back. And you're laughing at us saying we don't get it? Lol

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