BB / bottom bracket height feeling and frame geo in general

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6/22/2018 2:34 PM

hi, im starting to ride more park and i have this frame : tt 20,6" ; cs 13.2" (slammed probably) ; bb 11.7" and classic 75° head tube angle and 71° seat... and 8" standover but we dont care. Im 5'9" just for info and I was looking for a new frame like 21" and something around 13.4" (the in between responsive and stable), im sure a it would be good but idk about 11,7 or lower bb height. what have u rode and what u think about the bb height ? I rode a fbm gypsy (probably the 3 but idk) from a friend which was 20,75 13,5 and 11,5 but i haven't felt a real difference, i was focused on the pedals feel bc im rly RLY picky with different pedal (more than frame and bars height lol).
So yh you probably dont care about details, but what u think about that bb height ? is it good to lower it or with a longer frame which is more stable, it will be so slow for the spin ?

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6/22/2018 4:12 PM

"classic 75° head tube angle" laughing

Why would you change the frame at all? If you're comfortable on your bike set up as is, I'd say to leave it. You'll actually make it harder to ride new stuff(park, in this case) by changing your setup as you'll also have the learning curve for riding a different setup.

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6/22/2018 5:51 PM
Edited Date/Time: 6/22/2018 5:52 PM

75° isnt the standard bmx complete angle ? i know dirt bike got lower angle
and i want to change bc in street that was good but sometime thats maybe a lil bit short and so responsive imo (for park), and i wanted to know if the bb height rly matter about responsiveness

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6/22/2018 6:21 PM

75 is pretty normal these days but the classics are 74-74.5, in my opinion, at least.

Personally, I prefer a twitchier bike on trails and in a park, just because it's what I'm used to but that's not to say I'd change parts just to get it more(or less) twitchy. It won't affect you nearly as much as you think. You don't need a certain setup for anything; BMXs are extremely versatile. If you're uncomfortable enough that you're thinking of changing the setup that works for everything else, you probably just need to get more comfortable in the park. People blame their setups for far too much, I think.

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My Sunday Soundwave V3 Build
Insta: @p.gibbons

"You can't educate pork"
- grumpySteve

"Life should not be a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside in a cloud of smoke, thoroughly used up, totally worn out, and loudly proclaiming "Wow! What a Ride!""
- Hunter S. Thompson

6/23/2018 12:20 AM

11.5 is low and horrible to me. I built 2 bikes, identical apart from BB height, and I could barely hop on the lower BB, it confuses me why so many street frames have such a low BB. 11.65 to 11.75 is the sweet spot imo. If you want stability, get a slightly longer chainstay. At your height a 21" toptube will feel a bit big probably. It also depends what kind of park riding you do. If you want to learn as many tricks as possible, you want a responsive frame. If you just want to go fast and boost everything, then you need a stable frame

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6/23/2018 2:44 AM

the difference between 20.6" and 21" is just 1 cm longer (im french so cm is a more concrete overview) and didn't thought that was so big (and like i said sometimes it really feels smart, my knees are rly rly close to my bars and i have a bars a lil bit forward, not chicago). i don't want to change frame immediately bc i can ride it a good time i think, there is no cracks or anything.
I was just thinking of the new one for next year probably, so maybe a 20,75 or 20,875 with around 13,3 or 13,4 cs with 11,7 will be good.
I will just try some other rider's bike to see

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6/23/2018 2:56 AM

and does one of you have or know someone who have a raw/clear coat/gloss clear frame ? i love how it looks but i saw some pics a topics talking about rust on it and rust on head tube, seat tube and BB weld looks rly terrible. Where i live in france it's not rly humid so u think it will takes how long to rust ? if i take care with spray (idk exact name) will it rust too ? (that's another topic i know but if u have experience about that)

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6/23/2018 3:09 AM

Even a quarter inch is a noticeable difference in tt length. But it's personal preference. If you're hitting your knees, that'll be to do with your posture more than frame size.

I've got a clear coated frame, no rust at all and I've had it over a year, and live near the sea. And I stripped the clear coat off the front end a couple of months ago. Just look after it and keep it clean

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6/23/2018 6:03 AM

thx steve i think my next one will be 20,75

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6/23/2018 6:48 AM
Edited Date/Time: 6/23/2018 6:50 AM

and one more thing about the tt length, if i custom a fbm frame with a 20,875 but i tweak the seat tube angle like 70,5° or 70° it will feel shorter but idk, i saw on this page https://bmxunion.com/blog/insight-bmx-frame-geometry/ that with a 9" standover the difference between 71° and 69° is 0,31" wich is a big difference, so i will do math (and correct me if im wrong).
for a 9" standover a 70,5° is around 0,08 but with a 8,5 standover ( (8,5x0,0775)/9) it will be 0,073 so the 20,875 tt feel like a 20,8 with a 70,5° and a 8,5" standover right ? i hate math but thats helpful haha
the only minor change with the fbm custom is that they have the choice between 11,625" and 11,75" bb height and no 11,7" but with a lil bit longer frame (rear a lil bit longer too) the 11,75 is maybe good no ?

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6/23/2018 7:09 AM
Edited Date/Time: 6/23/2018 7:29 AM

benn63720 wrote:

and one more thing about the tt length, if i custom a fbm frame with a 20,875 but i tweak the seat tube angle like 70,5 or 70 ...more

I've got a custom fbm frame, it's badass! But I really would suggest riding quite a few frames first in order to know exactly what you want. I stuck with 71° seat tube angle, because that's standard, and I saw no point or benefit changing it. If you pick a toptube length, that won't change (actual). But the effective tt will change when you play with stand over, head angle, and seat tube angle. If you make the seat tube angle steeper, it increases the effective top tube length (by moving your bb back). I really don't know if .5° will be enough of a difference to feel it though, and shortening the tt length to compensate might be over compensating. I went with 11.625" bb height, as that's the same (pretty much) as my old s&m cam wood v2. And I went with the 8.625 stand over, with a 21.125 toptube, 74.5 head angle, and 13.365 chainstays (my wheel sits about 13.5, but I don't like my wheel slammed) Photo

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6/23/2018 9:44 AM

yh the frame looks BADASS dude thats the look i want (black bars and brown leather seat) ! and yh i know for the seat tube angle it obviously doesnt change the tt length but affect the feel of the tt length (and so overall feel of the bike), and yh in theory with these calculations the 20,875" feels like a 20,8. I dont need to change it for the moment i will progress and next year i will probably order one between 20,75 or 20,8 (with 70,5 or 70 seat angle) and 13,25 or 13,375 cs i will see what is the best for me

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