BMX Geometry technical questions...

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10/30/2021 1:19 PM

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Long story short. I'm 40, live in the middle of nowhere and thinking about upgrading my frame. I understand all the concepts about the various length, IE a short chainstay 12.5" for example make aerial tricks more snappy compared with 13.5" which better suited for trails.

The current rig is a 2018 WTP Justice, 20.75" TT and this maybe a little short for me as I am 6' (183cm).

I have a long torso and short legs, as such I thought I'd be a smart arse and see if I can work out the perfect geometry from the provided spec's from the various manufactures using CAD software. However, this is becoming a little trickier than I first thought. I've noticed that may brands don't include the BB rise above the axel level, nor the angle between the seat tube and the TT.

Can anyone give any advice? Cheers
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10/30/2021 5:35 PM

Low bb is more stable

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10/30/2021 6:09 PM
Edited Date/Time: 10/30/2021 6:11 PM

Looks like Solid Works and also you seem to be an engineer.

You are just giving technical informations, and just geometry informations. Who are you? Are you new to BMX? What do you need it for?

When I started BMX, I did not know anything and was glad to get even the shittiest frame, I could afford. Nowadays, it is a much more different topic. You really need to tell, who you are and what you want to do with it, to help.

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10/30/2021 11:36 PM

bmx_strays wrote: Photo

Long story short. I'm 40, live in the middle of nowhere and thinking about upgrading my frame. I understand all the concepts ...more

BMX brands measure BB height wrong. Rather than it being from axle height, they measure approx from the floor. 11.6ish is about average with 11.5 being considered low and 11.75 getting on the high side.
The angle between the seat tube and TT is kinda irrelevant as it's not going to affect the rest of the geometry. You'd use seat tube angle, stand over height (which is also measured wrong, it's what BMX call seat tube length. Centre of BB to centre of tt), and TT length kinda predetermine the angle of the TT. I mean, you could have a low sling tt but it'd be weaker without some kind of brace. And it'll look wafty.

I'd suggest sticking pretty middle of the road at your height/age (I'm also 40, and 6'). 13.3ish chainstays, with your wheel not slammed so it's more like 13.5. 21tt, 75 ha, 71 SA, 8.5-9 so, 11.6-11.75 BB height. That'll be a good all rounder. And adjusting bar height will have an impact on overall feel (I'm currently using 9" rise on a front load with low stack headset. Tempted to go back down to 8.75)

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10/30/2021 11:41 PM

Double post yolo.
Basically, everyone over thinks this, you'll be able to ride any bike just as well. It literally just comes down to personal preference, and changing a few dimensions isn't going to be the difference between pulling a trick or not.

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10/31/2021 1:09 AM

Yes you could say I'm over thinking this, however there is a point to my exercise which I am sure some of you may appreciate. Not having the luxury of being able to try various frames, this seem the only logical solution I could formulate before spending my hard earned cash on a new frame.

Why am I changing my frame?

1. The WTP Justice is a Tri-moly, meaning the back end is made from hi-ten steel. One of the inherent issues with this design is it starts to bend (come out of square) when doing 360s/spins on the back... this I have noticed with my current frame.
2. Being new to BMXing, my riding style can be very much described as 90's downhill MTB position, IE arm reached out and arse over the back wheel. They say you cant teach old dogs new trick, so I'm interested in seeing if I can mitigate this with geometry.
3. As per general consensus, at 6', 20.75" DT is a little short. I've changed my bars to a 9.5", stems etc. but each change, though improving the situations doesn't get away from the fact that the frame is too small.

The aim I guess is to see if you can use engineering and maths to see if you can choose the perfect frame without trying it out first. Will it work? that's another matter?

PS, chain stay lengths, from the observations done so far, (ok only 5 bike frame have been inserted into CAD) I'm coming the the early conclusion the tire size and rim selection plays more of a role than one would think.

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10/31/2021 1:32 AM

You are definitely over thinking it. You'll get used to anything. From the description of your posture, I'd definitely say 13.5cs, which isn't long by any means. Making your bars higher won't make a bike feel longer, if anything they'll make it feel shorter unless you Chicago your bars, due to the angle of the headtube.

Rim type doesn't matter (strength and weight matter to some people, but again, it isn't a night and day difference), and tyre size is personal preference. Wide tyres are a trend that's starting to die out. Everyone I know is starting to go down to 2.25/2.3 as it's more responsive.

To put this into perspective, are you going to wear the same clothes every day? Have the same amount of change in your pocket? Check your exact psi before every ride? Weigh the same? There's so many variables that small changes don't make a huge difference.
If you change one thing, it'll have both positives and negatives, and might take a little time to get used to, but won't have a huge impact on what you can and can't do. Keep away from extremes, like really long or short cs, or really slack or steep ha, and you'll be fine. There's no way to know what you like without trying different things. And what might be the "perfect" geometry might feel shit to you. It's all personal preference, nothing more.

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10/31/2021 5:22 AM

.pegless. wrote:

You are definitely over thinking it. You'll get used to anything. From the description of your posture, I'd definitely say ...more

Nothing wrong with thinking about something, especially when spending money. Am I over thinking it, probably, though as I know how to do this, why not?

In agreement about tires, personally I'm thinking about swapping my Federals to KHE mac 2+ to help with reducing the rotational weight.

Going back to the frames, I do think using CAD can help and its nothing like your analogy with small changes. Here's and example; currently, in order to clamp my legs to the saddle to bar spin, the seat post has to be raised 5". Looking at both the WTP Battleship and S&M Tall Boy, both can have a 21.25" TT, there's only 0.55" difference in the stand over height, however the headtube height it over 1" difference. Not much, but if I continue with 50mm stem and existing handlebars (set parallel to the forks), then the actual difference is huge.

Photo

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10/31/2021 5:42 AM
Edited Date/Time: 10/31/2021 5:43 AM

Hi, may I ask you what you like/don't like about how your current bike responds as a whole? (I understand your frame is too short, my question is more about how you feel on your bike when riding than numbers)

Also do you ride or plan to ride something in particular (meaning street/park/bowls/dirt) or more like a little bit of everything/what's available?

Last one are you more into tech/grind stuff or airs, flowing around, aso?

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10/31/2021 9:40 AM

tang-po wrote:

Hi, may I ask you what you like/don't like about how your current bike responds as a whole? (I understand your frame is too ...more

There nothing I don't like about my bike, it does the job. I've got used to is of sorts. Though I'm aware that the rear triangle is off centre, so always aware when doing good sized table tops about how hard I land.

Don't like the weight as is a killer when your trying to spin and rotate and hauling all that weight it a killer, (No, I'm not about to bulk up, I'm 40 with kids, so not doing an American Beauty!)

Currently doing a mixture of stuff, started in June at the local pump track, skatepark (concrete) for a few months, flatland trick and trails. All depending on the weather and work and family commitments. I like big jumps and crap at airing out on vert.

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10/31/2021 9:59 AM

I’m 40 as well and got back into BMX when my daughter started riding about 4 years ago. I’ve messed around a lot with geometry over the years and as many folks will say, it just comes down to personal preference.

Mathematically, there are probably “ideal” geos that are optimal for my body type and riding style...but I’ve found my combination of parts just seem to gravitate to what feels good.

Example: I have three forks/three bars/three stems that I have been playing with to rebuild my coaster and cassette rides. The most comfortable steering combinations of each all have almost identical total rise heights (steerer tube length+stem rise+bar height): 434mm +/- 2mm. This is not something that I thought about prior to buying parts, they just happened to add up the that.

Same thing with chainstays. I’ve ridden multiple geos and realize that my sweet spot is 13”. It’s responsive enough to make it quick and snappy, but also gives me room for back-pedaling when I do ride pegs. I love manualling and the 13” chainstay feels really comfortable...not a lot of effort to bring the front wheel up without sacrificing too much stability.

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10/31/2021 10:27 AM

.pegless. wrote:

You are definitely over thinking it. You'll get used to anything. From the description of your posture, I'd definitely say ...more

bmx_strays wrote:

Nothing wrong with thinking about something, especially when spending money. Am I over thinking it, probably, though as I know ...more

I started riding BMX when I was 14. My analogy is more accurate than cad drawings. You can have good days and bad days. That can't be measured and calculated, or put into practice. Changes in temperature, wind direction, etc, all plays a part in how well you ride. It's about feeling comfortable above and beyond anything else. I've had a wide variety of different set ups, so I know what I like. Some measurements and cad drawings will never change that. So much of bike set up is trends too, they come and go, so what if your measurements are based on current trends and lack of experience? You won't find the perfect geometry without actually trying different things to see what you like. Even just a year or so ago bars were taller, with most riders now switching back to frontload stems or lower bars.
Don't forget, what you intend to ride will play a huge part too. You won't be doing technical street on a bike with perfect geometry for park, or trails, or vice versa

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11/1/2021 6:25 AM

tang-po wrote:

Hi, may I ask you what you like/don't like about how your current bike responds as a whole? (I understand your frame is too ...more

bmx_strays wrote:

There nothing I don't like about my bike, it does the job. I've got used to is of sorts. Though I'm aware that the rear ...more

Very positive thing that you like riding your bike as it is.
Maybe try to get the same kind of geo frame in 21 or 21.25, shouldn't be too hard to find.
As far as weight goes, you want to focus on rotational weight as it's the most noticeable when riding.
In my experience maxxis grifter tires do help with rotational weight, they're grippy and pretty strong for their weight range. But any tire that's light and decent on weight will help in that aspect.
Cheers!

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11/2/2021 6:55 AM

who tf cares you'll get used to anything you ride as long as it's not in the extremes

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