MegaSack DRAW - This year's winner is user - rgwb
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Hi Guys & Girls,
I don't proclaim to be a guru but I always thought that I sort of understood bike geometry but I'm scratching my head here.
So here's my conundrum... I had an On One Inbred 29er laying dormant in the garage and thought I'd build it up into a chunky winter road training bike. I threw some drops on it, some slicker tyres and started pedalling. I loved it! Then I took it to my local woods, done a bit of easy singletrack & fire roads and it was even better!
My problem is, the bike is a 19.5" (Large) and because of the drops, I've had to fit a 35mm stem which when coupled with the narrow bars, feels a little twitchy.
I'm looking to downsize the frame to a 18" (Medium) Inbred to give me a bit less reach and allow me a bit more room to fit a longer stem to slow the handling down a bit.
So if I downsize the frame (and therefore seat tube) by 1.5" to get a shorter effective top tube length, I will have to run my seat post 1.5" higher. Taking into account the seat tube angle, moving the saddle up will in turn, move the saddle backwards.
I've had a little play and moved the seat up by 1.5" on my current bike and this moves the saddle rearwards by about 15mm (never measured too accurately, in a darkish garage with an old tape measure). The smaller frame has a reduced seat tube length of 18mm. Does this mean by down sizing the frame will only reduce the reach by 3mm?!
Not sure how accurate this geo chart is but this is where I got my information: https://geometrygeeks.bike/bike/on-one-inbred-29er-vertical-dropout-2017/
Apologies for the essay!
Any ideas?!
Surely the seat tube will be the same angle on them both, so even though the saddle is less pushed into the frame on the larger size, the horizontal "estimated top tube" measurement will be shorter in the smaller size with the saddle at the same overall height?
Since you know the angle of the seat tube, you could use cos/sin/tan to figure out exactly how much the seat will move backwards. However assuming you also have to raise the handlebars (because the stack is lower) then the two will almost cancel each other out (72° Head tube angle vs 73° seat tube angle. Just calculate the difference 1° makes).
In theory the seat should end up exactly the same distance behind the bottom bracket (the seat tube angle is the same, so assuming the saddle ends up the same distance from the bottom bracket as measured on the line of the seat tube) the bike will be shorter by the difference in reach.
Can you buy inbreds anymore? If you have to buy a frame there are probably much better choices (like the Genesis drop bar almost a mtb frame, I forget the name, for instance)
Thanks for the replies!
looks like I was getting confused with how the effective top tube was measured. Told you I wasn't a guru!
I know there are probably better frame choices out there, but they start getting £££. I think it might be the genesis longitude that you're on about. This would be the ldeal candidate I think, but was hoping to pick up a used Inbred frame for ££ and swap all my bits across.
Again, thanks for the help!
I think you're overcomplicating this.
To start with, the horizontal distance from saddle to BB is known as saddle setback.
Given both sizes have the same seat tube angle, the set back should not change, you'll just have more exposed post, this doesn't affect the fit. You wouldn't run your saddle lower on a smaller frame? I think that's what you were suggesting?
Head tube lengths and head angle are supposedly contant as well, though I'm not sure if this is accurate, this means the reach difference is easier to work out as the only factor is top tube length.
So the shorter top tube will be significantly shorter reach and you can run a longer stem.
Before changing the frame though, do you have a setback post and can you use an inline instead? This effectively steepens the seat angle, reduces the saddle setback and means you might get away with a longer stem. I'd try that first.
I was knew the seat post would be 1.5" further out of the seat tube but was forgetting that the saddle would be the same height from the BB.
It all makes sense now!
Looking to pick up a different frame set is also starting to look like a better option. I was just trying to stick with the inbred as I could swap over all the components (QR front and rear wheels especially) to save cost but looking at the Genesis frame sets, even the new ones appear to be QR front and rear and same component sizes as the inbred throughout!
I was thinking Genesis Vagabond (or the Surly, etc equivalent). There's lots out there like it now. The geometry is built around dropbars with as much mtb DNA in there as possible. Being designed around drop bars, the reach should be right without throwing off the other measurements.
I was looking at the vagabond last night actually. Can pick up a 2020 Frameset for £345 brand new at the minute from one of the websites. This might be the way forward!