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Finding this very much trial and error, but since I have been riding my new bike hardtail 20" Voodoo Bizango after so long I haven't started to suffer fro lower back pain. I have another bike a YT Capra large and don't get the pain with this, I have tried altering the seat position/height, bar height and stem but it hasn't shifted it yet. Any advice?
<span style="font-size: 0.8rem;">How do the lengths compare? A bike that was a couple of inches too short put me in agony that no amount of adjustment could sort.  </span>
which length should I be measuring, Top tube I presume? Just measured it and they are both 60cm from middle on Seat tube to middle of TT, so frames are the same. However I have a longer reach stem on the Bizango which puts it at 68cm whereas the Capra is 64cm, would a shorter stem help on the Bizango or worth a try?
Is the stack and reach, from the vertical plane of the BB to the centre of the bars the same on both bikes?
Initially, my Wazoo made my then very weak lower back ache badly after ~40mins of riding. I swapped out the 80mm 6 degree stem for a 100mm 35 degree, which IIRC, rose the bars ~5cm. Helped loads back then in early 2015, when I was much heavier and much less fit.
Surprisingly, the 35mm 0 degree Corto stem I bought at the same time as the Knuckleball bars, didn't upset my lower back (I guess the 45mm shorter reach offset the ~25mm bigger drop to the newer bars that have 25mm rise IIRC).
When losing weight from summer 2016, I started sporadically doing some core exercises, like "the bridge" and attempting the 2min "plank" challenge.
As I started cycling for fitness in 2017, I think the extra cycling exercise (my only exercise besides my job) naturally helped to make my lower back a bit stronger, plus I still do the odd core exercises (although probably less than I should).
Since last October, I've been running a 110mm 17(?) degree Specialized stem from my old Tricross on the Wazoo, mainly because it felt like it made the front end feel far more stable while descending at speed, fortunately my lower back has been fine with it.
Handy tool for stem adjustments  http://yojimg.net/bike/web_tools/stem.php
Iv measured again and the only difference now is the stem length,, the headtube is about 15cm on the Capra and 12cm on the Bizango however I have quite a few spacers on the bizango which gets it to the same level as the Capra. the Stem on the Capra is a 50mm and on the Bizango an 80mm
Saddle Angle.... I used to get back pain on all bikes from riding over many years.. 5 years ago I thoguht that I mostly used the saddle with effort for climbing and flat. I tilted the saddle forward alot so when the bike points up the saddle is kinda flat again. This also rotated my hips forward and straightened my back which also helped transfer the weight directly down through the axle to the floor instead of arching my back, keeping myself from sliding back on the saddle and trying harder to keep the front wheel on the floor. The bars need to be set properly also to save wrist or shoulder pain. When crouching down in front of your bike look at the bars. If they flare up like a plane flying they need to be rolled back to flat or close(depends on you) It may look odd when above and on the bike but bars are designed to work for comffort and control.. Control will be become better if you notice small differences.. These things I have changed on many bikes for people and made a difference for them.
How tall are you to need a 20"?
6ft 2 🙂
If you haven't been riding as much lately, it could simply down to losing strength in your core - whilst bike fitters will make out they can fix all ails, spending more time doing some core fitness exercises and stretches might be all you need?