Originally Posted by
mtbvfr
Hi Folks,
I have some questions for the Bike Fit gurus.
The bike in the photo has a 130mm/5° Rise stem. The frame is a 19" 1990 Diamond Back Ascent. I have marked the dimensions of the bike's actual Top Tube length as well as the length from the centre of the rails to the centre of where the steerer tube would be if it extended that far.
I have uneven leg lengths and to help counter that the Right crank (180) is 5mm longer than the Left. My Right
Femur is 5mm longer than the Left and the Lower Right Leg is 3mm longer than the Left.
I'm always having to push my butt backwards and the only thing I can think of as to the cause of that is that my knees/legs want to be further forward to be better positioned over the cranks.
However, moving the saddle forward means my back is more curved and constricts my breathing.
This current configuration is more comfortable than previous iterations where I tried shorter and taller stems with which I found I was having to push my butt backwards more often.
The current bar has a 35° back sweep which doesn't make for a good position for the bar-ends when standing out of the saddle. However, Ergotec has a 23° bar which shouldn't be too much different to a 25° bar which I found to be ok when standing out of the saddle.
This current bar has a 31.5mm mounting diameter and 0mm rise which affords a pretty comfortable aero position when placing my hands on the bar next to the stem.
I would prefer the stem to be no more than 120mm long.
Vital Statistics
Inseam: 84.5cm
Thigh: 41.0cm
Arm Length: 61.0cm
Torso Length: 60.5cm
Foot Length: 26.1cm
Height: 5" 10" = 177.8cm or thereabouts.
So, what Effective Top Tube length do you think would be more suitable for me?
Apart from having a custom frame made, were any frames made in the 90s with longer Top Tubes than what I am commonly seeing?
I wouldn't want a bike with a taller Seat Tube and I would prefer a lower Bottom Bracket too.
Thanks!, MTB.
don;t think I'm guru material, but I do have my thoughts on 'fit'. I'm not really well versed on current, modern MTB fit as well as for 'road', but it seems your current fit and 'desires' tend more towards a 'road' fit, rather than 'enduro'/all mtn type riding - which is much more upright and 'lower' center of mass, using droppers to adjust according to pedaling vs COG requirements.
You talk about being 'Aero', so I'm assuming a more 'road/old-school XC race' position; not the modern ALL TRAIL position. Low/No lug tires also seem to indicate that...
I won;t question your measurements - and also assume the differences between legs is accurate. And, yes as Iride01 mentioned, a pelvic tilt can also cause incongruencies - can happen from 'below' (your leg length differences) or 'above' (my scoliosis and compressed spine disks).
Anyway, saddle position and 'sliding forward'. 'Sliding forward' is usually a natural attempt to 'get over the gear', meaning you're feeling that you're not able to get 'full power' for the entire pedal stroke - IN THAT GEAR... if you're not sure... try this experiment... get into a gear, riding at an effort which causes you to clearly 'push forward'. NOW drop into the next lower gear (assuming these gears are relatively close... not always possible in mtb gearing)... IF, in lower gear, you're now not having the urge to push forward, it's because you can power that gear at whatever cadence you can reach same speed, is 'easier'... Example - road TT, most good TT riders will use the max gear they can power right to the max of their 'power'/cadence balance - ideally to produce the highest speed. It's RIGHT at the tipover point where next larger gear slows you down ... small changes in road profile, apparent wind, even 'effort' will cause 'slowing'', hence at that border, the constant want/desire to 'slide forward' to 'Get Over the Gear' better... which is what you're doing.
...so - leg length differences bring issues, and at best some compensation - one leg wants further back, one wants further forward, often what happens is favoring one leg over the other - very natural.
the key adjustment position is the saddle, setback and extension. since you've already 'compensated' with different crank lengths, best to pick a 'standard' measure point, like center of BB for both measurements (extension and setback) - your 62cm measurement doesn;t really measure anyhting useful. The Key is your sitzbones placement relative to the pedals/cranks.
My own reference for setback is horizontal setback of sitzbones relative to BB (using 172.5 cranks) - which for me is 300mm +- 2mm. so depending where I 'sit' on the saddle, I set the saddle so my sitzbones are 300mm behind the BB... saddle height fron center BB - I have determined, for me, to use as a 'start' std point - 88% of my 'cycling inseam' measurement - my cycling inseam is 88cm (34.6 inches), so my saddle height is 78 cm start (yeah, really weird, those numbers...). If I'm gonna be riding 'Big' gears a lot, like 'track' or 'crit race', I'm gonna move the saddle up 3-4mm and reduce setback 3-4 mm...
FYI, I'm now 5' 9.5" (176-77 cm), cycling inseam of 88 cm (34.6 in), my femur length is 440mm +_2mm, wingspan of 182 cm. At 25 yrs old I was 6'0", most all of the 'loss' is due to serious spine/disk compression and quite a bit of scoliosis (apparently genetic, runs through my mom's side of family).
Yeah Really LOONNG legs and arms with a torso length of an average person 5'3" LOL!
so, after all that, I would suggest : find/measure where you're sitzbones find themselves on the saddle, maybe use the BB to get that start/standard setback (for future use) .
commonly 'setback' is used from saddle nose horizontal to BB... so for my saddles, I measure where the sitzbones might be and then add the need additional as 'saddlenose to BB
example - my Spesh Phenom saddle puts my sitzbones 225mm from nose, I add additional 75mm distance from nose to center BB to get my 'setback' set.
FInally - try moving your saddle UP 3-4mm, don;t change setback yet. see if that gives a better balance of power for leg extension and reduces the need to 'push' forward on hard efforts.
it is a 'compromise' - there is no one position cures all
... as for the adjustment of your torso and hand position - that's up to what you want and how it works... again, there is no ONE fit which cures all.
I like a hand position which allows me to tuck my elbows in more, less outward bracing, so more back sweep works for me - but then I don;t really bomb knarly DH, Enduro type riding.
I'm old and break easily - tired of being in constant 'injury recovery mode' - so cautious when it gets really hairy. LOL!
Ride On Yuri