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Questions Re: Effective Top-tube, Head-tube Angle, and Stem Length

KYHikerBikerKYHikerBiker Posts: 9
edited November 2017 in MTB buying advice
Hi all!

I'm looking for a new bike after 6 months away from riding, and finding a bike I can work with has been quite a challenge.

I'm a mere 5' 8.5", but I've found only one size M bike ever (an 18.5 Trek Superfly) that allowed me to ride without back discomfort/strain, which usu. sets in the night after a ride and lasts for a couple days.

I've owned two size M bikes, and I've ridden a half-dozen or more at shops around my region; all of them cause back strain that worsens the longer/more frequently I ride. I've become perceptive enough that I can usu. predict if this back discomfort will occur, based on how the bike feels when I ride in the standing position.

Recently, I rode a size L Salsa Timberjack, and voila! The bike felt great in the standing position, and I had zero post-ride back strain.

I did, however, have achy and snappy elbows and shoulders, I'm sure due to the bars being too far away.

So I'm trying to figure out whether I have a comprehensive enough understanding of effective top-tube (ETT) to make some reasonable predictions about my range of possible adjustments on the Salsa Timberjack.

Here goes: first, my old Trek Superfly had an ETT + stem length of 685. If I make the ETT + stem length of the T-jack identical to the ETT + stem length of the old Trek Superfly, can I expect the reach-to-the-bars (notwithstanding stack height) to be the same, in the seated position, on BOTH bikes?

My understanding is that ETT takes into account seat-tube angle, but I wonder if the tighter seat-tube angle on the T-jack might necessitate moving the seat farther back than I did on the Superfly, in order to get the right position over the pedals.

Secondly, assuming I can bring the bars back enough on the T-jack to get a good, seated, reach-to-the-bars, can I predict that the T-jack's range of adjustment will "tolerate" the specific stem-length reduction I'll need to make?

In order to get the T-jack in line with my old Trek, by my estimation, I'll need to reduce the stem from 70mm to 45mm, possibly even down to 40mm. I know that mountain bikers hail short stems these days, and I'm not a bit hesitant to use one; I'm just not sure if the T-jack in particular would "tolerate" a stem this short. The head-tube angle = 68.7, whereas my Trek had a head-tube angle of 69.6. The Trek did well down to about 70mm of stem length, but it got a little wacky around 65mm or less.

Cheers, and many thanks to any bikers who might help me with some input.

Posts

  • I might add that I've tried riser bars and various stem lengths on the size M bikes I owned, but still, only the 18.5 Superfly allowed me to feel good in the standing position and ride free of back strain.
  • robertpbrobertpb Posts: 1,866
    It's where the 3 points of contact that are the most important if you want a pain free back. So seat angle is less important than where the saddle is in relationship to the pedals, then where the hands are in relationship to where your backside is.

    Get that right and you can adjust most bikes to suit provided you start with the right frame size.
    Now where's that "Get Out of Crash Free Card"
  • robertpb wrote:
    It's where the 3 points of contact that are the most important if you want a pain free back. So seat angle is less important than where the saddle is in relationship to the pedals, then where the hands are in relationship to where your backside is.

    Get that right and you can adjust most bikes to suit provided you start with the right frame size.

    I feel like my back problems originate from inadequate standing position on the various bikes.
  • robertpbrobertpb Posts: 1,866
    Why would you stand on a bike it's for sitting on.
    Now where's that "Get Out of Crash Free Card"
  • JBAJBA Posts: 2,749
    edited November 2017
    I don’t understand why you are concerned about the ‘standing position’. You should be sat on the saddle as much as possible.
    According to the Salsa size guide you should, going by your height, be on a small or, at a push, a medium Timberjack.
    Unless you have abnormally long arms and/or legs a large is way too big for you.
    Maybe your bad back is because you are riding a bike that is too big and you are continually over-stretching?

    You need to go to a decent bike shop and get advice on what size bike you really need.

    As a comparison I am 6’ tall with a 31” inside leg. I ride a Medium Giant Trance with a reach of 425mm. The Large Timberjack has a reach of 463mm. There is no way I could ride that and remain comfortable.
    “Life has been unfaithful
    And it all promised so so much”

    Giant Trance 2 27.5 2016 ¦ Sonder Broken Road 2021¦ Giant Revolt Advanced 2 2019 ¦ Giant Anthem 3 2015 ¦ Specialized Myka Comp FSR 2009
  • robertpb wrote:
    Why would you stand on a bike it's for sitting on.

    Bike James disagrees:

    https://www.bikejames.com/strength/why- ... -pedaling/
  • JBAJBA Posts: 2,749
    Biking James has taken a small bit of data relating to road bikes and tried to apply it to mountain bikes. The geometries of the types of bikes are totally different, as is the gearing and style of riding.
    “Life has been unfaithful
    And it all promised so so much”

    Giant Trance 2 27.5 2016 ¦ Sonder Broken Road 2021¦ Giant Revolt Advanced 2 2019 ¦ Giant Anthem 3 2015 ¦ Specialized Myka Comp FSR 2009
  • JBA wrote:
    Biking James has taken a small bit of data relating to road bikes and tried to apply it to mountain bikes. The geometries of the types of bikes are totally different, as is the gearing and style of riding.

    Indeed. And I won't pretend I developed a riding style based on Bike James!

    I just tend to ride in that position a lot (much less so on the road), and I thought it was pretty typical to ride standing when on trails.
  • 02GF7402GF74 Posts: 1,294
    robertpb wrote:
    Why would you stand on a bike it's for sitting on.

    Bike James disagrees:

    https://www.bikejames.com/strength/why- ... -pedaling/

    Doesn't matter what that says, a bike has a saddle so is meant for sitting, as above. Sure it is OK to get out of the saddle to accelerate or climb up hills but that is a tiny amount of the riding time.

    You don't need to read nonsense like that but use your noggin and try for youre.

    Do a short 1 mile ride. Youll do it fine sitting but I'll bet you would not be able to do it sanding the whole way.
  • mattyfezmattyfez Posts: 638
    Back pain and elbows snapping sounds a bit extreme.

    Do you have an underlying health issue? I'm very tall and can tolerate my much shorter girlfriends bike for about an hour before I get seriously uncomfortable.

    Standing position is fairly much irrelevant.

    This is quite a good video about bike fitting, it's road bike focused but the same principles apply to mountain bikes unless you want to do the red Bull rampage, it's a good general guideline..

    https://youtu.be/oxNznrlRXGU

    That bike james site sounds like absolute guff to me.
    You only stand off the saddle when you want a short burst of power, it's not a normal riding position.
  • The RookieThe Rookie Posts: 27,748
    JBA wrote:
    Unless you have abnormally long arms and/or legs a large is way too big for you.
    Wrong way round, bike length is predominantly set by torso length, so for the OP a larger frame would suite if he had short legs (therefore a longer torso than average for his height) and the normal correlation of arm leg length would mean short arms as well.
    I'm 5'9.5" but have short legs and arms, from experience I know I need a bigger frame to match my torso. I'm taller sitting down than a mate who is 6'1" and nearly as tall as another mate who is 6'4", but he has freaky long legs.
  • mattyfez wrote:
    Do you have an underlying health issue?

    Not to my knowledge, though certainly a reasonable question.

    I can throw two+, 18-hole rounds of disc golf at a sustained, sweat-dripping, water-guzzling pace any day I get a chance.

    Last weekend, I backpacked 14 or more miles over three days with a 45 lb pack (I pack too much. Something I gotta work on).

    No back discomfort whatsoever.
  • KYHikerBikerKYHikerBiker Posts: 9
    edited November 2017
    Deleted post
  • The Rookie wrote:
    JBA wrote:
    Unless you have abnormally long arms and/or legs a large is way too big for you.
    Wrong way round, bike length is predominantly set by torso length, so for the OP a larger frame would suite if he had short legs (therefore a longer torso than average for his height) and the normal correlation of arm leg length would mean short arms as well.
    I'm 5'9.5" but have short legs and arms, from experience I know I need a bigger frame to match my torso. I'm taller sitting down than a mate who is 6'1" and nearly as tall as another mate who is 6'4", but he has freaky long legs.


    This appears to be the case. A shop owner took some measurements and brought this apparent fact to my attention. He said I have short legs with a longer-than-average (for my height) torso. Unfortunately, he didn't have any bikes on hand that would serve well for a test-ride.

    He did say he'd put me on a size L frame without hesitation and work from there.
  • JBAJBA Posts: 2,749
    The Rookie wrote:
    JBA wrote:
    Unless you have abnormally long arms and/or legs a large is way too big for you.
    Wrong way round, bike length is predominantly set by torso length, so for the OP a larger frame would suite if he had short legs (therefore a longer torso than average for his height) and the normal correlation of arm leg length would mean short arms as well.
    I'm 5'9.5" but have short legs and arms, from experience I know I need a bigger frame to match my torso. I'm taller sitting down than a mate who is 6'1" and nearly as tall as another mate who is 6'4", but he has freaky long legs.

    I appreciate that and understand that frame length is usually more important than height in an MTB frame but if you look at the geometry of the Timberjack there is quite a difference in the stand-over heights between a small and large frame (over 3 inches) and that has to come in to play as well doesn't it?
    “Life has been unfaithful
    And it all promised so so much”

    Giant Trance 2 27.5 2016 ¦ Sonder Broken Road 2021¦ Giant Revolt Advanced 2 2019 ¦ Giant Anthem 3 2015 ¦ Specialized Myka Comp FSR 2009
  • The RookieThe Rookie Posts: 27,748
    How does standoever effect rider comfort when in the saddle?
    Yup I frequantly have minimal or negative standover on bikes that fit me perfectly.
  • jempijempi Posts: 58
    The standing / in saddle discussion did reminded me to this historical moment:
    [
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