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1/3/2019 12:06 AM

Primoz: you get it!

A good seated position will get you up without too much pain, a good standing attack position will get you down fast and safe. Both can be combined.


1/3/2019 3:07 AM
Edited Date/Time: 1/3/2019 3:12 AM

Primoz wrote:

A longer reach needs to be balanced out with a much steeper seat tube angle. With a tall rider, this moves the seat forwards by about 5 cm or more. You can easily keep short chainstays and still achieve a much more forwards biased CoG. Granted, your front axle will be moved forwards as well, but still, the weight distribution wont be affected as much as it might seem. That's the key here.

A steep seat tube angle (78°-ish) with 420 mm ish chainstays on an almost 1300 mm wheelbase would give you a better distribution (more weight on the front) than a large 2015 Giant Reign for the same rider (who would, at 190 cm, need an XL frame - me for example).

Plus i still think the rear axle should be as close as possible to the rear wheel for your feet to drive the rear better. Just like you have your hands relatively close to the front axle. You don't steer the rear through the handlebars and you don't steer the front with your feet (much).

I struggle to understand the corelation you're making with short CS's, weight distribution on downhills and seat-angle prevalence. Steeper SA is definitely helping A LOT when climbing and this I why I went for the "forward offset seatpost idea" to compensate for crappy geometries. But there is another great allied on your quest to great seated position when climbing, and it's definitely not short cs's since it's what you seem to favour on your enduro bike. It's personal preference even though I'm quite at the opposite of this thinking tbh. Plus every time it goes techy (flat ground included) you tend to get off the saddle to better deal with it and to avoid comical bouncing or unpleasant kicks on your butt, so seated position above all may not be the best starting point when considering a MTB geo.

The short CS's argument hardly convince me TBO. It only works at it's best in conjunction with short front ends since you can easily shift your weight forward to a point where your weight is far enough on the front to compensate fort the short-cs's main detriment, which is a weight toward the rear axle. It makes for a very playful ride but also a twitchy one as the speed increase. Lots of 26 and 1st gen 27.5 bikes where designed this way and short CS on "modern long reach" geo seems to be a reminiscence of this era IMO.

To picture my argument I mentally draw a triangle between both wheel's centers and the rider's center of gravity. To simplify even further we're gonna assume it's somewhere above the BB in standing position (the position you have when hitting technical stuff, hence when being balanced matters most). Given your numbers, 1300mm WB with 420mm CS means a HUGE inbalance ratio. I'm pretty sure it would climb as horribly as it corners with all that weight on your rear wheel. Better have a 2 ply tire and loads of damping on the shock !

If you increase reach and steepen SA accordingly your seated position will stay good but not your standing position (it moves rearward = more muscular force and weight shifting to apply on the bars) AND when going uphill, since the unbalance created by your short CS is even more exagereted.
If you increase reach + steeper SA + longer CS then your seated position is still good on flat, better uphill (less wheel lifting) AND your standing position is more balanced.
This is why Pole spec's it's machine with 455 CS and there is an ongoing debate between them and Vorsprung on Youtube on this very topic, where Vorsprung considers that CS as long as 490mm would make it corner even better. (Maybe a proof that we may have reached the point of too long reach)


1/3/2019 4:16 AM
Edited Date/Time: 1/3/2019 4:16 AM

Like i said, a forward offset post is not a solution, since you're masking the issue of a slack seat tube angle, which, frame-wise, comes with a short reach (to still give you a normal top tube or cockpit length). A forward offset post would mean a very short (uncomfortable) cockpit, which would have to then be offset with a longer stem. And those seem to be going out of favor.

As for weight distribution, here are some numbers regarding my L Reign 2015 (too small for me actually) the upcoming Bird AM9 in XL and a geometry i have been throwing around for a 27,5" 160 mm enduro sled with a very steep seat tube angle.

The numbers are as follows (wheelbase value is caluclated, within 1 mm for my Reign, 6 mm too short for the AM9, but the calc was done very quickly now, it could be optimised somewhat; CoG X_pos. is X position of CoG in regards to BB; StA_actual is seat tube angle at my seat height of ~800 mm from BB, so actual value for me, not marketed).

Bike Chainstay Wheelbase (calc) CoG x_pos. StA StA_actual % Rear weight
Reign 434 1216 31,3 73 71,4 61,7
AM9 440 1285 57,1 76 75,1 61,3
Custom 425 1279 68,9 76 76 61,4

EDIT: Vital filters multiple spaces, so it's not formatted properly in the actual post :/

So you can see that even with a much longer chainstay, the AM9 differs only slightly from the custom geometry since it also has a slacker effective seat tube angle. And a shorter front centre as well. And the Reign, with a longer chainstay and even with a much shorter front centre, has the most weight in the rear. And i was trowing around numbers with even more extreme rear weight distributions (under 60 %)a while ago. On the other side, a ~1100 mm wheelbase with a 425 mm chainstay Commencal Meta 5.5 from 2008 has an even worse (more rearwards) weight distribution.

Granted, all of this is based on flat ground and being sat down. But i'd hazard a wager that rotating the bike towards an uphill would exaggerate the issue the Reign has due to a slack seat tube angle. Plus when stading up, if you actually do lift yourself up, you would pull yourself forwards. And with longer wheelbases you do tend to ride more towards the front to weigh the front more (longer chainstays give you more slack in this regard). But i remember going straight on in the first corner with my Reign when i got off the Meta, where you had to ride over the rear because the bike was so short.

Plus, technical climbs usually need to be done with the rider moving forwards on the seat to move the weight forward. While with ultra steep seattubes everybody raves at how well the bikes climb. And if you look above,t he weight distributions can be completely OK even with shorter chainstays. And the flat techy stuff, i tend to drop the seat and move the the middle of the bike to be able to pump things well, again moving the weight forwards, which is different to going downhill, where you tend to move it rearwards. So you'd keep your balance without any issues.

Since i haven't done many rides with different bikes (it's impossible to get an XL demo bike in Slovenia, let alone for free, and i don't want to ride other people's bikes or even bug them for it) and i'm not in a place where i could buy 5 bikes a year (honestly, who is?), i sadly can't comment on what i prefer more, longer or shorter chainstays.

Oh, BTW, this holds true for XL sized frames, as i mentioned many times. This is because with the average XL frame from a big company, the weight distribution is horribly biased rearwards with the slack actual seat tube angles. With smaller sizes the story is completely different.

Different sized frames will need to look differently, not designed by using a coper machine with a scale set to them like they are now. And not with the same rear ends, since antisquat values differ between sizes as well.


1/3/2019 9:30 PM

If the seat angle is steepened so that it reduces cockpit length and subsequently requires more reach, what happens when the rider is out of the saddle? The Ideal out of the saddle/descending reach number is then likely compromised for climbing.

I'm far more willing to adapt my technique to aid climbing than ride a bike that is too big for me to control on the limit descending.

I'm sure that would be the consensus.


1/3/2019 11:44 PM

JCL wrote:

If the seat angle is steepened so that it reduces cockpit length and subsequently requires more reach, what happens when the rider is out of the saddle? The Ideal out of the saddle/descending reach number is then likely compromised for climbing.

I'm far more willing to adapt my technique to aid climbing than ride a bike that is too big for me to control on the limit descending.

I'm sure that would be the consensus.

It is the other way around: a longer reach is better for descending, with more room for weight distribution so you can ride calmer, the steeper seat angle makes it possible to get forward in a comfortable position (for some), and the weight far up front for climbing and CoG further up front when climbing, meaning you can sit in your comfortable position while climbing steeps without getting out of the saddle or sliding to the front of the saddle. I ride a Pole, and I can ride calmer without correcting my position both up and down. Only issue is the long chainstays make the bike somewhat a chore to pop a manual, but you get the front wheel up if you want still. My manualing technique sucks anyway.


1/4/2019 12:35 AM

That makes sense, while moving the weight to the extremes requires more effort and more movement, you can be more precise in general. A very short wheelbase bike will cause you to weight either wheel too much too quickly just by making a small mistake or getting hung up on something.