Enduro/AM - The Weight Game

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Enduro/AM - The Weight Game
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Posted: Nov 21, 2019 at 15:10 Quote
How can a bike have a huge rear-center elongation but not have that present as pedal kickback? If the elongation is huge and the chain is taut, that would necessarily result in driving the rear wheel as Sherb described, or kicking back into the pedals. Would it not?

Posted: Nov 21, 2019 at 15:13 Quote
I'd imagine the idler is taking up the increase in chain length. At this point I can firmly say I have no real idea what was causing it.

Posted: Nov 21, 2019 at 15:14 Quote
R-M-R wrote:
thuren wrote:
This is fun I blocked a few numbers but I don't mind showing this at all this is fun. I'm in patent pending stage so all good!

Extreme as I can get the realistic in-use number... Sag, 30t chainring, small rear cog, blah blah... In the larger rear cogs AS goes down to normal numbers.

Thank you for the chart. Observations and thoughts:

• Centre of mass: It's high. That's roughly where I might place an XL rider's centre of mass, but remember to include the bike's mass.
• Sag: How much are you using on the model you shared? Based on the axle-to-crown of the Fox 36 and assuming, due to the head angle, front and rear are matched, it looks like 20%; based on the Ohlins pictured in prototype photos, it's about 30%. Your website mentions 25%; this makes sense for a linear leverage curve on flat ground, but it's considerably lower than where the bike will settle when climbing.

Good stuff!

That first pic was normal flat ground upright position(25% rear 30% front sag), small rear cogs...

Here's steeper climb simulation 35% rear sag 20% front sag tucked position CG, in the 42t rear cog....


Posted: Nov 21, 2019 at 15:21 Quote
sherbet wrote:
I'd imagine the idler is taking up the increase in chain length. At this point I can firmly say I have no real idea what was causing it.
Wasn't the idler in a fixed position with the pivot though? If that's the case it can't take up any chain growth and only serves to change the angle at which the chain is pulling from pedaling forces. If the rear center is growing then there would still be chain growth driving the rear wheel. Maybe kickback isn't the appropriate term but I can't see how the rear center could elongate without resulting in driving the rear wheel like you described.

Posted: Nov 21, 2019 at 15:37 Quote
badbadleroybrown wrote:
How can a bike have a huge rear-center elongation but not have that present as pedal kickback? If the elongation is huge and the chain is taut, that would necessarily result in driving the rear wheel as Sherb described, or kicking back into the pedals. Would it not?

Without an idler, yes. With an idler, no, because the chain doesn't run directly between the chainring and the cassette, so that pathway isn't the relevant one to consider.

When pedaling, kickback occurs because the rider is keeping the chain in tension, so your feet are directly connected to the chainstay elongation effects. Your feet won't rotate backwards, but their forward rotation will be slowed to some degree, which feels jerky.

When coasting, the chain will only go into tension if the chain elongation is more rapid than the rate at which the rear wheel is spinning the cassette forward. This doesn't happen because the impact would have to be unrealistically fast.


thuren wrote:
That first pic was normal flat ground upright position(25% rear 30% front sag), small rear cogs...

Here's steeper climb simulation 35% rear sag 20% front sag tucked position CG, in the 42t rear cog....

It's nice to see you've given some thought to various scenarios. My thoughts on the model inputs:

• 25% sag on flat ground makes sense for a linear motion ratio curve, especially with a coil shock.
• If you measure your own fork sag, are you at 30%? My flat-ground fork sag is usually closer to 20%.
• Super steep climbs can push the rear sag well over 40% and the front sag drops to almost nothing - maybe 10%. Moderate climbs will, obviously, be somewhere between the scenarios for flat ground and super steep climbs - close to your climb values, though I'd go a little less on the fork sag.
• On climbs, the centre of mass for the combined system moves rearward due to weight transfer. The rider doesn't adopt the hunched forward, climbing position on flat ground, so it's important to correct for the rotated frame of reference.

Posted: Nov 21, 2019 at 15:50 Quote
badbadleroybrown wrote:
sherbet wrote:
I'd imagine the idler is taking up the increase in chain length. At this point I can firmly say I have no real idea what was causing it.
Wasn't the idler in a fixed position with the pivot though? If that's the case it can't take up any chain growth and only serves to change the angle at which the chain is pulling from pedaling forces. If the rear center is growing then there would still be chain growth driving the rear wheel. Maybe kickback isn't the appropriate term but I can't see how the rear center could elongate without resulting in driving the rear wheel like you described.

Idler is inline with the pivot, so as the chainstay lengthens, the idler automatically decreases chain length as the chain is running a more straight line to the back end as it goes into the suspension.

Posted: Nov 21, 2019 at 16:05 Quote
sherbet wrote:
Idler is inline with the pivot, so as the chainstay lengthens, the idler automatically decreases chain length as the chain is running a more straight line to the back end as it goes into the suspension.
Ah, got it

Posted: Nov 21, 2019 at 19:47 Quote
sosburn wrote:
Circe wrote:
dchill wrote:


not if it's her's and his' bikes

Yup! Luckily we're the same height, so we can easily trade bikes.

Ahhh i wish my gf biked

Buy her an Ebike.. My wife had zero interest and now asks to go for rides. She rides her Turbo and I'll ride my regular bike. It's good.

Posted: Nov 21, 2019 at 22:22 Quote
swan3609 wrote:
sosburn wrote:
Circe wrote:


Yup! Luckily we're the same height, so we can easily trade bikes.

Ahhh i wish my gf biked

Buy her an Ebike.. My wife had zero interest and now asks to go for rides. She rides her Turbo and I'll ride my regular bike. It's good.

That's sweet.

Posted: Nov 22, 2019 at 7:31 Quote
R-M-R wrote:

• 25% sag on flat ground makes sense for a linear motion ratio curve, especially with a coil shock.
• If you measure your own fork sag, are you at 30%? My flat-ground fork sag is usually closer to 20%.



Yep with the coil shock, I designed the bike to be run at 25%, and still work great even at 20%.

Yes fork sag with a 160mm fork is designed to be a true 25%, and closer to 30% with a 170mm(as on print).

I always hated having to run my bikes in the past with 30-40% rear sag, and maybe 5-10% front sag, to have the bike hold better geo with aggressive riding/terrain. Big part of the Crowbar was being designed to run more balanced sag front to rear, mainly so using most of the fork travel on a regular ride was possible. Example..... On my Stumpy and Enduro I used to run 95-110psi no tokens in my Fox 36, and on the Crowbar I run 75-80psi with 2-3 tokens, to have the bike handle proper.

Posted: Nov 22, 2019 at 7:49 Quote
thuren wrote:
R-M-R wrote:

• 25% sag on flat ground makes sense for a linear motion ratio curve, especially with a coil shock.
• If you measure your own fork sag, are you at 30%? My flat-ground fork sag is usually closer to 20%.



Yep with the coil shock, I designed the bike to be run at 25%, and still work great even at 20%.

Yes fork sag with a 160mm fork is designed to be a true 25%, and closer to 30% with a 170mm(as on print).

I always hated having to run my bikes in the past with 30-40% rear sag, and maybe 5-10% front sag, to have the bike hold better geo with aggressive riding/terrain. Big part of the Crowbar was being designed to run more balanced sag front to rear, mainly so using most of the fork travel on a regular ride was possible. Example..... On my Stumpy and Enduro I used to run 95-110psi no tokens in my Fox 36, and on the Crowbar I run 75-80psi with 2-3 tokens, to have the bike handle proper.

I've observed a considerable shift in bike set ups of EWS riders and world cup downhillers over the past few years. Seems to me as if the downhillers are going less and less balanced, running a much more rearward bias of the bikes, whereas EWS riders are tending towards the more balanced setup that you are referring to.

Whether this has just been a result of myself paying more attention to this detail; a change in the style of the races; a change in the understanding of the most effective bike set up; or a combination of all three, I'm not certain. I've certainly began to prefer running a higher fork sag, including for the steeper terrain I've ridden, but it does require a change in technique.

Posted: Nov 22, 2019 at 8:21 Quote
bobj wrote:
thuren wrote:
R-M-R wrote:

• 25% sag on flat ground makes sense for a linear motion ratio curve, especially with a coil shock.
• If you measure your own fork sag, are you at 30%? My flat-ground fork sag is usually closer to 20%.



Yep with the coil shock, I designed the bike to be run at 25%, and still work great even at 20%.

Yes fork sag with a 160mm fork is designed to be a true 25%, and closer to 30% with a 170mm(as on print).

I always hated having to run my bikes in the past with 30-40% rear sag, and maybe 5-10% front sag, to have the bike hold better geo with aggressive riding/terrain. Big part of the Crowbar was being designed to run more balanced sag front to rear, mainly so using most of the fork travel on a regular ride was possible. Example..... On my Stumpy and Enduro I used to run 95-110psi no tokens in my Fox 36, and on the Crowbar I run 75-80psi with 2-3 tokens, to have the bike handle proper.

I've observed a considerable shift in bike set ups of EWS riders and world cup downhillers over the past few years. Seems to me as if the downhillers are going less and less balanced, running a much more rearward bias of the bikes, whereas EWS riders are tending towards the more balanced setup that you are referring to.

Whether this has just been a result of myself paying more attention to this detail; a change in the style of the races; a change in the understanding of the most effective bike set up; or a combination of all three, I'm not certain. I've certainly began to prefer running a higher fork sag, including for the steeper terrain I've ridden, but it does require a change in technique.

That's interesting man.

Something that might influence that is there's a lot of overforking goes on in the EWS.

Lot of bikes with 150-160 out back and 170-180 up front. Yeti SB150 springs to mind (150/170). That might lead me to set my fork up softer than my rear end because I want to use full travel.

Whereas in DH the bikes tend to either be balanced travel wise (200/200) or have even more travel at the back (e.g. V10 (215), Supreme DH(205)).

And some of the earlier Fox 49s were only 190 travel I believe, so a 29er v10 might have 215mm out back and 190mm up front, underforked 25mm. I'd probably have the rear softer than the front too.

Posted: Nov 22, 2019 at 8:36 Quote
thuren wrote:
I always hated having to run my bikes in the past with 30-40% rear sag, and maybe 5-10% front sag, to have the bike hold better geo with aggressive riding/terrain. Big part of the Crowbar was being designed to run more balanced sag front to rear, mainly so using most of the fork travel on a regular ride was possible. Example..... On my Stumpy and Enduro I used to run 95-110psi no tokens in my Fox 36, and on the Crowbar I run 75-80psi with 2-3 tokens, to have the bike handle proper.

Interesting perspective. I get what you're saying, but I actually like the firm spring and minimal ramp-up on my fork because it stays higher in the travel for a given bottom-out force.

Posted: Nov 22, 2019 at 9:48 Quote
In the days before big negative springs in forks, I ran like 30% out back and 5-10% in the fork.

I keep trying to set my bike up like this with newer forks and it just doesn't seem to work. Fork is so much stiffer than shock that it is really hard to get it to move. I finally relented and have been much happier with more even sag front to back.

Current fork is a 2017 36 with Luftkappe. Last bike that worked good the old way (30 rear, 5-10 front) had a 2014 Pike with the OEM air spring.

Posted: Nov 22, 2019 at 9:51 Quote
skerby wrote:
In the days before big negative springs in forks, I ran like 30% out back and 5-10% in the fork.

I keep trying to set my bike up like this with newer forks and it just doesn't seem to work. Fork is so much stiffer than shock that it is really hard to get it to move. I finally relented and have been much happier with more even sag front to back.

Current fork is a 2017 36 with Luftkappe. Last bike that worked good the old way (30 rear, 5-10 front) had a 2014 Pike with the OEM air spring.

Have you considered a coil?

I should add to my previous statements that I'm on a coil sprung fork and so far, so good.


 
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