Advanced geometry nerding

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1/3/2019 12:20 AM
Edited Date/Time: 1/9/2019 7:33 AM

Just watched this for Vorsprung suspension.



Really cool videos, both part 1 and this. Some good and interesting points by Steve.

Bonus is the discussion between Steve and Leo from Pole in the comment section.

Ps. Kind of interesting that Steve is on the Deviate and loves it. The mtb-media kind of excused this bike for being a niche bike for the alps. Noting conservative geo. I thought when i read the reviews that this wasent the bike it could be, but was i duped by what the mtb-media dictates as the be-all end-all geo? And yeah, its a gearbox bike, but that´s a different discussion.

Any thoughts on this?

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1/9/2019 7:50 AM
Edited Date/Time: 1/9/2019 7:57 AM

See Also: The Internet Was Wrong; Short Chainstays Suck

Seriously, if you get into the data, you'll see fork offset helps the front/rear bias, but nothing helps as much as increasing your rear center as your front center grows.

I believe this so whole heartedly, I will be making my own seatstay for my Sentinel this year - that creates a (GASP) 18" long CS length.

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1/9/2019 10:05 AM

jeff.brines wrote:

See Also: The Internet Was Wrong; Short Chainstays Suck

Seriously, if you get into the data, you'll see fork offset helps the front/rear bias, but nothing helps as much as increasing your rear center as your front center grows.

I believe this so whole heartedly, I will be making my own seatstay for my Sentinel this year - that creates a (GASP) 18" long CS length.

That's rad! How much fork offset will you be running?

Chris Porter seems to prefer long chain stays AND short offsets, while other brands such as Pole don't seem to feel fork offset is as important. I think I read a post somewhere from Leo at Pole where he said he's tested both offsets and didn't feel a significant difference.

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1/9/2019 10:14 AM

jeff.brines wrote:

See Also: The Internet Was Wrong; Short Chainstays Suck

Seriously, if you get into the data, you'll see fork offset helps the front/rear bias, but nothing helps as much as increasing your rear center as your front center grows.

I believe this so whole heartedly, I will be making my own seatstay for my Sentinel this year - that creates a (GASP) 18" long CS length.

Marshall Willanholly wrote:

That's rad! How much fork offset will you be running?

Chris Porter seems to prefer long chain stays AND short offsets, while other brands such as Pole don't seem to feel fork offset is as important. I think I read a post somewhere from Leo at Pole where he said he's tested both offsets and didn't feel a significant difference.

37mm

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1/9/2019 7:11 PM

jeff.brines wrote:

See Also: The Internet Was Wrong; Short Chainstays Suck

Seriously, if you get into the data, you'll see fork offset helps the front/rear bias, but nothing helps as much as increasing your rear center as your front center grows.

I believe this so whole heartedly, I will be making my own seatstay for my Sentinel this year - that creates a (GASP) 18" long CS length.

In case bike designers ever ski, that is obviously, noone increase front center of 🎿, always 🎿 grows in both directions

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FUN

1/9/2019 7:33 PM

Marshall Willanholly wrote:

That's rad! How much fork offset will you be running?

Chris Porter seems to prefer long chain stays AND short offsets, while other brands such as Pole don't seem to feel fork offset is as important. I think I read a post somewhere from Leo at Pole where he said he's tested both offsets and didn't feel a significant difference.

https://www.mbr.co.uk/news/chris-porter-dream-bikes-379753

As of Sep 20 '18, Chris Porter is on a Geometron G16 in XL with 29 front, 27.5 rear. Quote from the article, "I’m running the Mojo Rising adjustable offset crowns on a [Fox] with a 29er front wheel and I’ve settled on 50mm offset for the best compromise between massive lean angle, hero drifts and carves on known trails, and fast, unplanned direction changes on blind trails." I recall elsewhere, that he found that the 51mm offset forks steer sharper more because the wheel is more inboard on the corner, than due to the mechanical trail.

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1/9/2019 9:04 PM

I am getting a steel FS frame custom made (3.9kg w/o shock) with geo based on balancing FC and RC. Hoping I have the numbers right to get the right amount of weight on each wheel, lowering the need to "hunt for balance and traction" or "compensating for the bike's weight bias": 445 RC to 845 FC, with 62d HA and 160mm travel F/R (monopivot). Had to go custom because I wanted a short seat tube (for max seat slammage) and mixed wheel size. Seat angle 80-81d, reach 490 for a 590 ETT. I'm 170cm/5' 7" with 76cm/30" inseam.

ROS9 was my first taste of balanced geo, but I craved more capability.

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1/10/2019 6:02 AM

At 5’7 and a 490 reach I’d be hunting for a small person to ride on my shoulders

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1/10/2019 8:55 AM

Why move the weight forwards with long chainstays when we can move it forwards with steeper seat tube angles on alrger bikes? And keep the stays reasonably long/short. Same effect (more weight on the front) and a shorter (but still long) bike.

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1/10/2019 11:15 AM

ninjichor wrote:

I am getting a steel FS frame custom made (3.9kg w/o shock) with geo based on balancing FC and RC. Hoping I have the numbers right to get the right amount of weight on each wheel, lowering the need to "hunt for balance and traction" or "compensating for the bike's weight bias": 445 RC to 845 FC, with 62d HA and 160mm travel F/R (monopivot). Had to go custom because I wanted a short seat tube (for max seat slammage) and mixed wheel size. Seat angle 80-81d, reach 490 for a 590 ETT. I'm 170cm/5' 7" with 76cm/30" inseam.

ROS9 was my first taste of balanced geo, but I craved more capability.

Sounds like an awesome bike. I hope you put up a review with your thoughts after you've gelled with it!

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1/10/2019 11:18 AM

Primoz wrote:

Why move the weight forwards with long chainstays when we can move it forwards with steeper seat tube angles on alrger bikes? And keep the stays reasonably long/short. Same effect (more weight on the front) and a shorter (but still long) bike.

Steeper seat tube angles will move the weight forward only whilst seated. Long chainstays will balance the bike out when standing.

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1/10/2019 11:22 AM
Edited Date/Time: 1/10/2019 11:28 AM

Primoz wrote:

Why move the weight forwards with long chainstays when we can move it forwards with steeper seat tube angles on alrger bikes? And keep the stays reasonably long/short. Same effect (more weight on the front) and a shorter (but still long) bike.

Fred_Pop wrote:

Steeper seat tube angles will move the weight forward only whilst seated. Long chainstays will balance the bike out when standing.

Steeper seattube means longer effective top tube and a longer cockpit. Thus you wil stand up, not back.

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1/10/2019 11:26 AM
Edited Date/Time: 1/10/2019 11:28 AM

Primoz wrote:

Why move the weight forwards with long chainstays when we can move it forwards with steeper seat tube angles on alrger bikes? And keep the stays reasonably long/short. Same effect (more weight on the front) and a shorter (but still long) bike.

Fred beat me to it.

seat tube angles have very little to do with the way a bike *really* handles when going downhill. The relationship between hand placement (handlebars), head tube angle, front center, rear center, foot placement and overall height is what we notice when descending.

Its funny, it seems most bike designers used to be a certain size, so a lot of the numbers were based around a good handling medium frame. To add, back in the day, an XL is now a medium in terms of actual length (if not a small!). Hence the whole rear center thing wasn't as pivotal (no pun intended).

As bikes have gotten crazy long, the internet kept short chainstays a thing. And for a lot of applications, this still makes sense. If I am not racing, just having fun, want to manual with the flick of a wrist (persay), a 16.x" chainstay still has its place.

If I'm racing however, or want to push the bike hard and move as little on the bike, you HAVE to vary the rear end length or you will be at a disadvantage compared to smaller riders on smaller more proportional bikes.

The bummer is, without doing some magician SC Syndicate type stuff, your travel too will start changing as you push the CSs (or SSs) longer.

For some bikes, this could be a good thing (Sentinel - where I'll be trying it). Other bikes, not so much...

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1/10/2019 11:30 AM
Edited Date/Time: 1/10/2019 11:31 AM

Primoz wrote:

Why move the weight forwards with long chainstays when we can move it forwards with steeper seat tube angles on alrger bikes? And keep the stays reasonably long/short. Same effect (more weight on the front) and a shorter (but still long) bike.

Fred_Pop wrote:

Steeper seat tube angles will move the weight forward only whilst seated. Long chainstays will balance the bike out when standing.

Primoz wrote:

Steeper seattube means longer effective top tube and a longer cockpit. Thus you wil stand up, not back.

Top tube is one of those "who cares" measurements. Again, when you are standing up you feel the distance between the BB and the handlebars. The rest really doesn't matter.


EDIT: To add, a steeper STA will actually make for a shorter top tube length, should you care.

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1/10/2019 11:32 AM

Oh, didn't know we are talking about dowhnill bikes, i thought the topic is about bikes that get pedalled on. Which is done sitting in the saddle for ~80 to 90 % of the ride.

I agree, the seat tube angle has little to do with 'downhill handling'. But it's stupid to optimise for that when you do it for ~15 % of the time and exert even less energy than that while doing it. But hey, that's my train of thought.

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1/10/2019 11:52 AM
Edited Date/Time: 1/10/2019 2:07 PM

Too rearward of a weight bias, and your front wheel is more prone to lifting and being squirrely--doesn't feel planted in the rough, prone to washout when turning, and lifts on steep tech climb (esp on impact with roots and stones), often throwing you off your intended line. The rear wheel also sees higher impact forces due to the extra weight, suffering extra damage. You counter rearward bias through a shorter front center and/or longer rear center.

Too forward of a weight bias and the front is hard to lift, and prone to diving on drops and jumps. There's a high tendency to go over the bars. It's safer to ride, *if* you defensively take a position to avoid going over the bars (getting weight back and low), but less "rad" to ride. You counter forward bias through a longer front center and/or shorter rear center. If you can't shorten the RC any more, then lengthen the FC with more reach and/or longer HA.

Once the idea of getting RC and FC properly proportioned becomes widely accepted, bad geo bikes will begin to disappear. XC 29er HTs for short people are horrendously forward biased (long CS, steep HA)--it's not uncommon to hear beginners on these bikes say they avoid using the front brake, as the fear of going over the bar is huge. Tall riders who get on long travel bikes with short CS will often encounter scenarios where they "almost died", likely because the bike was going one way and they wanted to go another, until the front wheel landed and managed to cut back onto a ridable line (not off the side of the trail). They might feel that downsizing feels faster and safer, since the weight bias isn't so far rearward--others just go with it and ride the backwheel and revel in the thrill of losing control, but making it look stylish. Tall people might be riding short travel bikes super well and become judgmental of shorter riders going longer in travel to do the same, when both riders could just be intuitively settling into a bike with good balanced weight bias, through FC and RC proportions.

Personal preference determines what the sweet spot balance is, but generally if the bike is very close, it doesn't take much effort to adapt. This "advanced geo" video encourages you to understand that the geo dictates rider position. Riders are adapting to the bike's balance by dynamically shifting their weight fore and aft. Backwards to unweight the front so it pitches up onto/over trail features without the bike forcing them back, and forwards to or allow the rear to pitch up (or put weight on the front and pitch it forward-down). Get the geo right and you can almost just stay in a static position, making it so the rider's CoM is being pulled by gravity directly centered between the tire contact points with the ground. Can just detach from the bike, staying level with the horizon, allowing the bike to independently pitch up and down. I'd go as far as saying it's perfected geo if that static position is a natural/comfortable standing position that requires minimal energy (core muscles) to maintain.

Many long travel 26ers were forward biased, with downsizing being popular. 27.5 tended to be dialed in 18.5/L. 29ers often demanded upsizing. Kona got 29ers "right" with their "XC/trail bikes" (Process 111 29, Honzo), at least in certain sizes (L). Far far better handling than the typical weight weenie bikes that the mainstream was buying up. The problem remains that the balanced feel tends to only happen in 1 size on many bikes, since the RC (horizontal chainstay length/rear center) stays the same yet the front center (FC) lengthens by about 25mm between sizes, hence why some people can feel a big difference in comfort, fatigue, and intuitive handling on certain sizes when they ride back-to-back. Also a problem that the geo changes according due to suspension movement, which is being addressed with linkage designs altering the axle path, and other design philosophies that emphasize consistency.

Sorry for the wall of text. xD This all could've been simplified by saying a 420mm chainstay may work best with a certain range of front centers (depending on rider height, travel and axle path/HA), say adding up to a 1150-1190 wheelbase. Calculate for all lengths of CS: 435mm CS with 160 travel and a 63.5 HA would be balanced with a ~1235mm wheelbase (435mm with 125mm travel and 66 HA ~1220mm). Then you can choose a bike based on travel and wheelbase, to match whether you prioritize speed and stability, or flickability and responsiveness.

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1/10/2019 1:56 PM

Primoz wrote:

Oh, didn't know we are talking about dowhnill bikes, i thought the topic is about bikes that get pedalled on. Which is done sitting in the saddle for ~80 to 90 % of the ride.

I agree, the seat tube angle has little to do with 'downhill handling'. But it's stupid to optimise for that when you do it for ~15 % of the time and exert even less energy than that while doing it. But hey, that's my train of thought.

I'd argue Vital is skewed more toward the "lets get to the top" mentality. Nobody is here really worrying about their strava times to the top, and most are willing to sacrifice a bit for the fun part.

That said, nothing I'm suggesting with respect to rear center would make the bike worse going uphill. I actually find a longer rear center to climb better, as the front end will stay planted more easily. Moreover, a longer rear center doesn't by itself have anything to do with rear center.

You can (and should) have both, a steepish STA and a long RC...err long RC if you have a long FC.

Point is, long rear centers on bikes with long front centers are actually being optimized for both climbing and descending.. .



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1/10/2019 1:58 PM

Just wanted to make a blanket statement here - the video is VERY good. If it takes you a few times to watch both part one and two, its worth it.

Everything being discussed here is articulated in a very objective data driven way. It'll help you understand why increasing the wheelbase is akin to lowering the bike, why short front centers may lead to more OTB moments. Why short rear centers are easier to manual etc etc etc.

Anyway, I just wanted to point this out as I feel we are all commenting based on a headline, not based on the content - which is among the best I've seen in a "Mountain Bike 301" type video.

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1/10/2019 7:01 PM

Well I'd say most on this site prioritize descending prowess over anything else and are willing to make compromises to reach that goal.

Long chain stays actually make for a good climbing bike with a nice steep seat tube. 445mm chain stays on one of my bikes climbs better than a the shorter 436 CS bike.

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