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pjs32000

Geometry help and education needed

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I swapped bikes with a friend the other day and noticed a couple of handling differences that stood out.  I'm a relative noob with this geometry stuff and being an engineer my brain is desperately trying to understand which geometry variables are resulting in the output differences I noticed.  I've excluded the models of the bikes to remove any possible biases.  These bikes are different sizes and price points so it's not a true apples to apples compare but I'm hopeful that those with a better understanding of geometry can help to educate me a bit which will go a long way when it's time for my next bike purchase.  The complete geometry specs that were available for each bike are attached.  Both are 29" full squish running tubeless.

Riding characteristics... bike 2 was slightly easier to initiate cornering.  Just after initiating a corner, bike 2 wanted to lean over the rest of the way very naturally into the corner with very little effort.  This was the most noticeable difference.  Bike 2 also felt a bit more planted and confident throughout the entire corner once leaned over.

 

Geo.jpg

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Just looking at the numbers and your description I would say wheelbase, handlebar width and head tube angle would all be determinants in cornering.

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to start getting a real idea how a bike compares to another, you need to look at the effective reach and stack. that is, how far is the bottom bracket from the grips. also known as: where are your feet relative to your hands? two bikes with identical reach and stack measurements can have very different characteristics based on where your stem and handlebar puts your hands. Lee McCormack calls this RAD and you can perform a repeatable measurement with the method Pete Verdone has developed to get an idea of what your body proportions are. I plugged his calculator into a Google spreadsheet that I can share with anyone who wants to try it, drop me a line.

to measure the Ride Area Distance of a bike, find the space at the center of the handlebar between the grips, as if your put your hands on the grips and found the direct midpoint between your fists. measure from the center of the BB to that point. that's the bike's RAD. it changes with different handlebars, stems, etc. but it remains the same no matter what the suspension is doing. This tells you how your body fits on the bike when you are wrangling over trails. saddle position is irrelevant for this purpose (although still important) because you're almost never sitting down when handling terrain like this really counts.

RAD is only a start. when you get into front-center, rear-center, BB drop, etc. that tells you more about how the bike handles based on what happens when you shift your weight around on the bike. but if you want to compare those things, you need to start with and apples-apples comparison but setting up the bikes with the same RAD, or at least comparing them.

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6 hours ago, pjs32000 said:

Just after initiating a corner, bike 2 wanted to lean over the rest of the way very naturally into the corner with very little effort.  This was the most noticeable difference.  Bike 2 also felt a bit more planted and confident throughout the entire corner once leaned over.

The front tire and its pressure can make a big difference with this too.

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The front tire and its pressure can make a big difference with this too.
Yes. The tires and pressure make a large difference and impacts on cornering as well handling.
Also when you rode these 2 bikes I assume you rode them on the same trail and corner to make the cornering comparison more accurate?

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You don’t include fork offset for bike two. I wouldn’t be surprised if it was a 44mm offset.


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1 hour ago, Teamsloan said:

You don’t include fork offset for bike two. I wouldn’t be surprised if it was a 44mm offset.


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This

A Shorter offset fork can change the dynamics quite a bit.

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Also different forks with differing travel, sag and compression/rebound settings will feel different cornering depending on speed, terrain etc. Also you don't have stem length or handlebar width for bike 2 unless they are the same that would also impact how a bike feels cornering. 

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I think we threw too many variables at him. “Purple bikes also corner better when turning left.” Oh well.


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Here are the answers to many of the questions above.

  • Bike 1 handlebars are 760, bike 2 are 780.  Bike 1 used to be 780 but were cut down, even at 780 it didn't feel like bike 2.
  • Bike 1 tires are 29 x 2.3.  Bike 2 tires are 29 x 2.4.  Both inflated to about the same psi, ~19-20 front and slightly more in back.
  • Rode both bikes on 0.6 loop at Walnut, an easy trail that I've ridden a lot and know well.  I noted the difference on flat corners.  I mostly ride intermediate stuff in town... WC, Brushy, SN, SATN, etc.
  • Fork details are quite different.  Bike 1 is 120mm travel 51mm offset.  Bike 2 is 150mm travel 42mm offset.
  • Bike 1 stem is 45mm, bike 2 stem is 50mm.
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It's no mystery that these two bikes handle very differently. Two degree slacker head angle plus shorter fork offset combine to give bike 2 way more trail. Also, equal rear center but 7.5cm longer front center for bike 2 will distribute the riders weight much more toward the rear. The mystery to me is why these differences combine to make cornering on bike 2 easier to initiate. I would have expected the exact opposite.

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It's all pretty subjective. Coke vs Pepsi

Op; I'd go demo some bikes and buy the one that feels best. We can't solve this riddle for you.

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1 hour ago, ebflo said:

It's no mystery that these two bikes handle very differently. Two degree slacker head angle plus shorter fork offset combine to give bike 2 way more trail. Also, equal rear center but 7.5cm longer front center for bike 2 will distribute the riders weight much more toward the rear. The mystery to me is why these differences combine to make cornering on bike 2 easier to initiate. I would have expected the exact opposite.

That's interesting about the front center, I will have to look more into that.  When on bike 1 the front tire feels like it wants to wash out a bit which causes me to scrub a little speed in turns.  The front can feel more squirrely in loose over hard flat turns which I ride often (Walnut Creek moondust for example).  This could also be simply because the bike is more agile and reacts to steering inputs more quickly than bike 2.  I didn't get that losing the front feeling as much on bike 2 but to be fair I haven't had as much time in the saddle on bike 2 either.  I had guessed that may be my weight is more forward on bike 2 and keeps the front wheel more planted, which is the opposite of your suggestion based on the front center specs.  For reference bike one is running a Maxxis DHF tire and bike 2 a Maxxis High Roller II.

After the comments about fork offset above I read a couple of articles which stated that bike 2 with the shorter offset and more slack head angle has more trail which generally means "greater stability and lazier, slower steering" and longer offset or steeper head angle means "quicker steering response" (their words).  Bike 1 is definitely more twitchy and agile in steering inputs, the difference is that bike just doesn't lay over as effortlessly after the initial cornering input like bike 2 does.  I'm almost thinking that the offset might be causing the steering agility difference but the head tube angle might be making bike 1 a little less stable through the entire corner.  I also read that the longer trail generally pulls the tire back under the rider resulting in more rider weight over the front wheel.  Based on this I was relatively convinced that's what's driving the difference but I hadn't thought the front center or that bike 2 has more fork travel.

I'm not looking to buy a new bike right now, this is just something I wanted to understand better because (1) I see lots of online comments about bikes having a "good geometry" with little explanation of what that means and what makes it good and (2) if I can pinpoint some of the things causing bike 2 to be a little more stable in turns I may look into how to upgrade bike 1 to get a similar feel.

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You’re lacking enough data in this analysis to obtain a reliable answer. You only have two examples to compare with entirely too many variables to make any kind of data driven conclusion. I would suggest taking the previous advice and widen your examination to include many more sources...

…go demo bikes…gather data.

Only then will you have enough to start seeing patterns that you can draw conclusions from.

...this concludes my engineering speak.

Ibis’s own marketing material for the Ripmo describes their geo choices as keeping the head angle slack to add stability while keeping the offset shorter (44) to keep the handling quick. Your description of “bike 2” sounds like my bike.

Too bad Outerbike isn’t happening this year. You’d fill up your spreadsheet for sure.


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The 2.3 vs 2.4 tires will also make a noticeable difference, more so if the 2.4 has aggressive tread pattern up front.   If you're planning to ride chunky trails often (brushy,  BCGB, RPR,....), I'd go with 130-140 suspension and wheels that can fit 2.4-2.6 tires.  My 130 suspension with 29 wheels (2.5 rear 2.6 front) is great for these types of trails.  I have no idea what the geometry on my bike is.

 

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7 hours ago, pjs32000 said:

 

I'm not looking to buy a new bike right now, this is just something I wanted to understand better because (1) I see lots of online comments about bikes having a "good geometry" with little explanation of what that means and what makes it good and (2) if I can pinpoint some of the things causing bike 2 to be a little more stable in turns I may look into how to upgrade bike 1 to get a similar feel.

Honestly, bike geometry is "learned". Geometry really matters for the first 10 or so rides, and then after that, your body learns how the bike handles and it compensates. A great example is people who talk about pedal strikes on their bikes. This is a really common problem on new bikes but then it goes away. Your muscle memory takes over and you learn how to ride it. 

At a micro level every bike is different and every bike has its own handling skills, but at a macro level, you get used to 80-90% of the variances.

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7 hours ago, pjs32000 said:

I see lots of online comments about bikes having a "good geometry" with little explanation of what that means

Preach. Nothing grinds my gears more than seeing the bro’s on pinkbike all jazzed up in the comments because a new bike has a 63.5 degree head angle because that is literally the only thing they know how to somewhat interpret. 

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while working in a shop, we had a customer return an entry level XC hardtail because, "the slack angle is wrong." I am fairly certain that the customer read something about "slack angles" on the internet and, without any idea what that means, decided his perfectly good bike was not good enough for some esoteric reason. I recommend that we generally don't get too caught up in one or another aspect of a bike until you've ridden a lot of bikes and taken a lot of notes.

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2 hours ago, AustinBike said:

Honestly, bike geometry is "learned". Geometry really matters for the first 10 or so rides, and then after that, your body learns how the bike handles and it compensates. A great example is people who talk about pedal strikes on their bikes. This is a really common problem on new bikes but then it goes away. Your muscle memory takes over and you learn how to ride it. 

At a micro level every bike is different and every bike has its own handling skills, but at a macro level, you get used to 80-90% of the variances.

Truth.  When I got my new bike I was pedal striking on 1/4 and DD quite a bit even though BB height was almost identical and the slacker HT and longer fork made steering feel different.  However, after a couple of weeks no more pedal strikes than on the old bike and steering felt natural.

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Geometry = .75 x Bike Geo + .25 x Component Geo (*my* geo)... but don't dismiss that 25%!  I'll spare you the full rabbit hole, but I tried to improve upon my *perfect* geometry and it didn't work out. At all. Mostly,

I had my bike pretty well dialed for 3 yrs and then I convinced myself that I could improve my technical riding skillz with improved geometry.  Specifically, I wanted to shift my cockpit rearwards to help with really steep climbs and do more technical climbs seated.  I happen to be at low end of the height range for my bike size (XL), which has slightly longer chain stays over the S, M, L sizes, and which I felt constrained my leverage, primarily.  Also, being at the low end of the range gave me the sense that I could expand or shift my cockpit without drastically altering the handling. 

Originally tried swapping my 50 mm/6 deg stem for a 35 mm/0 deg, but didn't have enough steerer tube to compensate for the lost stack and rise.  So, I extended my Fox 36 from 150 to 160 mm with a replacement air shaft assembly, then switched to the shorty stem. This was in Aug 2019.

Fast forward to now and I'm back on the 50 mm/6 deg stem and keeping the 160 mm travel.  Technically, my riding is probably better than last summer, but it's hard to tell, between improvements in skillz and fitness, erroding trail conditions, fuzzy pre-change memory, etc, etc.  I did learn a lot - the hard way - but feel like I 'lost' the better part of a year dickin around with this.  I definitely like the increased travel and .5 deg slacker head angle! My bars are back at their previous height but my saddle is a few mm closer to the bars.

Once I made the initial change, I realized I couldn't simply replicate *my* geometry, just shifted back, which initiated a seemingly healthy effort to reestablish *my* new geometry.  While I kept saddle height constant, *my* geometry was strictly bar height and saddle fore/aft.  What I 'forgot' was that my bb didn't change location, so moving my saddle back lost that over the pedal feeling that I like and that the steep eff seat tube angle enabled. So I had to shorten my cockpit, which reduced my leverage for manuals, etc.  It also meant that my hands and feet got too close, which meant hitting my knees on switchbacks, less comfortable and less efficient out-of-saddle riding, and way worse climbing steeps since my hands were effectively lower and too close to my chest. It also put my center of mass higher, which seemed to compensate some for manuals but was less stable through rock gardens and jra. So I went through many, many iterations, changing saddle fore/aft and bar height, but never really getting it right.

What I've learned:

-I'm very sensitive to *my* geometry.  More so than to the bike's.

-If it ain't (too) broke, don't (try to) fix it (too much) - sometimes the optimal set up is only recognizable after you jack with it, but then again, why jack with it if it's working?

-Bike geo is important, but less so than fit and your *my* geo. That first 75% is the easy part of the solution (ie, a great ride).  The last 25% is *my* geo and that's what really matters and can easily compensate for any perceived shortcomings of one bike's geo over another.  Which is why I don't rely on demos when shopping bc I'm not likely to get *my geo* consistent on bikes I'm comparing.  I may think one bike manuals better, for instance, but it could easily be a 5 mm difference in bar height.  Then you add in suspension set up, tires, etc - it's a crap shoot because you can't isolate variables. I think you buy based on the what feels right for you and your riding (bike geometry, price, components, suspension design, colors/decals, etc) and you'll adapt to it, especially with good *my* geo.

-*My* geo is different for different bikes.  Transferring your dimensions from old bike to new is a good place to start, but part of finding the right *my* geo is aligning your balance and leverage points with those of the bike's.

-What feels good in the driveway rarely translates to the trail.

-Tinker with One. Variable. At. A. Time. That was hard for me on multiple levels.  About the time I started this odyssey, I had to replace both my shoes and my saddle because of wear/breakage and neither were still available (though I eventually found the saddle, phew).  I was having knee pain on one side with the new shoes, but thought it was due to saddle location. So I was trying to factor into *my geo* adjustments, but was chasing my tail, it turns out, when I realized my cleats were too far back.  That's one level.  The other being occasionally changing both bar height and saddle fore/aft, which was great when it improved *my geo*, but frustrating when it didn't (duh).

Hope this helps someone.  Kinda helps me to write down...

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