modern bike geometry

[mack_turtle]

Naked's framebuilder notes on a prototype

 

interesting perspectives here. good stuff to try to wrap your head around so you know why any aspect of a bike goods good or bad to you. could help you make an informed decision about your next bike purchase, tweaking your current bike, or if you have ambitions for a custom frame some day.

I found the following bits of particular interest:

Front-Centre is KING!
This is the big revelation (along with steering axis below). I have always moved front-centre around as a resultant of bike fit, as I put most of my faith in handling characteristics of head angle and trail. No more! We have all been riding mountain bikes that are way to short in the front. I had always assumed that the slack head angles required to move the front wheel out would result in a chopper feel and too much wheel flop. I realize now that as long as the stem length is kept to a minimum, the whel flop virtually disappears. What is left is a very stable feeling bike that demands to pushed harder. A normal front-centre for us was in the 70cm range. Even a few years ago, it was not uncommon to be as low as 65cm. Now, I'm thinking 77-830 is the sweet spot, even in our tight twisty west coast trails. I think the biggest advantage is that you are now balanced on a much longer see-saw, so every bump is felt less and keeps you away from both tipping points a little longer (think: less likely to go over the bars). I thought tight climbs would suck, but this was also easier. The long resulting longer wheelbase obviously is far more stable through chunder. Cornering feels awkward at first. This is because positioning is identical to the bikes I have had for the last 10 years but the front wheel is 10cm further out in front so I have to get used to initiating a turn 10cm sooner. When done properly, you can really load the front wheel and carve hard without the feeling of jack-knife. This is also about steering axis.

Head Angle
This is basically unimportant. This obsession with this number has got to stop. It is all about where this lets you get the front wheel placed. This number changes drastically depending on how your fork is set up. On this bike, the head angle varies from about 64.8 to 71 depending on how much travel is used. One thing that can be said is that a hardtail should have a MUCH slacker starting head angle than an equivalent duty full-squish. On a hardtail the head angle is always steepening under compression, where as a full suspension can either steepen or slacken. In most cases a fully tends towards more rear compression and so tends to slacken more during use. I can see pushing head angle out to 61 degrees (unsagged) if needed on a hardtail, but no more than 65 degrees on a fully. After that, you are starting to get too much fork binding on all but the steepest descents. 

[yeah, about that. everyone fretting over "my bike has to have a ##-degree head tube angle" is likely benefiting from how that angle puts the front wheel further out, not necessarily how it steers. this is cool if your main goal is not going over the bars on a downhill. that might compromise other aspects of handling if not done carefully. not all of us ride down hills 100% of the time, so we need a compromise.]

Steering Axis
Needs to be as direct as possible. For most mountain bikes this means super short stems. It is not so much the short stem that is important but the resulting hand placement relative to the steering axis. A stem with a 32mm reach on a 780mm bar with normal sweep gets you pretty close to being in line with the steering axis of the front fork. being in front of the steering axis in the old days especially with shorter bars and long stems is what gave us the feeling of wheel flop and the horrible jack-knife scenarios.

[I have not heard stem length discussed in this context before. interesting perspective. coincidentally, with a handlebar with 12 degreess of backsweep and a 50mm stem, my hands are about in line with the steering axis.]

 

Lower bars
The long front-centre gives you so much more stability and less chance of pitching over the bars, you no longer need to have a high hand position. I can see a real return to wide flat bars, with "riser" bar looking dated real fast. Slacker head angles helps lower the bars. It will be interesting to see what people do to keep bars low enough as the forks get longer and longer.
[I am often surprised to see modern bikes long travel forks, a low BB, an upright stem, spacers, and a riser bar. if that works for you, especially if you're tall, go for it. I keep lowering my handlebar (measured relative to the bottom bracket) and it only gets better the lower it gets

[ATXZJ]

Pretty much why i bought a process 111 when they came out, and I've stuck with that geometry since. It just feels (right) to me compared to what i'd owned prior.  The sentinel i built had a 64* HTA,  35mm stem, 800mm bars, 42mm offset fork and had almost no noticeable flop. That pretty impressive considering its sporting what would have been DH HTA a couple years ago. The long reach/front centre of these bikes combined with short stems and low standover gives me the confidence to push harder.

FWIW, even my current "XC" bike has 66HTA with 40mm offset fork and 40mm stem 780 bars. Concerning lower bars, my rigs usually have one 2.5mm or 5mm spacer under the stem because i prefer low stack on all my bikes. If i want more, i usually just buy a bar with more instead of adding more spacers. Low stack and standover helps my feel like im slung into the bike and not over it. Totally agree on HT HTA needing to be slacker for compression and sag. You generally lose a degree just from sag. 

 

Interesting to read those notes and definitely illustrates why the new geo bikes are popular.

Edited October 19, 2018 by ATXZJ

[mack_turtle]

I always wonder if these longer bikes actually make the reach longer, or if the short stems, wide bars (which result in more backward sweep at the ends), slack-er head tube angles all result in a fit that is about the same as a previous bike fit with a short TT/ long stem. if so, that makes sense because riders are not getting taller or suddenly getting longer arms. but if the effective reach is actually getting longer, at what point are you just making the bike "too big"? and if the bike is "too big" are we making up for that with ginormous tires and fretting over suspension settings to make up for the increased skill needed to pilot a freight train of a bike? (that will be the topic of a future blog entry, stay tuned!)

If I were to ride a current Kona or Guerrilla Gravity model, I would have to shoehorn my 5'9" self onto a small frame to be able to wrangle the bike the way I like to. Iperhaps I prefer a somewhat compact fit because my brain learned to wrangle a bike from riding BMX.

I rode a borrowed medium Honzo (which has an extremely long reach) on some trails in Indianapolis a few weeks ago. (yes, Indy has at least one park with "hills" and a fun, challenging trail system.) it was set up with a 120 mm fork, I think, and a super short back end. however, it has some weird, narrow "alt bars" and a huge stack of spacers under a 70mm or so stem. it felt like a beach cruiser that was comfy, but definitely not confident in the turns and felt dead in the air. I don't think I got a fair chance to really critique it because the cockpit was set up so weird. 

Edited October 19, 2018 by mack_turtle

[mack_turtle]

then there's this- http://www.peterverdone.com/i-dont-make-bicycles-i-make-weapons-systems/

!

PVD is reportedly 5'10" but the reach on his frame is 75mm longer than a medium Honzo, which is already long. it's more than 10 cm longer than most of the bikes I have ever ridden. I wonder if the radically short stem and handlebar sweep make up for that or if it's really that much longer. I asked him that but he never responded.

Edited October 19, 2018 by mack_turtle

[RidingAgain]

Interesting to read this kind of consideration and take a look at what my bike frame build is.

On the seat tube angle, I'd read recently that the trend was towards a steeper angle, but the PVD seems to have headed in the opposite direction.

Edited October 19, 2018 by RidingAgain

[mack_turtle]

It's highly likely that the slack STA on Verdone's latest bike was chosen to negate the offset placement of the seat tube in front of the BB.

 

 

If he put a conventional 74 degree angle or a steep 77, the saddle would end up waaaay forward. I'll bet that if you measured the angle from the BB to the top of that post, it would seem closer to normal. With a custom frame, you know where you want the saddle to end up and can plan accordingly.

Edited October 19, 2018 by mack_turtle

[throet]

3 hours ago, mack_turtle said:

Front-Centre is KING!
This is the big revelation (along with steering axis below). I have always moved front-centre around as a resultant of bike fit, as I put most of my faith in handling characteristics of head angle and trail. No more! We have all been riding mountain bikes that are way to short in the front. I had always assumed that the slack head angles required to move the front wheel out would result in a chopper feel and too much wheel flop. I realize now that as long as the stem length is kept to a minimum, the whel flop virtually disappears. What is left is a very stable feeling bike that demands to pushed harder. A normal front-centre for us was in the 70cm range. Even a few years ago, it was not uncommon to be as low as 65cm. Now, I'm thinking 77-830 is the sweet spot, even in our tight twisty west coast trails. I think the biggest advantage is that you are now balanced on a much longer see-saw, so every bump is felt less and keeps you away from both tipping points a little longer (think: less likely to go over the bars). I thought tight climbs would suck, but this was also easier. The long resulting longer wheelbase obviously is far more stable through chunder. Cornering feels awkward at first. This is because positioning is identical to the bikes I have had for the last 10 years but the front wheel is 10cm further out in front so I have to get used to initiating a turn 10cm sooner. When done properly, you can really load the front wheel and carve hard without the feeling of jack-knife. This is also about steering axis.

How is "front-centre" measured? Don't believe I've seen this before. I'm trying to understand the relationship between this and being able to run lower bars. I've always based higher bar height on comfort and getting more "poppiness" out of the bike, not so much fear of going OTB. What advantage do lower bars give you - better cornering and climbing? 

BTW I was checking out your blog post on Reach & Stack, and found that very interesting. I've got 2 FS bikes and have been playing around with the bar height on them given their different geometries. One bike has longer reach and stack and is limited in the amount of spacers on the steerer tube, which means I had to compensate with riser bars to get the desired height, even with a relatively short stem. The other one has a much more cramped cockpit, necessitating a longer stem and low-rise bars to put me in the proper position. I like riding both bikes, but definitely get more playful and daring on the bike with long reach/stack, short stem, and 1/2" bar rise. 

[mack_turtle]

Front- center: draw a vertical line through the BB. To horizontal  distance from that line to the front axle is the front-center. The distance from that line to the rear axle is the rear-center.

If take any bike design and increase the fork length, offset, or reach, you get a longer front-center. The author is saying that head tube angle and fork offset at not that big of a deal, but the longer front-center that results is.

Edited October 19, 2018 by mack_turtle

[mack_turtle]

27 minutes ago, throet said:

The other one has a much more cramped cockpit, necessitating a longer stem and low-rise bars to put me in the proper position. I like riding both bikes, but definitely get more playful and daring on the bike with long reach/stack, short stem, and 1/2" bar rise. 

Are you comparing the handlebar height relative to the saddle or the BB? The first is important as it relates to seated peddaling comfort, but the latter is essential as it pertains to standing and wrangling the bike. If one bike has a lower BB drop (relative to the axles, not the ground), then the handlebars might be the same height.

 

try this- stand your bike up in a corner as straight as you can get it. it will be hard to get those to be accurate, but try to get it as close as possible. measure from the floor to the grip, then from the floor to the center of the BB. subtract the BB height from the grip height. that's your effective stack height.

now measure from the wall behind your bike to the grip. then measure from the wall to the center of the BB. subtract the smaller distance from the longer- that's your effective reach.

now do that on your other bike. are they really that much different?

easier method: just measure from your BB to the midpoint between your grips. that's what Lee McCormack would call your RAD- Rider area distance. it's the hypotenuse of the triangle formed by your effective reach and effective stack. if that distance from your feet to your hands is radically different, you will have two very different-fitting bikes. depending on the way that centers you on each bike (where your center of gravity is relative to the tire contact patches), that will determine how your bike handles. fit versus handling- two different but connected things.

 

this is a helpful tool to compare two different bikes and equalize the fit- http://bikegeo.net/

Edited October 19, 2018 by mack_turtle

[throet]

20 minutes ago, mack_turtle said:

Are you comparing the handlebar height relative to the saddle or the BB? The first is important as it relates to seated peddaling comfort, but the latter is essential as it pertains to standing and wrangling the bike. If one bike has a lower BB drop (relative to the axles, not the ground), then the handlebars might be the same height.

Both. For comfort I'm referring to bar height relative to the saddle and for "poppiness" I'm referring to bar height relative to the BB. The method I use when making adjustments is to get out of the saddle and find a comfortable half-squat / half-pushup position. When in the saddle I try to make sure that I've got enough bend in my arms to maintain a loose grip with good steering control.   

[mack_turtle]

10 minutes ago, throet said:

Both. For comfort I'm referring to bar height relative to the saddle and for "poppiness" I'm referring to bar height relative to the BB. The method I use when making adjustments is to get out of the saddle and find a comfortable half-squat / half-pushup position. When in the saddle I try to make sure that I've got enough bend in my arms to maintain a loose grip with good steering control.   

that's probably more effective than all the mathematical crap I just told you to do. but if you wanted a way to verify your suspicions about how the bikes compare, it could not hurt. 

[ATXZJ]

2 hours ago, mack_turtle said:

It's highly likely that the slack STA on Verdone's latest bike was chosen to negate the offset placement of the seat tube in front of the BB.

 

 

 

If he put a conventional 74 degree angle or a steep 77, the saddle would end up waaaay forward. I'll bet that if you measured the angle from the BB to the top of that post, it would seem closer to normal. With a custom frame, you know where you want the saddle to end up and can plan accordingly.

 

2 hours ago, mack_turtle said:

It's highly likely that the slack STA on Verdone's latest bike was chosen to negate the offset placement of the seat tube in front of the BB.

 

 

If he put a conventional 74 degree angle or a steep 77, the saddle would end up waaaay forward. I'll bet that if you measured the angle from the BB to the top of that post, it would seem closer to normal. With a custom frame, you know where you want the saddle to end up and can plan accordingly.

Virtual STA is probably closer to 72-73* on that bike. As it looks like hes only going by actual STA. 

Would like to add the shift towards steeper STAs that has become popular was also necessitated by shorter CS and the need to do something with the tire under full compression. 

Modern honzo with anything longer than a 40mm stem is blasphemy. Mine has a 35mm with an angleset shortening the reach a bit.

Edited October 19, 2018 by ATXZJ

[AntonioGG]

Have you guys used imagej?  I started using it for my bike fits.  I capture video of myself on the trainer with some joint markers, then get snapshots from the video and measure angles.

 

https://imagej.nih.gov/ij/

[mack_turtle]

My issue with most bike fitting systems like that is that they seem to give you a great position for sitting and pedaling. If you want an efficient position to crank out miles on a flat roads, that's fine.

It does nothing for how the bike fits under you when you stand up. I find that a perfectly comfortable position to sit and spin the cranks is a compromised, even dangerous, position when you go to wrangle the bike on a trail.

Edited October 20, 2018 by mack_turtle
Spelling

[AntonioGG]

I just asked if you have used the tool since you guys were discussing effective STA from a picture.

nevermind

[AustinBike]

Man, it needs to stop raining. Look what we have resorted to....

[mack_turtle]

1 hour ago, AntonioGG said:

I just asked if you have used the tool since you guys were discussing effective STA from a picture.

nevermind

Sorry, I was out and about when I read that and assumed it was some sort of fitting system. suffice it to say I am leery of fitting systems. Imagej looks like a useful tool for other things though.

[throet]

So Intense just announced the 2020 Primer, which is new for the first time since they introduced the bike in 2017. The new 29er model, in lowest setting, adds about 2 inches in length and slackens head tube angle by about 2 degrees. Chains stays are slight longer and reach is slightly longer. They're offering the bike in both 29er and 27.5 version with different geo for each, which I suspect spells the end for the Spider. There's also a Primer S version for more aggressive riding that goes even slacker on the HTA. I'm sure these are nice bikes, but can't imagine the new models being as poppy / playful as the original. 

https://intensecycles.com/collections/trail 

[mack_turtle]

Still confused about this. No part of my skeleton grew 2" in the past few years, so why would I want a more stretched out, huge bike now? I guess I'd have to try it, but I am one to avoid sitting on my bike when stuff gets rough. If I can't bunnyhop and manual a bike on the trails, that geo is rubbish. What's the point of a bicycle that you can't wheelie?

[throet]

6 minutes ago, mack_turtle said:

Still confused about this. No part of my skeleton grew 2" in the past few years, so why would I want a more stretched out, huge bike now? I guess I'd have to try it, but I am one to avoid sitting on my bike when stuff gets rough. If I can't bunnyhop and manual a bike on the trails, that geo is rubbish. What's the point of a bicycle that you can't wheelie?

Agree. Keeping my 2017 Primer and still loving it. Seems the masses are going for more stability vs. rideability. 

[mack_turtle]

Just now, throet said:

Seems the masses are going for more stability vs. rideability. 

AKA hang on and let the suspension do all the work. Just buy a motorbike already and get off my lawn!

[bestbike85]

17 hours ago, throet said:

Agree. Keeping my 2017 Primer and still loving it. Seems the masses are going for more stability vs. rideability. 

This is definitely what’s happening. I just did a major shift with my bike, the opposite way of the industry. 

I reduced reach by 10mm, reduced wheelbase by 35mm and increases my HTA 1 degree steeper. I felt my old bike was super stable and capable, but it was built with light duty trail suspension. So I went opposite on this new one with much more trail focused and playful geometry, but much more burly suspension. I think this change is for the better to do it all from trail to park laps and all in between. 

[Ridenfool]

18 hours ago, mack_turtle said:

AKA hang on and let the suspension do all the work except the pedaling. Thus, preserving the rider's energy that would otherwise be used covering for the shortcomings that not having suspension will require.

FTFY

[ATXZJ]

19 hours ago, throet said:

So Intense just announced the 2020 Primer, which is new for the first time since they introduced the bike in 2017. The new 29er model, in lowest setting, adds about 2 inches in length and slackens head tube angle by about 2 degrees. Chains stays are slight longer and reach is slightly longer. They're offering the bike in both 29er and 27.5 version with different geo for each, which I suspect spells the end for the Spider. There's also a Primer S version for more aggressive riding that goes even slacker on the HTA. I'm sure these are nice bikes, but can't imagine the new models being as poppy / playful as the original. 

https://intensecycles.com/collections/trail 

Looking at getting a new primer frame for a mullet build for the wife. She likes the rollover of the 29" front wheel but has to run a small frame and gets a lot of rear tire buzz when the seat is dropped.  Inside info: the primer 29 and S frames are identical so you can get some different color options if going frame only. Not stoked on price, but they'll drop to normal intense direct prices by winter.

As far as geo goes

IMHO, modern geo works, maybe not at the 5mph switchback heaven that we have here, but it works where any speed, flow, elevation change or actual fun is involved. I've owned more than enough bikes to have learned this. Central texas is a whole different animal from the terrain others riders are testing bikes on so the older, steeper, higher BB and shorter WB stuff actually does well here.

The one bit of modern geometry that everyone should be able to agree on is STA. The newer 76* + stuff feels make me feel way more planted on the bike in the climbs. Also, suspension design and shocks have come a LONG way in the last couple of years as well.

Edited August 21, 2019 by ATXZJ

[throet]

1 hour ago, ATXZJ said:

IMHO, modern geo works, maybe not at the 5mph switchback heaven that we have here, but it works where any speed, flow, elevation change or actual fun is involved.

When I see somebody manualing on a Pole Machine, I'll believe this.