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hardtail SS geometry

3K views 45 replies 11 participants last post by  mack_turtle 
#1 · (Edited)
[sorry, I just realized that I made a redundant thread on this topic a few days ago!]

I have been on what seems like an endless quest for the perfect frame. I don't have the budget for a custom frame, and I don't know what I would want anyways, so I don't mind buying a different off-the-shelf frame every few years. Regarding geometry, my experience tells me that:

1. everything is connected. every 5mm or 0.5 degrees of change in one dimension affects the rest of the bike in subtle ways that don't manifest until you're ridden a different bike for while over a variety of terrain;
2. your body, skill, mentality, and riding style change, so the "perfect" bike for you now might not be so perfect in a few years;
3. everything is a compromise. a bike that is capable at slow-speed rock-crawling is most likely going to be twitchy at speed;
4. different overall ride characteristics are good for some terrain, riders, and ridin styles than others. so the perfect geometry for you might be a major liability for someone else; and
5. despite what people will say, a difference of a degree here and there and a few mm here and there makes a big difference in how a bike is wrangled over the terrain.

That said, I'd like to delve into rear-end geometry and how it affects riding a singlespeed hardtail/ rigid bike.

I am on a bike with a moderately short CS length with a lot of BB drop. that is, the distance from the axles to the BB is more than the average mountain bike. I can make the BB a tiny higher by using a longer fork and get it further from the ground, but the overall BB height and drop is "low" because of the design of the frame and can't be changed much.

specifically, the frame I have now (2016 or so Jabberwocky) has a moderate chainstay length (430mm) and a low BB (68mm.) this means the rear-center measurement is actually about 425mm. I am starting to think that rear-center and front center measurements are too frequently overlooked.

This low-slung bike feels very stable and holds momentum well, but I am staring to think it becomes a liability when I have to stop, suddenly change direction, and lunge the bike over a ledge, which is a constant issue on my local terrain. I am also starting to suspect that i "don't worry about rock strikes" because I know exactly where the pedals are and ride accordingly-- conservatively! I'd like a remedy that, but I don't think I can change any of the components of the bike. I think I want a bike that handles more like a trials bike, but is stable enough to maintain momentum on technical XC terrain.

One way I have been able to wrap my head around this is to take it to extremes. How would a bike handle (pedal strikes aside) with the same CS length and a 100mm BB drop? a 25mm BB rise? What about a ultra-short 415mm CS with a 100mm drop, or a 25mm rise?

The point I am getting at is: what's your ideal compromise between a rear end that turns on a dime, and gets your center of gravity low enough that you don't feel like the bike is going to toss you at speed as it concerns BB drop, rear-center, and chainstay length? (and leaving BB height as an afterthought, for now)

[this is a ramble that will probably be edited at some point.]
 
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#3 ·
Stache is not a style of bike I would be into (not steel and a rigid fork would majorly throw off the geo) but it does have a low BB at 65mm drop and a short CS at 420. That's a rear- center of 415mm. Interesting data point. Probably indicates that a compact rear end is what intrguies me.

Honzo is similar but that front end is stupid long. Do not want.

I am still amazed that they could cram bike tire in such a compact rear end!
 
#5 ·
3. everything is a compromise.
Indeed

One way I have been able to wrap my head around this is to take it to extremes. How would a bike handle (pedal strikes aside) with the same CS length and a 100mm BB drop? a 25mm BB rise? What about a ultra-short 415mm CS with a 100mm drop, or a 25mm rise?
That's a loaded question, and I'm not going to pretend I know all the answers. I think Walt would be an interesting resource for a question like this. You should invite him to chime-in to this thread.

Regarding your current setup: shorter cranks will help with pedal clearance, but there's other minor tradeoffs that come with that. Also stuffing the biggest tires possible into your frame will be the only other way to raise the BB a few mm without adding a longer fork. Maybe try a 2.6" in front?

Concerning extremely short chainstays, low BB's and short cranks, this is an interesting read.

http://forums.mtbr.com/26-27-5-29-plus-bikes/paradigm-drift-omg-i-built-myself-bike-1078271.html
 
#7 · (Edited)
to clarify, I already know what fits the static measurements of my body regarding where my feet and hands go within one centimeter. I can't stretch or contract the bones in my arms or legs, so I can play with handlebar position by means of stem dimensions to get that to work to maximize available range of motion. if frame has a top tube that is too long or to short to make that fit with a reasonable length stem, it's not going to work from a fit stand point.

For example, I am 5'9" and I would have to ride a small Honzo with a 50mm stem, slammed on top of the headset with flat bar in order to get the effective grip reach/ rise that works for me, then I would need a ton of exposed seatpost. obviously, that bike was not designed for my riding style.

currently, my fit is perfect with a medium Jabberwocky, 50mm stem, low-rise 12 degree 760mm handlebar, and a 480 a-c rigid fork or 100mm squish. it rails on smoother sections of trail and steady climbs, but it's a big of a pig when things get chunky.
Some say that bikes like mine are "stable." I think it's too stable. It feels like it's velcroed to the ground. It fights getting in the air and over stuff. I have to wonder if tucking my rear wheel in a bit or getting the BB higher would help.

what I am trying to figure out is: where should the BB be, relative to the wheelbase, to suit my riding style? that's a matter of BB drop and chainstay length. my suspicion is that a shorter CS and higher BB would suit me in that regard.

throw into that the fact that I have no desire for anything more than 100m of travel (rigid prefered), and that excludes a lot of the market. it's a niche withing a niche, within a niche.

Regarding your current setup: shorter cranks will help with pedal clearance, but there's other minor tradeoffs that come with that. Also stuffing the biggest tires possible into your frame will be the only other way to raise the BB a few mm without adding a longer fork. Maybe try a 2.6" in front?
shorter cranks and bigger tires would raise the BB higher off the ground, but it would not change the BB drop. the BB and the axles would still be the same vertical distance from each other. I am only partially interested in getting the BB higher off the ground. my main interest is considering how the rear-center and the BB drop affect handling.

are people actually running 165mm cranks these days just to avoid pedal strikes? that sounds like a lousy compromise.
 
#8 ·
I checked in with Walt. to paraphrase, although he was brief, he said I could get my front end to feel lighter by raising the BB (relative to the axles, not the ground, I presume), raise the handlebar, or shorten the chainstays. I can do option 2 effectively, but the other two would require a new frame to make a difference. hope that helps for anyone else who is confused.

he also noted that most of the long/low/slack bikes on the market today are designed to stay planted on the ground. that's great if that's how you want to ride, but it's not my riding style and not terribly useful on my local terrain unless you have tons of suspension and voluminous tires to make up for it, or prodigious levels of skill and power.
 
#9 ·
Good luck with that...

Almost all production (hardtail) bikes have gone to long front ends, short stays and slack head angles. Even the XC racing hardtails. They climb really nice, and track downhill like rockets, but the fronts are heavy and tend to droop when air born unless you really pull them up.

If you got a old Vassago, you need to cherish it because the new geometry on hard tail bikes is very different from yesterdays hardtails from a 3 years ago
 
#11 · (Edited)
So what you're saying is: I need a time machine to go back to before everything became an "Enduro brah" bike.

Or just hibernate for a year or two until they come to their senses.
I ended up with Spesy Hardtail Epic, (still havent found a decent Singlespeed yet). It is very different than the turn on a dime bikes I had. I will say I can roll in some stupid steep stuff with it and have plenty of confidence. Everything has gone "G2" geometry. Its not bad, especially if you have fast wide open stuff. I would bet it may be perty good with a rigid fork since it would prolly tighten up the steering a little more.
 
#13 ·
I hear you on the terrain. I live in tight, twisty, and need to change direction quickly, area.

Better get used to it... I spent some serious time on a Stache. As Mike said, it was a good bike. It was loads of fun. I could jump it, land it sideways, knowing it was going to crash and it would auto correct every time. Bike held a line like Nobody's bizness. Unfortunately it wasn't the easiest to pry off that line when I needed it dart over here or there. I found myself riding on the outer edges of my normal lines and still wanting to crank more steering in to it.
 
#14 ·
at risk of going on a long curmudgeonly rant, I will say that I think this discussion goes to the crux of a philosophical debate that I've been having with myself. I am winning this debate!

should a bike accentuate your skills as a rider, or make up for your shortcomings? can a bike do both? it seems like most of the bikes coming out are appealing to my lazy side- more plush suspension and giant tires that glide along the trail as if its on rails.

*yawn*

I want to suffer for my riding enjoyment godammit! give me a bike that goes where I tell it go go and not some pantywasted "rails" bullshit! I want to feel the trail and let it remind me of how small and insignificant I am in the face of the cosmos, not obliterate it under compressed-air pillows. keep your sissy-seatposts and enduro "sleds," I want to ride a damn bicycle!
 
#20 ·
I want to suffer for my riding enjoyment godammit! give me a bike that goes where I tell it go go and not some pantywasted "rails" bullshit! I want to feel the trail and let it remind me of how small and insignificant I am in the face of the cosmos, not obliterate it under compressed-air pillows. keep your sissy-seatposts and enduro "sleds," I want to ride a damn bicycle!
I agree it's fun to work the bike instead of letting it do the work. It's rewarding to earn your speed. Plus I know I'm still building skills because of that bike.
 
#15 ·
I’m not familiar with specific measurements from one frame to another, but have you looked at a Nimble 9?

I’m a similar size and I ran the medium N9 SS rigid and with a 100mm fork with great success.

My latest version has a 140mm fork with B+ wheels and I like it even better. I’ve been thinking about doing a 79er + version with a rigid fork. Which by an eyeball/rough calculation method would keep the geo close. Or you could run something with a shorter AC and keep the front end lower.

I’ve always felt that the N9 has the most “sensible” of the new geo bikes. Doesn’t have any of the super slack head tubes or front triangles. Everything is in proportion. I like a bike that I can chuck around, get some air with, and generally ride it like a big BMX.

The N9 is the most fun bike I’ve owned, by quite a bit.


Sent from my iPhone using Tapatalk
 
#18 ·
I'm not familiar with specific measurements from one frame to another, but have you looked at a Nimble 9?
I almost bought a N9 a while back. it seemed very over-built and porky for my needs, but the geo with a shorter fork would drop the BB to -73mm and steepen the STA to 76 degrees. that's weirdly steep and low. if it works for you, that's fine. it might be an option with a 120mm fork, but it would be pretty close to things I don't like now, but with a shorter chainstay.
 
#17 ·
sounds interesting, but I am partial to steel and chain tensioners are a pain. I appreciate Specialized's more conservative approach to geometry though. not everything has to be an enduro brah long low slack bike. I would be curious to know how the Carve SL rides compared to a steel bike.

The Log Lady flew under my radar. that might be more like it. I'll have to check previous discussions to see if regular 29er wheels fit in there.
 
#19 ·
For the purposes of comparison, the Kona Raijin is pretty close to what my custom frame is. My bike has slightly longer chainstays, and a longer reach. KONA BIKES | MTB | MTB HARDTAIL | Raijin

I also was interested in the Carver 420 Ti, not dissimilar Titanium 420 - Carver Bikes

^ both of which I kind of consider modern XC geo (ish) without getting too "trail".

Mack Turtle: I'm curious what you think of, and if you have previously owned an "old school XC" geo bike like my old Kona Unit, or the Karate Monkey of the same era (2011)? That bike was super nimble and responsive, almost twitchy. Great for tight trails, not so much for bombing burly descents. Just curious.

konaworld

https://www.bikeworldiowa.com/images/library/features/srly_geo_kmonkey_11_f.gif
 
#21 ·
For the purposes of comparison, the Kona Raijin is pretty close to what my custom frame is.

I also was interested in the Carver 420 Ti, not dissimilar Titanium 420 - Carver Bikes

Mack Turtle: I'm curious what you think of, and if you have previously owned an "old school XC" geo bike like my old Kona Unit, or the Karate Monkey of the same era (2011)? That bike was super nimble and responsive, almost twitchy. Great for tight trails, not so much for bombing burly descents. Just curious.
I foolishly bought a small Karate Monkey around 2011-12 when I should have bought a medium. I would probably still be riding it if I had bought the right size.

the Unit has always been on my radar, but I think I would hate the long chainstays on it. I am glad they have modernized is a bit, but that's still very long.

The Rajiin and Carver look right up my alley, but I'll need a save up for a long time before I can afford something like that. The Carver looks especially perfect for my needs, so I'll keep an eye on that for sure.

I've always wanted a ti frame, but never had the extra cash. foolish things like paying off a mortgage and saving for retirement keep getting in my way. If I can find a similar frame in steel/ cheaper, I would be into it now.

The closest thing I can find is perhaps the old ROS9. I had one for a few months and absolutely hated it for some reason. the other is the Kona Honzo, but the top tube on that thing is egregiously long so that I'd have to ride a small frame with a flagpole of a seatpost. it's obviously not made for my riding style.
 
#23 ·
if you're looking at ETT, they are pretty close. ETT tells me how the bike fits when I am sitting down and pedaling in a straight line, which is pretty useless on a mtb IMO. the reach and stack tell a different story.

I worked it out, and the reach and stack on a medium Jabber with a rigid fork is about 438 mm (which is quite long) with a 600 mm stack on a rigid fork. The reach on a medium Honzo with a 120 mm fork is 450. I have a 50 mm stem on my Jabber and a handlebar with a fair bit of backsweep to make it fit the way I like. I would have to ride a medium Honzo with a 35 mm stem and an inverted handlebar, or a small Honzo with a 50 mm stem and a ton of exposed seatpost.

shortening the fork on a Honzo would steepend the HA a bit and theoretically shorten the reach a tiny bit, but it would also put the BB back down to an uncomfortably low position. I am trying to rein in the BB drop for handling purposes. I'll have to check later on http://bikegeo.muha.cc/
 
#24 ·
what's your ideal compromise between a rear end that turns on a dime, and gets your center of gravity low enough that you don't feel like the bike is going to toss you at speed as it concerns BB drop, rear-center, and chainstay length?
I like short wheelbase bikes that have less BB drop. They're fun! They make me feel I'm more skilled than I am. I don't like the "in the bike" feeling, especially on a SS. Yeah, short/high bikes suck at higher speeds but I'm OK with that. With long low bikes, I can't weight each wheel the way I like without unweighting the other end of the bike more than I want. A lot of this is because I have a super short torso (like Walt). I believe Walt puts a big emphasis on the front-center dimension for each customer. If I ever go custom, I'm going to him (although I think he personally prefers a low BB height). I'm also pedal-strike prone, ride super rocky trails, and like 180mm cranks, so low BB height is a deal breaker for me.
 
#26 ·
an old KM could be an interesting experiment, but I would have to go searching for a fork and bodge something together for my front axle to fit a qr axle. my current components are somewhat modern, so axles, seatpost, and headsets would need to move around.

I am fairly certain that the current model of the Monkey checks all the boxes in terms of components compatibility and geometry, so that might be my next frame. I almost bought one when I got my Jabberwocky but chose the Jabber instead.
 
#31 ·
shorter stays, BB higher relative to axles, fewer pedal strikes, effective reach and stack about the same but with a tiny bit more spacers under the stem. (added bonus, my brake levers would no longer hit the top tube so I can do bar spinz!) I can probably run a 34/21 instead of 32/20 and not change the chain length but have a shorter CS. it should be more lively and fun for the rocky, twisty, 15-20 mile rides I do most of the time. for long slogs on gravel roads, I have a CX bike with 35 mm tires.
 
#33 ·
yes, compared to the Juice, the handlebar is MUCH lower on my Jabber, relative to the BB. I was trying to balance out the length of the TT by lowering the handlebar. it makes the bike feel very planted and stable, but it seems to want to do its own thing when I need to make major changes in trajectory. If I at least flip my stem over, that will raise it 1 cm or so, but there's not enough steerer tube left to put any spacers under it. I'll give it a try.
 
#34 ·
It's not steel, but have you looked at the geometry of Salsa's Timberjack?
Many of the trails I ride are similar to yours- tight, twisty, chunky stuff that most people won't even hike over. The Timberjack handles well enough to manage the techy stuff easily, but is still fairly stable at speed. If you were to toss a rigid fork on it, it'd be a switchback-slaying demon...
 
#35 ·
Timberjack is on my radar as well. I might tolerate my first alu bike.

edit- if I can find one. just called LBS and the Timberjack frame has been discontinued. I think I'll boycott Salsa purely because they stopped making the El Mar.

further notes: the KM is out of stock as well. at least they'll make more of those. ****, I need to buy one before tariffs make bikes more expensive than they already are!
 
#37 ·
Have you looked at the Soma Riff? Only 44mm bb drop with a 484mm (100mm corrected) fork.

I have a '12 KM. I like the bike, but I hate the track ends. My last build I had the chain length optimized for shortest chainstays possible, only to realize I couldn't remove the rear wheel without breaking the chain. Stupid. Anyway, I might try my KM with a 29+ front/B+ rear/longer fork to slack it out a bit. It feels a bit fast these days with the stock rigid fork.

I don't know if you're seriously considering another ROS9, but mine cracked at the headtube. Beware.

The Timberjack isn't available as a frame anymore? Guess I can cross that one off my list.
 
#38 ·
Riff is a 650B frame. I am kind of 650B curious at this point, but I don't have the budget for all that would be involved. I would have to buy new wheels, tires, and a fork to start. I had a Juice before and I liked it except I always hated the long chainstays. some people like em long, I like em short.

I had a '12 or so KM as well. the track ends worked great with a QR axle slammed at 32/20. I always had a 10x135 thru-axle hub in it, so if the chain was new and too tight, I could just yank the whole bolt out, no problem. I am just fine with track ends. the new version with a 12x142 should be just as easy to deal with. after riding BMX bikes most of my life, they are no big deal.

I had a ROS9 for maybe three months. I could not not it out of my life fast enough.

QBP lists all of the Timberjack frames as "discontinued," but they seem to have completes still. maybe they have a new "colourway" coming and will restock then. You never know with Salsa.
 
#40 ·
Of course not. What would be the fun in ending my quest? I just like knowing how different aspects of a bike change the fit and handling. I like to try something new every now and then and I always own just one mountain bike. Fortunately for my wife, I limit that curious impulse to bicycles.
 
#41 ·
It's somewhat the bike, right? Let's see...

The small KM was too small. Juice was a good solid bike but long stays can be a bummer. ROS9 had too short a top tube, or too tall a front end, or might need a 51mm offset fork to make it handle right...whatever, it ain't workin' for Mack. It had to go. But lessons learned: don't run your saddle too far back nor your bar too high, and be kind to your lower back. Then the Jabberwocky - best bike yet. Goldilocks geo. But the eye wanders. Maybe there's something else a wee bit better? Even if it's just different?

What did I miss?
 
#43 ·
Maybe there's something else a wee bit better? Even if it's just different?
This may have nailed it for me. I'm pretty happy with the fit of all my bikes, and none of them fit the same. The geo is different which makes them good, or less good, at certain things. Again, tradeoff.

I think that's a good thing, it wouldn't be as fun if they were all the same. I am also seem to be endlessly curious about bike geometry. I want to ride them all, even if I think a certain aspect of one makes it look horrible on paper. (Jones chainstay length- looking at you)
 
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