The only constant in life is change, and mountain bike geometry is constantly changing. Yep, things are still getting lower to the ground, longer from wheel to wheel, and pushed back even slacker.
In 2016, when Aaron Chamberlain took a look at the numbers, he categorized 27.5-inch trail bikes as having rear travel between 120mm and 150mm, and 29ers sporting between 120mm and 140mm of rear travel.
Those numbers are still in range for this analysis, but probably not for long. On some of these same bikes, we’ve seen big increases in travel, much slacker angles, and 27.5-inch models changed completely to 29ers, or at least adding in the bigger wheel size as an option.
Let’s talk briefly again about what some of these measurements mean and then look at how things have changed over the past two seasons.
Head tube angle: The angle of the head tube relative to the ground. A steeper head angle keeps your weight more forward which helps with climbing. It also responds to steering input more quickly. A slack head angle will push the rider’s weight back more, adding confidence on steeper downhill sections of trail, while slowing steering input.
Top tube length: The horizontal distance, measured parallel to the ground, between the center of the head tube and where it intersects the seat tube or seat post. As you go up in frame size, this distance will get longer.
Seat tube angle: The angle of the seat tube relative to the ground. More brands are steepening the seat angle to keep the rider over the center of the bike while pedaling.
Reach: The horizontal distance between the center of the head tube and a perpendicular line drawn through the center of the bottom bracket. Reach has mostly been getting longer on updated models.
Stack: The vertical distance between the center of the bottom bracket and the top of the head tube. This may impact the height of the handlebars.
Chainstay length:Ā Measured from the center of the bottom bracket to the rear axle. A shorter chainstay helps with snappy cornering and wheelies. A longer chainstay helps keep the bike more stable.
Bottom bracket (BB) height: The distance from the ground to the center of the bottom bracket. A low BB helps with cornering and keeps center of gravity low, but can mean more pedal strikes.
Wheelbase: The distance from rear to front axle. A longer wheelbase generally means more stability at high speeds, and a shorter one usually results in a more maneuverable bike.
Here’s a look at where the numbers have gone since the end of 2016, and what that means.
The numbers
2016 27.5-inch trail bike geometry
In 2016, the 27.5-inch trail bikes we looked at (all size large) had an average of 142mm of travel up front, 135mm of rear travel, a 67-degree head angle, 74-degree seat able, and the average chainstay length was 429mm.
The average effective top tube length was 622mm, reach was 452mm, stack was 604mm, BB height was 338mm and the average wheelbase for a 27.5-inch mountain bike was 1,174mm.
Have things really changed all that much? Let’s look.
2018/2019 27.5-inch trail bike geometry
The averages for our updated models are 144mm of fork travel, 140mm of rear travel, a 66.5-degree head angle, a 74.6-degree seat angle, 428mm chainstays, and an 1,195mm wheelbase.
Effective top tube length for a large is 623mm, reach is 458mm, stack is 608mm, and BB height is 339mm.
What does it all mean?
It means that things have indeed continued to go slacker and quite a bit longer, but not lower.
The average front travel on these mountain bikes increased by only 2mm, but the rear grew by 5mm. The head angle slackened by a half-degree, the seat tube steepened by more than a half-degree, and the average reach grew by 6mm.
The average wheel base grew by close to an inch. This is certainly the most impacted area of geometry. The bottom bracket height moved up a millimeter, and the chainstay length shortened by a millimeter.
This could suggest that bottom bracket heights have gone as low as they can go, and making them lower isn’t going to help the ride. Or, it could be that with increased travel, bottom brackets are being pushed back up, and may slowly come back down again.
29-inch trail bike geometry
In 2016 when we last looked at 29er trail bikes, the wheel size was picking up momentum. It had broken barriers with skeptical consumers and mostly proved itself.
Average fork travel was 134mm and frame travel was 123mm. The average head angle was 68-degrees and seat tube angle was 74-degrees. The average chainstay length was 437mm, and reach in a size large was 446mm. Bottom bracket height was 335mm and the wheelbase average was 1,176mm.
Unfortunately this isn’t going to be a perfect comparison. The majority of the 29ers we looked at last time have not been updated, and we have a 27.5-inch bike that joined the club — the new Giant Trance. But, the mountain bikes that have been updated will give us a general idea of where geometry is heading for the 29-inch trail bike, and things look to be changing dramatically.
The average fork travel is 138mm, rear travel is 127mm, head angle is 66.4-degrees, and seat angle is 74.6-degrees.
The reach is 440mm, chainstay length is 433mm, bottom bracket height 342mm, and the wheelbase is a long, 1,202mm.
What does it all mean?
The average fork and frame travel increased by 4mm, head angles have averaged over a degree and a half slacker, and the seat tube is more than a half-degree steeper.
Average reach has increased by 14mm, chainstay length was cut by 4mm, and the average wheelbase has increased by over an inch. Bottom bracket height has also not gotten lower for the big wheels and has actually moved up by 7mm, perhaps due to longer travel amounts.
Regular Singletracks readers have probably noticed a swath of long-travel 29ers being released lately. It seems that manufacturers are really nailing down the geometry on these bikes to a T.
An analysis of both wheel sizes
The trail bike category in general is getting more travel. Obviously, right? I’d say this is an indication of good, progressive suspension design, that allows trail bikes to stay high in their travel when climbing, while still offering usable travel for descending.
The real shocker is how quickly average head and seat angles are changing. While 27.5-inch bikes saw head angles slackened by a half-degree, the 29ers changed by more than a degree and a half based off the samples we included. These bikes are edging much closer to the 66-degree mark.
Seat angles are also getting much steeper to make for a suitable climbing position. In both wheel sizes the averages changed from 74-degrees to 74.6-degrees.
While chainstay length didn’t change much for 27.5 bikes, 29ers are seeing shorter chainstays, by 4mm on average. Engineers are finding out what’s really possible for 29ers.
Wheel sizes are becoming a matter of preference more than ever and are not being defined by their capabilities anymore. Now, multiple brands are producing the same model in both wheel sizes.
When 29ers were re-introduced to the mountain bike industry, it was as a more cross-country friendly bike, maybe because designers couldn’t nail down the right geometry for confident descending.
That’s obviously changed, as lately multiple downhill bikes with 29-inch wheels have been released and raced. The big wheels are intended for speed and rollover, while 27.5-inch wheels are regarded as the more fun and nimble wheel size.
Funny, I remember the same thing happening with 27.5-inch and 26-inch wheels not too long ago.
In another two years, maybe we’ll look at geometry again to see how things have changed and if bikes are still getting lower, longer, and slacker… or longer and slacker… or maybe just slacker. Hopefully, we’ll still have two wheel sizes to choose from.
Coming back to trail riding after a long layoff, the new bikes amaze me. My ride has 27.5″ wheels and 150mm of travel on each end (Specialized), climbs well, handles technical trail with razor precision, and will normally handle any hit you’ve got the nerve for and ask for more. It is so much better than my old bikes that it puts a smile on my face every ride.
I’d like to try one of the new 29ers, but it’s hard to imagine it would be better overall than my current setup for the trails around here. Either way, it’s great to see brands across the spectrum getting specs dialed in. It’s a win for us all.
Hey mike, same experience as you. Never really stopped riding, but went from a ā99 stumpjumper fsr to a 29er ht, and thought both were great. But coming from older-school geometry. (Now riding a yt jeffsy), it took me a while to get a hang of the new geometry. It rides differently. For example, for a long time I had to remember to keep the chest low to weight the front, otherwise the front wheel would wash out in turns. I used to have to get behind the seat and hang on for dear life, now itās much more of an āin-the-bikeā riding experience. Also, things like wide bars are great, but unless youāre really using them to lean the bike over, theyāre not much of a value added and more of a nuisance.
Now that Iāve gotten used to it, I can ride my jeffsy on Bikepacking tous and lift service downhill runs, And enjoy it to the fullest.
I’m totally sold on modern geometry. I recently purchased a new bike with modern geometry and the size large bike with a stock 60mm stem fit pefectly. However I bought the size XL bike, switched to a 40mm stem and adjusted the seat forward which effectively made the seattube steeper. I can just sit and spin up the steepest climbs without the front wheel coming off the ground and I don’t need to get way behind the seat for steep descents. No more moving way foward or way back. Just stay in the middle position and ride. Long top tube, short stem, steep seat tube angle really make a bike much easier to ride. Why did it take mountain bike designers so long to figure this out?????
This got me thinking, has anyone else noticed there seems to be an increase in āclimbing adviceā posts as the LLS trend in bike design continues? Iām just noticing a lot of posts about problems with pedal strikes (low bottom brackets), wandering when climbing (wide bars) and wheelying (slack head tube angles and a general rearward weight bias)…
Donāt get me wrong, modern geometry is fun when things speed-up and modern trail bikes are more capable descenders than DH bikes were in the early 2000ās. It just seems really unforgiving of riders who donāt have their climbing fundamentals dialed.
It could also mean that the awesomeness of new bikes has led to a lot of growth in the sport. In which case, everyoneās a beginner at some point and disregard everything I said…
Great read! A point I make sure to consider when I fabricate my own bike frame is suspension travel as it relates to actual angles when riding. It’s relevant to remember that what reads on paper as a 140 mm travel, 66 degree head tube actually rides about 2-5 degrees steeper.
This is a fantastic outlay of information Matt. Thanks for all the number crunching and analysis.
Thanks, Brian! My head was definitely hurting after looking at all the numbers on spreadsheets for a while, haha
Nice analysis of the numbers. It used to be sooo easy picking a bike… small, medium, large. Pick one. š Now, well, I heard MIT is offering a degree in bike geometry numbers. With my most recent bike purchase, I got caught up in the numbers and way over analyzed. In the end, it came down to what fit “my” geometry. With so many options in dimensions available today, you can really get a custom fit IF you know what to look for. While all the numbers and options can be dizzying, the good news is, bikes fit and feel better today than ever!
Traian Grigorian,
I read that article, and I get it. But since I’m currently new bike shopping, I’m learning just how difficult it is to get proper test rides in.
Some shops have rentals/demo fleets, but only in the cheapest version of the bike. For many models, it’s difficult to get a test ride at all. The shops can order you anything, but you may not be able to test it first.
Considering I live in Colorado, I imagine it’s even harder elsewhere. Consumer direct — well, what do you have but geometry tables? Reviews seem to be overwhelmingly positive for everything, i.e. not very useful.
Brett, we have a story that might help out with this coming soon š
Matt,
How soon is soon? š I look forward to reading it.
Very interesting article, Matt!
The two stats that jumped out at me were the change in HTA and the “massive” shortening of the chainstays.
I’m all for slacker angles/more travel, but I wonder what the driving force behind the shorter CSs was. It could have been engineers were finally figuring it out, or maybe it was the industry mostly ditching the front derailleur. I think SRAM declared the FD dead after the 2016 models were released.
When you steepen the seattube, lengthen the toptube, and slacken the headtube angle, the wheelbase becomes longer. Shortening the chainstays shortens the wheelbase. Basically, wheelbase is added to the front and subtracted from the back so that the wheelbase doen’t get to long. The shorter the wheelbase, the smaller the turning radius, the more nimble the bike rides. A bike with a very long wheelbase would be harder to ride on sharp switchbacks.
I’m not sure how the STA plays into wheelbase, but the slacker HTA and longer top tube are definite factors. IMO, a longer front center and shorter chainstay is where it’s at!
Shorter chainstays tends to make a bike feel more playful and easier to manual. My point was I wonder what was more of a factor in being able to shorten the chainstays: the fact that the engineers finally figured out some voodoo magic or the fact that the FD is basically a thing of the past now. My money is the on the FD no longer being a concern when it comes to designing bikes.
“Jared13: Iām not sure how the STA plays into wheelbase”
I think by making the STA steeper, the rider is moved forward allowing the other changes in geometry to take place to lengthen the wheelbase. At least that’s how I visualize it.
Ah, so the steeper STA doesn’t directly lengthen the wheelbase, but it allows a longer front center to be comfortable.
Got it, thanks for the clarification!
I think there are practical limits of geometry change where negatives start creeping in. Too low BB equals too many rock strikes etc. One trend that is also overdone I feel is overly short stems. I use longer than normal stems to weight the front wheel for cornering traction and climbing personally.
Yep, it seems like BBs have reached a point of “low enough” and are creeping back up a bit, or staying where they are.
So, this is an old thread, and I’m an old crusty grump….but this discussion is painful. So many us probably rode when singletrack with a patch of very mild downhill or rollers was as awesome as you got, and we currently live in places where we don’t need trail, enduro, or really any long travel bikes. It seems like everyone who comments has Moab in their backyard. Wait…I can’t remember the last time anyone referred to Moab as a mtb mecca. My point is, my 99 FSR works great for most of what I’ll likely ever ride, and the 2002 Jekyll 800 is almost like cheating, it’s so plush. Both have some parts upgrades and rebuilds, but are mostly stock. If I’m feeling really salty, I can clean most any trail on my 1995 Performance M105 with True Temper double butted cromo, purple shiny bits, and a nicely rebuilt Mag21. It’s also only 23lbs and rides like a the best night of your adult life. It doesn’t hammer downhill the best with the overly long stem, but I can get down there. If I need to hammer downhill, I bring the fs rides. We’ve kind of lost touch with what this sport was. Groomed trails, chair lift rides up, bikes too heavy to lift onto my suburban, and challenging ability to climb are baffling. Yes, I’ve ridden some new, modern rides, and yes, we’ve come a long way, but I think the biggest advances are the parts that work every time and the incredible brakes and smooth drive train. So very few of us really need the 160 mm travel, dropper post, can’t ride a trail with roots bb height, and geometry something like Peter Fonda in Easy Rider, but we’ve drank the kool aid. Anyhow….Love you, love your show….if we meet on a technical trail that climbs as much as it descends, we’ll see who can hang. Sincerely, a 44 yo retro grouch. Ps….don’t get me started on wheel sizes.