A Note on Geometry
February 19th, 2008 by Tim GrahlWe field a lot of questions about geometry: “how long should my chainstays be?” or “what is the right trail for a 29er?”. These are great questions, but it’s important to understand that the way a bike rides and feels is the SUM of all of the angles and lengths that make a complete frame, and no one aspect of it can be taken into account without knowing the whole picture.
For example, chainstay lengths: Knowing ONLY the chainstay length of a bicycle, measured from the center of the BB to the center of the rear drop out, tells very little about the way a bike will ride or handle. Without taking into account BB height, the chainstay length alone is a very deceptive figure. If two bikes both share the same EXACT chainstay length, let’s say 17.5″, but one frame has a bb height of 11″ and another bike has a BB height of 12″, the rear triangle will be almost 1/2″ LONGER on the bike with the higher BB, measured horizontally. This, of course, will increase the overall wheelbase (assuming all other figures remain the same). Now it would be easy to draw the conclusion from the above example that the lower BB bike will have a shorter wheelbase and therefore will handle better, but this again would be a false conclusion. It is possible, with seat tube angles, top tube lengths and other geometry figures, to alter the ride quality of either of the examples above. Even getting information on BB height AND chainstay length for a given frame still doesn’t tell the whole story.
On rake, trail and head tube angles: Niner has done exhaustive testing in this area, and we’ve come to some conclusions: The 29er wheel is INCREDIBLY stable. It can handle a monster wide range of trail figures and still be completely stable at speeds. Recently, with the advent of longer offset forks, and lower crown to axle forks, we’ve been investigating EVERY possibility in trail figures and found that even a ridiculously low trail figure does not result in any perceivable twitchiness at speeds. The lower center of gravity of the rider in the frame and the inherent gyroscopic effect of the larger wheels make seemingly unmanageable trail figures no problem to control. That being said, there is a movement to longer offset rakes in forks and slacker head tube angles. We aren’t sold here. If you’re building a 29er frame for a very short rider, then the longer offset fork and kicked back head angle will help with toe overlap issues, but for the average height rider, once again, you have to take into account ALL geometry figures. The SUM of these figures CAN add up to a bike that handles well out on the trail, but it’s never ONLY about the longer offset and slacker angles. Once again, just taking into account these two things is very deceptive. If you ONLY slacken the head tube AND increase the rake of the fork, leaving all other geometry figures the same, both the increased rake and decreased head angle serve to lengthen the front-center and wheelbase. In order to keep the wheelbase the SAME, the top tube length would have to be shortened, changing the cockpit length of the bike.
As you can see, changing just one thing has a dramatic effect on everything else and everything else can be changed to make that one thing work. It’s about the SUM of the figures.
All we want to say is take geometry with a grain of salt and don’t close your mind off to other possibilities. Ultimately, marketing hype and your perception of how that hype is going to affect your ride will play more of a roll in your feeling on the bike than the actual numbers being thrown at you. There isn’t any ‘end all’ of bike geometry.
Chris Sugai and Steve Domahidy
Niner Bikes
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Tech Jargen - Thanks for the perma-twitch!
Very nicely SUMmed up.
“The 29er wheel is INCREDIBLY stable. It can handle a monster wide range of trail figures and still be completely stable at speeds.”
Niner is not sold on “longer offset rakes in forks and slacker head tube angles”.
Steep head tube angles and 38mm offset forks are not dead.
Having had the benefit of riding mulitple 29″ bikes before buying one, I am in the camp of steeper head angle with less fork offset. While I think Fisher’s 51mm offset really helps to improve the handling of their bikes over previous model years, I still find them too sluggish for my taste. Long live the steep head angle.
Niner continually talks of “stable at speed”. I agree their bikes are stable at speed and a blast to ride fast-not to mention exceptionally well made. It was great to have a chance to ride them at the BallyHoo. What about all of our technical single track with slow lines? I found their offerings very unstable at slow speed. I like a bike that doesn’t require all my attention just to keep it going in the required direction. Maybe out west with wide open spaces these bikes are really wonderful. Here in the Midwest, I vote for a slightly slacker angle.
RC: You bring up a great point. Regional styles exist and riders tend to gravitate toward certain geometry packages based upon local trails. I agree that there is a “Western”, Mid-Western, and Eastern take on what a good handling mountain bike is. This has been the case, for the most part, for as long as mountain bikes have been around.
So, to integrate that into this discussion, I would say that a 29″ers handling is not only a sum of it’s various measurements and angles, but a result of a particular regional riding philosophy. Even production bikes have this trait to a degree, although customs tend to really show this. Think Jones, Wolfhound, Badger, Quiring, Engin, and Indy Fab for various examples.
Niner is West Coast based and perhaps their design is influenced by that. I know that Raleigh’s is by being based in Seattle. Salsa Cycles is a great example of Mid-West type handling, and so on and on.
It may not be fair to “brand” a brand by regionalism, but it is clear that it has an influence in companies designs.
I don’t think it is “branding a brand”, I would say more like categorizing a geometery,
Here in the Seirra FS 26er are huge, but No one rides an FS 29er, because of the generally longer stays, and the serious climbs. (yes I know you can’t determine geo just by chain stays)
Where as FS 29ers are really popular in other areas of the country.
That’s interesting G-T, re the Salsa observation. To me, Salsa was always west coast and was the pioneer in the classic NORBA geometry (71/73). My 23 year old Salsa handles as great as any new bike.
That’s what I was thinking Mike - Salsa’s headtube angles are even steeper than the Niners - Salsa Mamasita was 73 last I looked at the size L (Niner 72) so even quicker.
Lots of “serious” climbing here in California. Long stays might be a disaster on 26ers but the traction of the 29″ wheel more than offsets the longer stays required. In other words, a 29er with long stays still grips better than any 26er with the shortest stays attainable. Could be there are regions in the country where there is more retro-grouchiness than in other regions.
Long stays if you want to sit out a climb and not tumble back over.
Short stays when you get out of the saddle to sprint them up and not spin the rear tire out.
Oddly, between long and short may be half an inch or less, while the difference in seat tube angle can make up for that with a single degree. A steep seat tube angled bike, you’ll use more seat setback for. Look where your azz ends up relative to the rear axle.
Show me a long-stay 29″ you can spin out, and I’ll show you a short-stay 26″ that I can spin out.
Mike, Greg: I was talking about 29″ers.
Not Salsa’s previous 26″er history. I think you guys are astute enough to see the difference there.
What drives me nuts is when people dismiss different geometry ideas (chainstays especially), even for different riders on different trails in different parts of the country. Especially when they bash geometry that they have never even tried.
29er has such a grip over some riders that they cannot fathom that their bike, with it’s geometry, is the perfect set-up for everything, everywhere. It’s as though any criticism would diminish them in some way. Sorta like the Ford/Chevy thing.
I love my 29er and I know that it’s not designed for riding out West.
I absolutely love my Niner and I am East Coast with tight twisty singletrack all around me. Niner’s rule!
I agree with niner, steeper head angles are better for 29 inch wheels. I just did a demo on a Specialized 29 enduro that has a 69 degree head angle. I was left inimpressed with how slow the turning was and the weight of the bike at 29.7 lbs, pigish at best.
My point: Ive allways liked the steeper head angle especially on a 29er. Niner has listened and thats why they will continue thrashing the big boys in the market.
You can’t really draw a conclusion from the Specialized since their offset is the other end of the spectrum from 08 ally Fisher
Chris and Steve cite a 1/2 ” difference in effective CS length between 11 and 12″ BB heights. I measure something closer to 5/32″ (.156 or about 4mm) on a 16.75 stay. It’s a factor, but not as dramatic as they state.
And effective CS can be deceptive. Lower BBs will result in a shorter effective measurement, but this is not going to make your bike much easier to wheelie.
Two quotes from the opinion voiced by Niner…
1) “it’s important to understand that the way a bike rides and feels is the SUM of all of the angles and lengths that make a complete frame, and no one aspect of it can be taken into account without knowing the whole picture. ”
2) “…there is a movement to longer offset rakes in forks and slacker head tube angles. We aren’t sold here.
I would say, with sarcasm, that statement #2 seems to conflict with the sensibility of statement #1. But wait, hmm, Niner doesn’t make bikes like described in statement #2. Is this all an education or a sales pitch here?
It may be a thinly veiled sales pitch, or it may just be a plea for people not to get too wrapped up in one set of numbers. From my Niner experience they build a pretty neutral bike (even with less trail/rake then seems to the new holy grail) that gives some of that “in the bike” feel 29ers have long touted. I’m not sure why they felt the need to put it out there, other than to beef up their position v. the trends of the day. On that note I’d love to give one of the new Fishers a whirl to see how they handle. Who knows, it may work out great for me. You never really know until you actually use the things.
Personally I think it’s nice to be at the point where I know enough to know that I don’t need to know too much. I’m not sure it’s worth getting too wrapped up in all the numbers on a stock frame, unless you’re looking at the bike as a whole. A real test ride will always tell you what works for you. If you’re going full custom then by all means obsess over every angle. Does that make sense?
I’ve been researching A LOT of frames for my first 29er. I’ve got all the parts already, including a White Bros rigid fork; all I need is something to hang them on. With all these choices, I am coming back to the geometry question.
A lot of people seem to think that a steeper head angle (around 72 deg) with increased fork rake to counter the trail problems brought on by the larger wheel is the way to go. But then I look at the Fisher 29ers and they all have pretty slack head angles, less than 70 degrees!
I’m also intrigued by the relatively low BBs on the Misfit Dissent. You don’t have to be a genius to understand the merits of BB placement, but on a 29er a difference of 20mm is quite a bit.
I really appreciate all this discussion…