[jacksonpt]

OK peeps... edumacate me on geometry, especially HT angles. How much difference does it make for overall rideability?

I live in upstate NY... lots of roots/small rocks, and lots of hills (but no mountains)... very little flat ground. I've been riding a Yeti 575 with a HT angle around 68* for the last 7 years or so. I've also got a Surly 1x1 SS with a 120mm fork that I've spent some time on (not sure the HT angle with this fork). Both bikes have been great, and are a blast to ride.

I recently bought a Niner Air 9 (hardtail 29er) with a HT angle of 71* and have not been happy with the ride, especially when riding aggressively - the front end often "bites" into rocks/roots when my old bikes would have rolled over it. I've played around with saddle position, bar position, etc and just can't get the feel I'm looking for.

I've never paid a ton of attention to geometry - I'm a pretty average guy (height, weight, proportions), so I've never really had to... every bike I've ever had has been great "right off the shelf". The HT angle on the Niner seems to be the most likely culprit for the poor ride. From what I understand, a "racier" bike is going to have more aggressive geometry (faster, more responsive), while a "trail" bike will be a little more lax (more forgiving, for lack of a better word).

Unfortunately, I can't get any of the bikes I'm interested in to demo, so I'm pretty much forced to buy and hope, which is what I did with the Niner. Before I do it again, I'd like to have a better idea of what might get me back to the ride quality I'm used to/looking for.

 

[HardtailHack]

Wow I had no idea they even made bike with a HA that steep, haven't owned a normal XC bike since 2000 and all my frames since then have been 67.5 or slacker. For day to day XC they are fine but you do have to lift the front from time to time in crazy tight corners.

Sorry I didn't really add anything helpful but I was just amazed at your head angle, must be a crazy skittish bike.

 

[jacksonpt]

FWIW... I just went out and did a visual comparison of the 3 bikes... the Surly definitely has a more relaxed HT angle than the Niner, VERY close to that of the Yeti.

 

[mrbigisbudgood]

Keep in mind that 29ers have more aggressive HTAs to counter the gyro effect of the larger wheel. A 29er with a HTA of 68 would feel super sluggish.

 

[jacksonpt]

gyro effect?

 

[mrbigisbudgood]

Blah blah blah science and math and numbers and equations blah blah.....

Attempting to not use enginerd language.

The rotational force in the X axis (wheel spinning) results in resistance to rotation in the Z axis (turning handlebars). The larger wheel diameter results in higher forces required to turn the handlebars. Making the HTA more steep makes the bike easier to turn.

 

[jacksonpt]

I get it. I don't like it, but I get it.

 

[Sandwich]

I think it may also have something to do with "trail", if you're familiar with that term. Basically, because of the increased height of the wheel, 29ers run more trail. My understanding is, to combat that, designers ran steeper HT angles to bring trail back to 26" numbers, and that's where you get a good handling or natural feeling bike from.

Check out the vid in this post: http://reviews.mtbr.com/650b-bike which will hopefully explain it a little better than I can. Gary fisher's G2 geometry tried to fix this issue by running higher offset forks (I think, they're intentionally vague) with a slacker HA to reduce trail and wake the bikes handling up. Like I said, they don't explain it well and just say "better".

For the niner, it sounds like you're running that narrow edge between sleepy and razor sharp. I suggested in the other thread that you try a works reducer, if it's compatible, to slow down the HA. I will say that my 72* Canzo was unfathomably poor handling. My 69.5* scott scale was one of the best, most naturally handling bikes I've owned. The components on each bike were the same, though the scott was a large and the canzo a medium, so I gained an inch in the top tube. I ran a shorter stem to compensate, so that could have had an effect, but I believe it has more to do with the CS and HTA than 30mm of stem.

 

[oldfart]

29ers have a steeper head angle to keep the trail measurement similar. Nothing to do with gyro effect. If so a DH bike with heavier rims and tires would need a steep angle than a XC bike with the same wheel size. Trail is the horizontal distance from the center of the contact patch of the tire to where the steering axis intersects the ground. If you draw that out on paper, you can see that by increasing the wheel diameter while everything else remains the same increases trail, which increases stability and decreases steering quickness. So to make a 29er handle in a similar fashion to an equivalent 26er, you need a steeper head angle, or more offset or rake on the fork, or some of both.

 

[sanjuro]

You have to understand something about hardtail design: most of them are stuck in the 90's.

Here is Ned Overend's bike from the 90's. BTW, he is 5'8".

In comparison, here is his bike from today:

Notice the drop to the bars from the saddle and how he used to ride with a 13cm stem and it is like 8cm?

That's the problem with most hardtail designers and riders: it is like they want to use road bikes on the dirt.

Especially with a lot of Niner owners. My neighbor is a Cat 2, and he does 40 mile rides to the Marin Headlands, which is entirely fireroad. His bike has a rigid fork too, and I took him on one of my rides, nothing too crazy, and while I was cheating using a Blur LTc, he thought I launched down the trail.

I've had some interesting discussions with framebuilders, and I've come to the conclusion that you can mess around with their original design. I'm not saying put on a Monster T and see what happens, but if your frame is built around a 80mm fork, try 100mm.

 

[JohnE]

Not even going to try to mash numbers and nerd talk, but in Neds case, does the fact that he now rides a SS have anything to do with it? I am faster on my SS (Old Kona Unit) in twisties and technical stuff, only place I lose speed is climbs...always attributed that to the narrower rear end contributing to the overall stiffness, allowing it to use more of the energy more efficiently...while the narrower wheels make a tighter gyro...

 

[sanjuro]

Not even going to try to mash numbers and nerd talk, but in Neds case, does the fact that he now rides a SS have anything to do with it? I am faster on my SS (Old Kona Unit) in twisties and technical stuff, only place I lose speed is climbs...always attributed that to the narrower rear end contributing to the overall stiffness, allowing it to use more of the energy more efficiently...while the narrower wheels make a tighter gyro...

That was the best side pic of one of his bikes. I would have used his trail bike.