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Discussion Starter · #1 ·
I've confused by posters talking about using a steep HTA and/or increasing fork offset reduce the trail to the "right ammount" for a 29er. What is the right ammount of trail? Is the goal to match the trail in a 26er?

I'm riding a Spyder 29er and I feel like it climbs and manuvers well in most cases. It seems a little unsure on steep decents. If I swapped for a Titus 29er (slacker HTA) and used a fork with more offset than the Reba to match the trail in the Spyder, would I improve my downhill handeling and keep the climbing ability of my Spyder?

I'm well aware that the strengths of the rider, rear suspension, type of riding, and individual preferences muddy the water. But all that aside, what other factors of bike geometry play a role here? Asumming a frame size is adjusted appropriately, is it possible to come up with the ideal geometry for a 29er?

Anyone brave enough to tackle this question?
 

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I don't think there is an answer. the right amount of trail is the right amount of trail.

I don't think trail has a vast amount to do with climbing ability, except insofar as the steering wanders uphill.

however it does have a lot to do with stability since a bike's ability to self-steer is almost entirely related to the trail

ie a bike with a lot of trail will stay upright even with no rider on it but a bike with zero or less trail can be simply unrideable

the quickness of the steering is the trade-off

where you fall personally in that equation is just that - personal

one size does not fit all

anyway, that's my take. Happy to be contradicted as ever!
 

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I agree, there's no one "right" amount of trail. More trail means quicker handling and less stability. Less trail (as with your Spider) means the opposite. Pick your poison. My bikes range from 58mm to 86mm and I like them all. The 'cross bike with 58mm can be very lively and fun on the right trail, but it's hard to steer through technical terrain with any precision and becomes downright scary on steep downhills. My KM has 74mm trail with the tires I typically use on it, and it's nicely balanced everywhere I've tried it. My Vulture's handling has been absolutely telepathic from the first mile I owned it, with a surprising 83mm of trail (I think that must calculated pre-sag, because it seems quicker than the KM sometimes). The Leviathan with 86mm is still pretty lively but rock solid on the downhills. Of course, trail isn't the only variable that affects handling, so there are other factors going on here.

Trail is strictly a function of wheel diameter, head angle and fork offset. So a longer offset fork (good luck finding one with suspension) will quicken your Titus's handling slightly. So will a fork with a shorter A-C height, since that will steepen the head angle. Again, slightly.
 

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Discussion Starter · #4 ·
Let me make sure I'm getting this straight. If the 71 HTA of the Titus (more trail than the spyder) means "quicker handling and less stability", putting on a fork with more offset would tip the balance back toward more stability? What about the SRM fork thrash tested in Dec. MBA?

If the steeper HTA of the Spyder equates with more stability why do I feel like it performs confidently except on technical downhill. Other posters have expressed similar opinions (see 29er shootout)?

Like you say, other factors are involved here, but I would value any other insight you could share with me. Thanks
 

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GlowBoy said:
More trail means quicker handling and less stability. Less trail (as with your Spider) means the opposite. Pick your poison.
Could be wrong here but I think this is backwards. A slacker HTA and/or less offset in the fork generally equates to a higher trail number and a more stable ride at speeds (downhill). A steeper HTA and/or more offset in the fork equates to a lower trail number and provides quicker steering, but at the cost of a less stable descender.

The OP brings up another question though. Does it matter how the trail number is achieved? In other words, will the handling be different on a bike with a steeper HTA and less offset as opposed to a bike with a slacker HTA but more offset in the fork? I'm guessing a slacker HTA might provide more toe clearance on a smaller frame but I could be wrong on that.
 

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trail is not just a question of head tube angle, but fork rake too

90deg HTA is a vertical head tube, which is VERY hard to ride and has very little trail (just that from the fork rake)

45 deg HTA is something like a chopped Harley, sticking way out in front, would probably ride in a straight line all day even without a rider on it, and has a massive amount of trail

the lower the HTA, the higher the trail for a given fork, the more stable the ride

within the few degrees we are talking about it doesn't matter too much how the trail is achieved. The stability comes from the tendency of the wheel to flop over when the bike is tilted.
 

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JAKEtheDOG said:
Could be wrong here but I think this is backwards..
You're right, I wrote that first part backwards.

I too wonder if it matters how trail is achieved ... a slacker HTA compensated by more fork offset allows for the same trail with more toe clearance, which is why it's talked about so much in the 29" world. The slacker/more offset bike would have more 'wheel flop', but I don't know if it otherwise handles differently than the steeper/less offset bike with the same trail.
 

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Glow Boy - I agree on the wheel flop issue - if you achieve short trail on a slack HA by having loads of fork rake the wheel will definitely tend to 'flop' more into corners. Whether this bothers you is a personal thing.

I generally aim for around 70-75mm of trail to give a nice balance of responsive handling and stability for general trail riding.

I've found it works well to actually have a bit less trail on a 29" wheeled bike than 26". Thanks to the better inherent stability at speed of the larger wheels, you can run less trail to have nice responsive handling through the twisty stuff, while maintaining high speed stability. This can also mean it's easier to change line at speed - when often on high tail bike they hold a line well at speed, but can be harder to initiate a turn.

I try to design bikes so that the trail changes appropriately with different configurations. With a stack rigid fork to aim for right around 70. Put on an 80mm suspension fork and it stays about the same (when sagged) as the general pattern of riding is likely to be similar. A 100mm fork and a big tyre will put it up over 80mm - which makes things a bit more manageable if you're doing the sort of riding where you want 100mm of travel. Alternatively put on some cross tyres with the rigid fork and you're down around 60, 23mm road tyres and you're about 55 - a good figure for more road like handling.

Sam
 

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eyebike66 said:
If I swapped for a Titus 29er (slacker HTA) and used a fork with more offset than the Reba to match the trail in the Spyder, would I improve my downhill handeling and keep the climbing ability of my Spyder?
As others have said, a good climbing bike is about more than trail, although I've definitely had bikes with too much trail that wandered all over the place on steep uphills. What would really improve your downhill handling is a Hopey Steering Damper. I think it was Sparrow/Tim R. who said it really doesn't matter what your trail is -- big or small -- just install a Hopey and dial in the perfect handling for you. I might still be riding my old Moots YBB if I had put a Hopey on it back when I ruined its crisp handling by installing a "modern" tall suspension fork. My current ride is a 29er with a 72 HTA and 50 mm of fork rake and has a trail of about 65 mm. With the steering damper installed I still have a wickedly quick handling bike, but the Hopey allows me to keep it on the straight and narrow even on rough fast downhills.

I personally like my trail coming from a significant rake on the fork. I've ridden an old road bike with a steep HTA (74 degrees!) as a "cross bike" and it had very little rake to the fork -- it was a challenge to keep the front end tracking true. My current cross bike with a 72 HTA and 50 mm of rake -- and the exact same trail as the old road bike (57 to 62 mm depending on the tire) -- handles wonderfully on the roughest of terrain, going exactly where I point it. My experience is that increasing the front center is good and doesn't adversely affect cornering anywhere, even on the tightest switchbacks. I also don't experience wheel-flop -- for me that's only been a problem on bikes with a large trail measurement, not those with a large rake. Finally, after sheparding a tandem through plenty of off-road miles, it's my considered opinion that wheelbase is not a concern when it comes to figuring out if a bike is going to work well in tight singletrack. So, increasing wheelbase by increasing fork rake and decreasing HTA is not a detriment to handling.

YMMV
 

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eyebike66 said:
Can you please explain a little bit more what wheel flop is an when I would notice it?
Wheel flop is really easy to understand. It is the tendency of the front wheel to flop over when the bike is leaned to one side. Try it with your bike stationary. Lean the bike over while holding it by the saddle only. The front wheel will flop over in the same direction.

This is important, because if it happens when the bike is moving, it steers the bike upright again. Try the same thing while pushing the bike forwards by the saddle. Tip the bike over slightly to one side. The wheel flops over, and steers the bike into the lean, so it pops upright again.

This is the most important factor in bicycle stability. More important than the gyroscopic stability of the wheels, by a long way. (It makes even bikes with tiny, tiny wheels stable). It is why you can ride some bikes with no hands, and others are much more difficult. It is why some bikes can be pushed and ride along on their own, still upright, and others can't.

Wheel flop is determined by trail. So the higher the trail, the more wheel flop, the more stable the bike.

"Stable" also means "less lively steering" since the bike will fight your attempts to do certain things. So there's a trade off between nervous, lively handling, and stable, sluggish handling, again mostly determined by trail. What is 'right' for any particular rider and application is a judgement call rather than an absolute.
 

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This is all very interesting. Makes me wonder what my geo is now. My frame has a 72* HTA, but I think that's when the A-to-C is 470mm. I'm using a Reba @ 100mm, so that's maybe an additional 15 to 20mm higher. What's that do to my actual HTA? What's that do to the actual fork rake of 38mm?

Is the longer the fork is affect actual rake numbers?
 
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