# Mechanical Trail Dimension -- What's it feel like?



## zipzit (Aug 3, 2005)

In a recent posting, http://forums.mtbr.com/showthread.php?t=572779 someone asked about a fork with a variable rake.. One comment there sparked a lot of thought... Rather than go off topic, here is a new post..



Walt said:


> -Zero/Negative trail is very bad for mountain biking. Duh. It's pretty funny, though. Especially if you can trick an unsuspecting friend into swapping bikes with you.


Walt, What does that feel like? Can you describe?

My reason for the question is this...What does too much / too little trail feel like?

--I know if I'm crusing my bike on the street, that if I have lots of trail I can easily take my hands off the handlebars and cruise all day. Is that a good thing for an off road mountain bike? (Or wait.. maybe this is the criteria... skilled rider, adjust trail so bike won't track hands free, not so skilled rider, adjust trail so bike does track hands free...?)

--Once in a while I'll be cranking hard then hit a corner that the bike just doesn't want to track right.. Its like I have to finish the corner wide in order to make it 'feel right'. The feeling reminds me of trying to ride across a fast moving stream about 8" or so deep. When you do that the bike wants to turn downstream. Its kinda like that in certain turns. Bike wants to go wide on the turn exit. Does that mean I have too much mechanical trail (or that I just suck at mountain bike skills ? I already know that!)

I've seen lots of studies that talk about mechanical trail numbers, but nothing that really tells how things feel.

thanks for your input...
zip.

ref: http://www.johnforester.com/Articles/BicycleEng/Kvale%20Geometry.pdf and 
http://en.wikipedia.org/wiki/Bicycle_and_motorcycle_geometry#Mechanical_trail


----------



## pvd (Jan 4, 2006)

When mechanical trail goes to zero the front wheel can flip around backwards with no effort. When MT goes negative, the wheel wants to flip around backwars. Both very bad things.


----------



## restlessrider (Nov 29, 2007)

the easy way to experiment with negative trail is to turn your fork backward with your stem forward.


----------



## Vlad (Feb 7, 2004)

I just remembered an article from Mountainbike magazine in it's heyday. They had an article about John Castellano, right around the time when the bike press was going crazy over the Ibis Bow Ti. Anyway, Mountainbike magazine ran pictures of Castellano riding his Bow Ti with an Amp fork mounted backwards. Any idea why he would do this? I can't imagine that bike handling or suspension action was better with a backwards fork.


----------



## flatfoot1 (Sep 8, 2009)

*Trail*

If the fork is backwards, trail increases by slightly less than the offset of the fork.


----------



## fallzboater (Jan 18, 2004)

flatfoot said:


> If the fork is backwards, trail increases by slightly less than the offset of the fork.


Without checking the math, I think it would increase by slightly less than _twice_ the fork offset, but the important point is that less rake (even negative) or shallower HA = more trail.


----------



## ted wojcik (Mar 12, 2006)

You're right.


----------



## Walt (Jan 23, 2004)

*Well...*

So you know how on even the most unstable bike, you can take your hands off the bars once you're up to speed at least for a few seconds and keep going straight and keep your balance?

Not with a negative trail setup, friend!

Imagine instead that if you let go with even one hand, the bike will try to violently steer off the trail/road/whatever. Imagine that the smallest bumps and rocks fling the bike violently off line. IMO, it feels like you're fall-down drunk and trying to ride, sort of. You can do it, but it takes all of your concentration and focus just to stay upright and on line.

There's no practical way to make yourself a negative trail bike without a custom fork, really, at least that I can think of (well, maybe you could put a 50mm rake road fork on a downhill bike...), so you'll have to take my word for it.

-Walt


----------



## dr.welby (Jan 6, 2004)

Vlad said:


> Mountainbike magazine ran pictures of Castellano riding his Bow Ti with an Amp fork mounted backwards. Any idea why he would do this? I can't imagine that bike handling or suspension action was better with a backwards fork.


It was an experiment in anti-dive. I think he modded it so the trail stayed the same, but by flipping the fork the axle path moved forwards instead of backwards, creating anti-dive.


----------



## pvd (Jan 4, 2006)

dr.welby said:


> ...creating anti-dive.


Uh oh. Here we go again.


----------



## dr.welby (Jan 6, 2004)

pvd said:


> Uh oh. Here we go again.


Only if you take it there, ha ha!

I seriously considered putting in a disclaimer, btw.


----------



## zipzit (Aug 3, 2005)

I get that too short a mechanical trail is a bad thing.. (never mind negative...) If lots of trail is good and stable, how much is too much?

What I'm trying to figure out is much more subtle that that. My current bike sits at 79mm mechanical trail (calculated) I can NOT ride comfortably for a long time on the street without hands on the bars. At the same time, there are definitely corners (off road) where on occasion I get stuck going wide to maintain 'balance / feel'. It doesn't happen often, maybe once in 25 hard corners.. .I never really paid attention to it before, but now I'm focusing on next frame geometry. (Oh, and I'm using ~2006/7 vintage Roxshox reba 29er forks with no plans to swap them out..if I want to increase trail, I will increase my head tube angle.. current bike = 72 deg, proposed HA for next bike = 70.5. That angle gets me to 89mm mechanical trail)

If I increase trail on the next frame via head tube angle, I know it would be more stable on the street, but that's not where I normally ride. Will I regret having more trail in the design of the next bike during off road cornering & off road handling? Will too much mechanical trail force me too wide on my corner exits?

Note: Wife has same brand / model frame as I in a different size. (Surly KM 1 med, 1 lrg)Different head angle by 1/2 degree. Her trail calculates out to 83 mm, but she is stable on street, hands free. Difference is probably the rider, she's way talented at balance activities (long time speed skater)... but who knows? (Heck yeah, I'm jealous. I'm a guy. With a reputation to uphold. )

thanks for any and all advice, zip.

I just had a thought.. If I make the frame say 1/8" longer ( theoretical top tube length), and decide that the head tube angle isn't working for me, and assuming fillet brazed joints.. can I melt off the braze, recut the TT and DT and re-assemble at a different HT angle? (or is that a really bad idea? And I'm assuming inexpensive paint job here, not powder coat...) That way I could try it and see if I liked it?


----------



## shiggy (Dec 19, 1998)

zipzit said:


> I get that too short a mechanical trail is a bad thing.. (never mind negative...) If lots of trail is good and stable, how much is too much?
> 
> What I'm trying to figure out is much more subtle that that. My current bike sits at 79mm mechanical trail (calculated) I can NOT ride comfortably for a long time on the street without hands on the bars. At the same time, there are definitely corners (off road) where on occasion I get stuck going wide to maintain 'balance / feel'. It doesn't happen often, maybe once in 25 hard corners.. .I never really paid attention to it before, but now I'm focusing on next frame geometry. (Oh, and I'm using ~2006/7 vintage Roxshox reba 29er forks with no plans to swap them out..if I want to increase trail, I will increase my head tube angle.. current bike = 72 deg, proposed HA for next bike = 70.5. That angle gets me to 89mm mechanical trail)
> 
> ...


There is more to easy no-handed riding than just the trail number. Component selection and balance, and especially overall frame/bike alignment.

Accessories on the bars can affect the balance. Maybe the headset is slightly tight. Slight twist at the head tube. Rear wheel a bit off center. Each could make it harder to ride with the hands off the bar. Even a change in wheel/tire weight can make a difference.


----------



## Walt (Jan 23, 2004)

*Short answers*

The trail number is not your no-hands problem. Most cyclists can easily no-hands on bikes with trail all over the map. Most road bikes are around 60mm and it's not hard to ride no-hands on them. Practice more!

Second, cutting off the tubes and rebrazing the head tube is a terrible idea for several reasons. You'll just have to take my word for it. You can add/subtract spacer from the fork to adjust the ride height and travel to mess around after the fact, to some extent, but I would just pick a number around 80mm and do it. Trail is not the end-all and be-all of the bike.

-Walt



zipzit said:


> ]
> What I'm trying to figure out is much more subtle that that. My current bike sits at 79mm mechanical trail (calculated) I can NOT ride comfortably for a long time on the street without hands on the bars. At the same time, there are definitely corners (off road) where on occasion I get stuck going wide to maintain 'balance / feel'. It doesn't happen often, maybe once in 25 hard corners.. .I never really paid attention to it before, but now I'm focusing on next frame geometry. (Oh, and I'm using ~2006/7 vintage Roxshox reba 29er forks with no plans to swap them out..if I want to increase trail, I will increase my head tube angle.. current bike = 72 deg, proposed HA for next bike = 70.5. That angle gets me to 89mm mechanical trail)
> 
> If I increase trail on the next frame via head tube angle, I know it would be more stable on the street, but that's not where I normally ride. Will I regret having more trail in the design of the next bike during off road cornering & off road handling? Will too much mechanical trail force me too wide on my corner exits?
> ...


----------



## .Zig (6 mo ago)

flatfoot1 said:


> *Trail*
> 
> If the fork is backwards, trail increases by slightly less than the offset of the fork.


Not true. 
If you took a fork offset of 42mm on a bike with a 65 degree head angle and a 764mm outer tire diam, your mechanical trail is 119.44. Reverse the fork for a negative 42 value (-42mm), all else equal, and your mechanical trail would be 203.44. Remember that the trail number is a function of the cosine of the fork angle (-) the offset.
pretty big change


----------



## compositepro (Jun 21, 2007)

That's some thread resurrection right there up with lazarus


----------



## .Zig (6 mo ago)

Still relevant , I believe.


----------

