Geometry Relationships- Mtbr.com

1. Geometry Relationships

Okay, I've been trying to read and understand the affects of head tube angle and fork rake on the trail of a bike.

Based on this link, I've derived that increasing trail will slow the handling of a bike.
http://www.anvilbikes.com/story.php?module=calculators

Therefore, a bike with a steeper HT angle and greater rake will have less trail and handle faster. Decreasing rake and keeping HT angle the same will reduce the trail and make for a slower handling bike. (I always assumed that increasing the rake will make the bike handle slower because the wheel base is longer). Conversly, decreasing HT angle and keeping the rake the same will also increase the trail and make the bike handle slower.

Wheel size also affects trail, as a larger wheel increases the trail, thus, a 29" wheel will inherently steer slower than a 26" wheel if HT and rake are kept the same.

I would then assume to make a 29" bike handle as fast as a 26" bike, you need to increase the rake of the bike and increase the HT angle.

Am I right so far in my understanding?

But, what affects will fork length have on trail? If two bikes have the same HT and Rake figures, but different fork lengths (one 438mm and one 468mm), will one handle faster than the other? (I'm also assuming HT length will be the same so that the bars are in the same place).

2. I think you've got it all very right and understandably explained!

As far as I (can) understand, fork length has no influence. You might as well move the fork/headtube intersetion up or down, rake and angle would stay the same. Odd, but thue, it seems.
Side note on the 438mm fork option, going suspension on a bike that works well with the 438mm, will totally chopper it out, shorter IS available, but only rigid of course. And older 420mm Pace would be sexy.
The 468mm is totally 75-80mm suspension corrected, and the bike could probably still take 100mm of plush, or be quickened with a 450mm'ish rigid.

I'd be interested to have a KM fork with just 38mm of rake, to compare it to my present 43mm one. Although the same length as my 38mm rake sussers, the KM fork does lighten up the handling quite a bit (again, it seems).

We need 43-46mm rake sussers, especially on smaller sized 29"ers. Those people, to get toe clearance, are forced on long toptubes and slack headtube angles and slow handling bikes, while a longer-rake fork would allow them shorter toptubes and steeper headtube angles for a bike with the same or better fit, and option to enjoy quicker handling.

3. Yeah, sounds like you've pretty much got your head around rake and trail. Fork length doesn't matter as long as HT angle and rake are the same. One thing to keep in mind though, is that if we are talking about the same actual frame, different fork lengths will change the HT angle, and this will have an impact on trail.

Sam

4. Well, if you look into details, a longer fork will put the whell more in the front compared to the center of gravity of the biker.
This can give a ligher steering, and less grip in the front for corners....

We already had got a lenghty discussion on this in an lod thread, can't remember the name, sorry.

5. No.

He specified that the bars would stay in the same place, so the crown to axle length is irrelevant. The front wheel will be in exactly the same place if we use those assumptions. That WOULD be a concern if we were to adopt Cloxxi's slack head angle/long rake setup for smaller frames, but probably not a deal killer unless the wheel was WAY out in front.

I think a couple of other interesting variables that don't get enough play are chainstay length and BB drop. These can have a HUGE effect on the handling of a frame, and in fact can be far more important than small changes in trail.

-Walt

Originally Posted by 20.100 FR
Well, if you look into details, a longer fork will put the whell more in the front compared to the center of gravity of the biker.
This can give a ligher steering, and less grip in the front for corners....

We already had got a lenghty discussion on this in an lod thread, can't remember the name, sorry.

6. Originally Posted by Walt
I think a couple of other interesting variables that don't get enough play are chainstay length and BB drop. These can have a HUGE effect on the handling of a frame, and in fact can be far more important than small changes in trail.

-Walt
Can you expound on BB drop. I've always assumed lower is better(COG) until pedal strikes become an issue.

7. Originally Posted by Walt
I think a couple of other interesting variables that don't get enough play are chainstay length and BB drop. These can have a HUGE effect on the handling of a frame, and in fact can be far more important than small changes in trail.

-Walt
Go ahead and give it some play.

My thought is that there is no reason for a 12.4" bb height or lower on a 29"er. The longer wheelbase should allow you to raise the BB, compared to a 26"er bb height, and maintain the same level of "twitchyness" and/or stability.

Larger frames should have longer chainstays. An XL size 29"er takes care of that cause you can't build it any shorter without bending the seattube.

8. Wheelbase?

Originally Posted by Walt
He specified that the bars would stay in the same place, so the crown to axle length is irrelevant. The front wheel will be in exactly the same place if we use those assumptions. That WOULD be a concern if we were to adopt Cloxxi's slack head angle/long rake setup for smaller frames, but probably not a deal killer unless the wheel was WAY out in front.

I think a couple of other interesting variables that don't get enough play are chainstay length and BB drop. These can have a HUGE effect on the handling of a frame, and in fact can be far more important than small changes in trail.

-Walt
I understand that raising and lowering BB height is going to have affects on Bar height, saddle position, and stability, but what is optimal? Remember Cannondale used to have the "East Coast" bikes with higher BB's? How did they ride? I'd assume twitchy.

Also, if the wheelbase is really long, does it matter how fast the front end is? A longer wheelbase will have a longer turn radius, so would that over ride the quick front end?

9. no to no :-)

Originally Posted by Walt
He specified that the bars would stay in the same place, so the crown to axle length is irrelevant. The front wheel will be in exactly the same place if we use those assumptions.
-Walt
If the bar is in the same place, i agree with you that this wont change the weight on the front.
But anyway this setup has still has an effect, because it needs a long stem. And a long stem make the steering less responsive to the bumps as the body weight gives a greater counter-torque on the steering....
It also give a different feeling in the turns has the path off the hands is not the same....

10. Maybe we're not talking about the same thing.

490 said, I think, that the fork with a longer c-a would also be on a frame with a shorter steerer and head tube in order to have the bars in the same place. In this case, I stand by my statement. The stem will be the same. The bars will be in the same place. The weight distribution will be identical. Handling will be totally unaffected.

Am I missing something?

-W

Originally Posted by 20.100 FR
If the bar is in the same place, i agree with you that this wont change the weight on the front.
But anyway this setup has still has an effect, because it needs a long stem. And a long stem make the steering less responsive to the bumps as the body weight gives a greater counter-torque on the steering....
It also give a different feeling in the turns has the path off the hands is not the same....

11. Yup...

Originally Posted by Walt
490 said, I think, that the fork with a longer c-a would also be on a frame with a shorter steerer and head tube in order to have the bars in the same place. In this case, I stand by my statement. The stem will be the same. The bars will be in the same place. The weight distribution will be identical. Handling will be totally unaffected.

Am I missing something?

-W
That is what I was asking. If the bars are in the same place because you lengthen the head tube, TT is the same, will the handling be affected, to which Walt answered "no". I was unsure of this. But why not build all forks suspension corrected? Why have a shorter fork? Does it ride differently?

12. Yep, low is good.

When I do a frame for a customer, I ask about a number of things that will affect the ideal BB height in order to decide how much drop to build into the frame (a little side note: framebuilders hate to talk about BB height, because it depends on too many variables outside of their control like tire size, inflation pressure, suspension sag, etc).

-What tires will the rider be running? Currently, almost everyone has a tire around 2", but if we get some beefier tires, this will become more of an issue.
-What length cranks will the rider be using? The longer the crank, obviously, the higher the BB needs to sit to avoid pedal strikes.
-What kind of riding and trails will the bike encounter in its life? Some folks ride buffed out singletrack every day, others are dropping off roots and rocks and grinding up muddy hills. You can probably guess which one will be happy with a super low BB!
-What fork or forks will be used? Some folks like to run a very soft suspension fork, others are on a full rigid setup. Sag has to be taken into account.

Basically, I go as low as I can, trying to balance out stability and pedal strike frequency. It's not that there's an ideal height - some folks are just smooth and can pedal through anything without hitting their pedals. Others can't keep the cranks off of rocks (like me) to save their lives. I'd say the average is about 60mm of drop for my frames. I've built as tall as 40mm, and as low as 70mm.

Here's what a low BB gets you:
-Extra stability. The lower you are under the axles, the less likely you'll be to get knocked off course by obstacles.
-Extra tire clearance (in some cases): with the same _effective_ (level) chainstay length, a lower BB will actually use longer chainstays. This makes it easier to fit in a bigger tire without having chainring clearance problems.
-Shorter effective chainstays. See above. You can also get the effective chainstay length a few MM shorter without compromising tire or chainring clearance. Both this and the previous bit are pretty minor concerns. We're talking about a few mm one way or the other. Note that shorter chainstays are not necessarily an advantage, but some folks want them for reasons mostly unrelated to their actual effects.
-Less endos. The big wheels help here too, but the low BB keeps your CG low, and helps keep you from going OTB. I have proven on several occasions that it is still perfectly possible to endo, no matter how low your BB is. But it's harder with a lower BB.

-Walt

13. My fingers are getting a workout!

The reason for the shorter forks was *mostly* that nobody offered fork blades long enough for a suspension corrected 29er until recently. Builders can be lazy people (coming up with your own fork design and blades is a pain), and for a customer who plans to run rigid all the time, a shorter fork is fine as long as the frame is built for it. A short fork is also lighter (most folks don't think about the fact that they'll add that weight right back in extra steerer length to get the bars to the height they want, though) and can be built a bit stiffer (for big folks) because of the shorter legs.

The second reason (which is VERY rare, in my experience) is that some shorter riders can't get their bars low enough with a suspension-corrected frame and fork. If you're a smaller person and NEVER plan to run suspension, a shorter (420mm or whatever) fork will help you keep your bars down low where you want them. I've actually never encountered someone who both fit on a 29er AND had this problem, but I'm sure they're out there.

-Walt

Originally Posted by bigwheelboy_490
That is what I was asking. If the bars are in the same place because you lengthen the head tube, TT is the same, will the handling be affected, to which Walt answered "no". I was unsure of this. But why not build all forks suspension corrected? Why have a shorter fork? Does it ride differently?

14. Thanks, thats what I expected, just the way you mentioned it before sounded like there might be more to it.

15. I've never managed to get my bars too low on a susp. corr. 29"er.

16. Originally Posted by Cloxxki
I've never managed to get my bars too low on a susp. corr. 29"er.
Yeah, but your desired saddle to bar drop just isn't for humans.

17. No torso, all arms, what can I do... :-)

18. Originally Posted by Walt
490 said, I think, that the fork with a longer c-a would also be on a frame with a shorter steerer and head tube in order to have the bars in the same place. In this case, I stand by my statement. The stem will be the same. The bars will be in the same place. The weight distribution will be identical. Handling will be totally unaffected.

Am I missing something?

-W
Obsiously, you are the native american speaker, and i'm not so i am the one that didn't understood everything right !
I do agree with what you wrote above.

What do you guys think of another configuration:
- let's say we are only modyfing the direction angle, with the same whellbase, same trail, same bar position.
So the change are in the direction angle and the stem lenght (bar to HT)
How is a change going to modify the behaviour ?

19. Huh?

Okay, its taken me a while to get my head wrapped around this as I've read a few different articles and some of the info was misleading, but, what is the "direction angle"?

20. Originally Posted by bigwheelboy_490
Remember Cannondale used to have the "East Coast" bikes with higher BB's? How did they ride? I'd assume twitchy.
The Beast of the East - 13+'' BB. I owned one. I used it for stock trials and technical XC. I didn't think it was twitchy. I really liked it. I have a tall BB on the Vulture and it's one of it's best features. I don't notice any handling issues with it but *do* notice how much I bang the crap out of my KM's cranks on the rocks I used to avoid. I may not be sensitive to the differences in BB height unless I literally have it smacked into me. The KM does seem to handle quite well tho.

21. Sorry, "direction" in french si steering in english

22. I would assume that a shorter, non suspension-corrected, fork would lose a bit of flex over a suspension corrected fork as a shorter spring is a stiffer spring. I was planning on having Wily build my fork non suspension-corrected because his forks look a bit long and thin.

How does having a longer head-tube affect frame flex around that area? Assuming you spread out where you weld the top and down tubes.

Moto

23. Walt,

What do you think of Grant Petersen's take on BB height and how it affects "bike tilt"? Seems to make sense to me, and corroborates what I've experienced on high and low bb bikes.

Sam

24. Not Walt, but I'd say that a high BB puts more distance between COG and the terrain (duh!), making any input, body english or unmeant disturbances, a less direct input to the bike's behaviour. So you need more body English to get the bike to do what you want. An in stress situations, about to crash, I think a high BB makes it harder to keep the rubber side down. DH bikes only have high BB's to clear obstacles and allow for lots of suspension sag. I know a DH Pro that rides with BB's that would even be low for XC, he loves it that way.

Imagine a typical MTB, but with somehow a second bike welded on top, on which the rider actually sits, BB height ~4ft and up. I can see how that bike would be really responsive at slalomming closely spaced obstacles, without even requiring body english, just very quick steering input. When it goes wrong, it just goes wrong without escape, but now thinking of it this way, I can see why some people say they like their high BB's for quick steering. It looks like it will do just that, but lose out on high-speed moves where the trail really starts moving in a different direction, rather than just slalom back and forth.

26. Cloxxki, you mean something like this?

Or even more extreme?

Looks like it could be fun - but I can't imagine the quick steering you seem to be able to picture. As soon as you tried to lean into anything surely that'd be very hard indeed....

Sam

That last one is just a wheelie machine! One pedal stroke and it'll help you dismount backwards, it seems. He had to use a small front wheel to not flip back before he reaches the seat trying to mount.

In both these examples, the virtual BB is moved back as it placed higher up. With a normal 50-50 weight distribution, I still think it would slalom very quick, in an odd way, barely leaning over.

28. It sure is - man there are some gems out there once you start looking......

Want some seriously negative trail?

Yes, you can still lean even with 2 feet of BB rise

Not sure whether this is UCI legal, but could be useful nonetheless....

And you thought SSWC was pretty crazy....

Check out more at cyclecide.com

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