Wednesday, October 24, 2012

29"er Geometry: Tuning Forks

Just over a year ago I posted this missive on 29"er Geometry. In that post I went back over where 29"er geometry had been, where it was, and where it looked to be going. My final conclusion then , (and still is), was that there is "No School" geometry these days for 29"ers. There just isn't a coalescing of thought on what the numbers should be like for 29"ers  today like there was 25 years ago with 26"er hard tail mountain bikes.

I've had several forks on this bike...
So, what we have today is the ability to change the front end geometry, and thus- the over-all handling characteristics of our 29"ers. This is an intimidating thought for a lot of mountain bikers out there. The geometry terms and how they work are confusing to many, and downright misunderstood by several folks.

But it doesn't have to be this way.

First things first- The Human species is a highly adaptable one, and the power of our brains to quickly learn and master new inputs and tasks is not to be underestimated. A rider can ride many variations of geometry successfully on many different types of terrain. The thing is, you have to "want to", and that may be influenced by preferences and perceptions to a high degree.

So take that all into mind while reading this post. Nothing I put forth here is going to reveal "the perfect geometry for 29"ers", nor should it. That said, I hope there is something you can draw from this to make your own considered judgments on what might be "right:" for your 29"er. This post will strive to show how you could "tune" your handling with simple fork swaps.

This is something I have done over the years with one of my bikes in particular: the OS Bikes Blackbuck 29"er single speed hard tail. this rigid Willits W.O.W. fork.
In 2008 I ran eight different forks with axle to crown measurements from 420mm to 510mm and offsets from 38mm to 51mm all on this one bike to gauge the effects of what front end geometry did to handling.

Head angle, bottom bracket height, and seat tube angle all changed with each fork, and I carefully measured each change to make sure I adjusted my cockpit to be as close to the same with each fork that I could. Then I tested each set up on various trails in the local area.

I posted my findings which maybe shocked some, and puzzled others. Basically what I said to start out here is the conclusion, but here are a few details for you to consider here. The terminology has to be understood first and foremost.
  • "Quicker handling" is basically steering that is less stable. The extreme of this would be a bike one has to really concentrate on to keep on trail, or you crash. Most folks want "quicker" to equal "easier to steer", but that can get confused with "stability", so be careful!
  • "Slower handling" is- you guessed it- more stable geometry. Essentially a bike that is so easy to ride, you can track stand it easily, and/or ride no handed over rough ground without the bike bouncing off line. Geometry can play into this, but so can wheel weight, rider weight distribution, and even tire pressures. "Slow" handling bikes tend to also be a chore to corner in tighter trails or switchbacks.
  • "Trail" is a result of a formula that takes into account the head angle, fork offset, and wheel diameter of a bicycle. A higher figure, (which is usually expressed in millimeters for bicycles), will indicate a front end geometry that is more stable, (slower), and a lower figure for trail would indicate the opposite, (quicker handling). You may have heard some folks in the rando world that will talk about "low trail forks" for better front end loading capabilities, and this is what they are referring to.  Mountain bikes tend to have higher trail figures for stability, and front end loads are not a concern here. 
  • "Axle to Crown" is a measurement from the center line of your front wheel axle to the base of the crown race on the fork. It is useful for adjusting the head angle of your frame without modifying the frame. For instance, you could use a fork on your 29"er that had anywhere from 420mm axle to crown to over 500mm and effect head angle changes of up to 3°-4°, which has a big effect on your Trail figure.
  • "Offset", or more correctly "Fork Offset", (which some refer to as "rake", which I liken to a gardening tool, but whatever...), is the distance the front axle is "offset" forward from the center line drawn through your steer tube to the ground. Where the axle is forward of this axis is measured as offset, but more here equals less. Yes- your front axle looks as if it is in front of your steer tube, but trust me- where the rubber meets the trail, that contact point is behind the center line of your steer tube if you draw a line straight down from the axle to the ground and compare the two points.  More offset = less stable Trail figures. So, if you push the axle further away from the steer tube center line, the tire contact point will get closer to the intersection of the steer tube axis with the ground equaling less Trail. (Of course, head angle and wheel diameter may negate this or enhance it.) Having a hard time visualizing this? Think of a shopping cart wheel. The axis where it pivots is your steer tube, the wheel's contact patch is behind that. Now tilt the axis, which is normally vertical, to be more like a bicycle head angle. See it now?
Okay- that's enough for one post. In the meantime, go knock yerself out playing with this Bicycle Trail Calculator . I'll come back again with another edition of "29"er Geometry: Tuning Forks" soon.


Steve said...

Iremember (and remember learning lots) those posts,I'm glad to see you coming back on this subject (especially as I shop for a budget new 29"er SS with a fork I do not care for-yes,one in specific) my memory more than most fades quickly,thanks for the links back :)

jonathansmith68 said...

Thanks for this post and for posting the link to the "Bicycle Trail Calculator". I had never felt too comfortable with my knowledge on the differences between the terms "offset", "rake", and "trail" and these items have definitely helped me be able to visualize these a little bit easier.

Roasta said...

GTed, how do you find the Slant six tyres and are you running them tubeless? I noticed the new STC logo for Sealant compatible on the Kenda site and there is a ~ 100g weight increase. I know of a guy recently that had non sealant specific tyres rupture (whilst not riding luckily) what are your thoughts experiences? Considering gooing-up an old set of Small Block 8's

Guitar Ted said...

@Unknown: I didn't like the Slant Six tires. I thought the profile was too rounded, they felt stiff, high in rolling resistance, and I thought they were too heavy for their size. Essentially, I found them to be a big let down from my expectations. The Smallblock 8 on the other hand was a wonderful tire, in my opinion. If you like Smallblocks, you also might look at the GEAX AKA tires, which are really great set up tubeless.

I never ran a Kenda tire tubeless because my friend in CA has had nothing but negative experiences and recommendations in regard to that. I just didn't want to try it based on that.

I am excited about the STC and the new Kenda model coming out called the Honey Badger, (yes- really), which should be out this coming Spring.

Roasta said...

Thanks for the reply comment GTed.
Honey badger looks interesting but I have always had more punctures with wide spaced knobs.I can definitely not complain about the small blocks in both 26inch and 29. 3500kms (2174miles) out of one rear and the front has plenty of life left - mainly commuting to work via road and also dirt plus regular single track sessions.

By the way for what it is worth thanks for all of your blog work. I appreciate all the time that must go into it. Since following you whilst living in the States and now back in OZ it makes my day to read your thoughts and live vicariously though all your bike builds and adventures.
PS: The pre-publish "prove your not a robot" characters and near on impossible to get correct sometimes.
Anyway enough off-topic high-jacking of your Geometry post.

Guitar Ted said...

@Roasta: Thanks! (And for the record, those comment verification characters make me go cross-eyed too!)