Geometry

I pretty much sucked at calculus, was ok at algebra, and somewhat good at geometry.  I’m no engineer by any means, i have a Master’s in Ecology but no longer use that knowledge other than to explore nature on my own.  I have thought myself to be an engineer at heart – always thinking of new parts for the bike that are needed or tweaks that would make existing parts better.  I’m sure most people getting into framebuilding are the same.

The funny thing is that the bicycle frame is one of those things that really can’t be changed that all that much from what has been filtered out over the last 200 years of its existence.  For different disciplines there are of course different frame styles – materials and geometry – but for the most part a bike is the same two triangle design that has always been and will always be.  It just works.

When you Google “bicycle frame geometry” you get a pretty good assortment of opinions out there on the internets.  Sheldon Brown’s website always comes up as a top contender, along with ‘experts’ espousing their beliefs, usually more about road frame designs.  I’m no expert, and have lots to learn still, but I’ll share what I have learned and want to create with my bikes. (If you disagree or find something I saw wrong, let me know please.)

The mountain bike seems to have more variation in geometry than any other type of bike.  It’s my opinion that MTB builders are still settling in on the ‘best’ geometry, especially when so many sized wheel bikes are being developed (26, 650b, 29ers, 36ers).  Road, track, and cross don’t have that much diversity in design – as far as I know.  Maybe time trial bikes are an exception to the road norm, but that’s like DH to normal all-around MTB riding.  The mountain bike is so ‘young’, relatively, that there’s still lots of exploring to do.  The funny thing to me is how history repeats itself – the ideas of the first builders are coming back in style.

If you look at the ‘first’ mountain bikes of Fisher made by Joe Breeze, they have slack head tube angles and lots of rake/offset on the fork.  This made riding over rough stuff smoother with a rigid fork – a slacker HT angle makes it purr over rough terrain even if it makes climbing worse because of more wheel flop at low speeds.  This didn’t matter much to those old schooler Marin dudes because they had maybe 3 speeds, 40lb bikes, and hiked most of the uphills.  They figured this out then and now modern downhill bikes have seriously slack HT angles and low seat heights like the old klunkers.

Many consider Rivendell (Grant) to have it all figured out, at least for more touring-oriented bikes.  Their geometry is copied by many – take a look at Surly’s frame specs (the Surly Cross-check is identical in geometry to the Atlantis).  Their forks have a lot of rake compared to most – check out the Bombadil “mtb” bike – it’s got a 71 degree HT angle and a massive 57mm of fork offset! This makes the trail less than most other mountain bikes (67mm vs. over 70mm). The new Bombadil looks a lot like an old school Fisher from 1980 (see photos below for comparison).

Rivendell Bombadil (c.2009)

Fisher MTB (c.1980)

The wheel flop at slow speeds is not due to the HT angle alone but is due to the increased ‘trail’ of the front end, if fork rake or offset is held constant.  Trail is a hard measurement to describe but what is can best be described as is the feeling of the steering of the bike – the behavior of the front end.  It’s an interaction of the HT angle and the fork rake/offset. Slacker HT angles (69 vs. 72 degrees) will be slower steering at slow speeds but more stable at higher speeds and in my experience are better “trail” bikes (as in dirt, not geometry) and do not wash out around corners as much.  But too much trail is bad (no pun intended), it makes the bike feel weird – just as too steep a HT angle will.  So builders of slacker HT angled bikes correct for these design differences by increasing fork rake (at least for custom builders).  If builders are making a frame for a suspension fork, there are not many options to play with the HT angle.  Suspension forks only come in a few offsets (44mm or 46mm for the most part, and 51mm for the Genesis or G2Trek/Fishers).  So to get the feel of the front end that most expect, most mtb builders choose HT angles between 70 and 72 degrees.  The vast majority of 26″-wheeled bikes have 71 degree HT angles; the vast majority of 29ers have 72 degree HT angles.  That’s because industry has decided that the MTB handles best with a trail figure between 70-80mm (it’s a narrower range than that, I just don’t know what it is).  There is definitely some variation in HT angles among builders – some like the more ‘responsive’ ride with steeper HT angles, and some like the tradeoff of a less jumpy bike on the downhills (myself included).  Trek/Fisher has come out with slacker HT angled bikes like the 69er and Fisher’s G2 fleet that have either 69 or 70 degree HT angles and convinced Fox to create a suspension fork with an offset of 51mm (solely for their bikes, it can’t be purchased solo).  So being different in frame geometry and still having a nice riding bike means riding a rigid fork and prior to that, experimenting with these interactions of HT angle and fork offset on your own.

Anyways, I’m rambling.

Soon, i will build an adjustable rake fork to test out all these theories for myself.  I know it’s been done before but I gotta try it for myself.  My first frame is a copy of the G2 geometry but for my body (short legs = short ST), and my second frame will be a Rivendell Bombadil copy.  Two ends of the spectrum should give some more insight into the ‘best’ geometry for all-around XC trail riding. I’ll make the adjustable rake fork for the Bombadil copy, with customized track dropouts for the front dropouts so that the axle can slide between about 40-57mm of offset.  We’ll see how it builds up…more on that later of course.

Here are some resources that really are pretty cool that help with visualizing geometry and the interactions of all the measurements:

BikeCAD applet – the standard if you don’t do the life-size drawings yourself. This version’s free but didn’t seem to work on Firefox (?).

Trail-U-lator – calculates trail for a given wheel size and their matrix of head tube angles.

GeometryCalc– a mini BikeCAD-like program that I’ve just browsed. It appears to just compare the bike geometry changes due to different suspension forks.  It has a nice big list of suspension forks to compare.


One Response

  1. I have been playing with spread sheets for a new custom bike. It is pretty complicated I just know I can feel the difference between the Fisher G1 on My Ferrous and the G2 on the Superfly. LIke the Ferrous better. A tenth of an inch or 2 difference in trail seem to make a pretty big difference to me.

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