Sherpa handling, Sherpa hacking, and a little bicycle science

[Danneaux]

Hi All!

For those wishing for the summary at the beginning, let me say Thorn (via their designer, Andy Blance) did a terrific job designing my Sherpa 560S. It is fun to look at why it handles so well, and how changes in fork rake and tire size can affect handling. In sum, it is largely pointless to substitute a fork with different rake, even though they are available. If one wishes to change handling, the most straightforward way to do so is by changing to wider/taller/larger tires or narrower/shorter/smaller tires in identical pairs or closely-matched pairs, front/rear.

The detail-oriented can read on...

I am terrifically pleased with my Sherpa, and have no intention of doing anything to alter the handling. It works beautifully and is the ideal un/loaded all-rounder I had been searching for. I could have built my own heavy tourer from scratch, but choosing to buy one ready-made from Thorn says something about how "ready-right" a solution it is.

As a hobbyist framebuilder with a longtime interest in the geometry of framebuilding, I became curious about the geometry of mine and, by extension, others (the geometry and trail of small-wheel folders is really interesting!). Though my comments are specific to my Sherpa 560S, purchased August 2011, they might or might not reflect on Thorn's general design philosophy at the time. I am spot-on the physical average in all dimensions for my age cohort, so perhaps my 560S can be taken as a proxy for the "average" Sherpa.

My Sherpa 560S' geometry
For those who are curious, my geometry follows (measured with high accuracy on my framebuilding fixtures; the measurements almost exactly match the figures from the Cycling Plus March 2007 article titled, "Super-Tough Tourers", available in PDF form from Thorn's own website (Reviews section)...
http://www.thorncycles.co.uk/thornpdf/CP194SherpaReview.pdf

(My measurements taken in metric units; inch conversions rounded to nearest whole 1/10th inch)
Danneaux's 560S Sherpa
Top tube: 56cm/22in
Seat tube (c-c): 50cm/19.7in
Chainstays: 44.5cm/17.5in
Wheelbase: 107cm/42.1in
Head tube angle: 70.5deg
Seat tube angle: 72.5deg
Fork offset: 52mm/2in
Trail: 63mm (62.93mm), given an overall wheel *radius* of 333.5mm w/Schwalbe Dureme 26x2.0 tires@5bar max. pressure for measurement
BB height: 295mm/11.6in with Schwalbe Dureme 26x2.0 tires
Standover height @ mid-top tube: 795mm/31.3in with Schwalbe Dureme 26x2.0 tires

Alignment of the rear triangle was spot-on, as was the head tube-seat tube. The fork is a bit wide at the dropouts, but the.dropout faces are parallel.

Handling/design philosophy and Trail
Although a comprehensive discussion of bicycle geometry and how it affects handling is beyond the scope of this posting, it is sufficient to introduce the topic by saying a major determinant of handling is trail (the result of head tube angle and fork rake; trail is the difference between the lines intersecting these points at the ground plane, and is affected by wheel diameter and tire size (a large tire will result in a larger-diameter and vice versa).

In turn, trail measurements fall into three broad categories: low, neutral, and high; each category results in specific handling characteristics for the bicycle, each optimal for a different purpose.

Here are some general characteristics of each trail category...

LOW TRAIL
Generally speaking, a low-trail bike will will require a deliberate effort to lean into a corner and to stand back up again on exiting corners. As speeds increase, the bike will tend to rise out of a corner on its own. At low speeds, the bike will tend to go straight and will do so with little rider input. This makes low-trail frames highly prized by fatigued randonneurs and by those who also tend to carry weighty handlebar bags low over the front wheel; low trail bikes don't need a lot of minding at speeds below 30mph/48kph and tolerate weight well in that location. Unfortunately, there's a downside -- at higher speeds, the stability goes away along with steering feel and there is a greater tendency to shimmy with or without a load.

NEUTRAL TRAIL
Most of the recent published studies on trail-affected geometry have been 700C-specific. With that wheel size, a "neutral" trail falls into the 56-59mm range with 57-58 being the golden mean, but can extend from 50-63mm, depending on builder philosophy and intended purpose.  The result is a frame that steers pretty much the same regardless of speed, one that will hold a line without much rider intervention and will corner neutrally (no bike-initiated diving into or climbing out of turns).

HIGH TRAIL
Like a low-trail bike (and unlike the bike with neutral geometry), the handling if high-trail bikes varies with speed -- but in the opposite way. High-trail bikes tend to exhibit light, lively handling at low speeds, which really helps when manhandling large touring loads at slow speeds on rough roads and goat tracks. They tend to take tighter cornering lines and are as happy to be steered as leaned, and stability/resistance to shimmy increase with speed. The downside here is if you're tired and grumbling along at low speed, a high-trail bike can require more conscious minding on the part of the rider in return for more sporty low-speed handling. At rest, a high-trail bike will exhibit a lot of "wheel flop" -- when leaned even slightly while standing across the bike, the fork will whip around like a horse biting a fly. It is nice to have stability increase with speed, but there can be a tendency to corkscrew into corners (tightening your line) if you're not used to the handling. You probably won't need to lean as deep into corners to turn, and you can "steer" the bike a bit like a bus with a flat steering wheel.

At 63mm of trail, this (my 560S) Sherpa on 26x2.0 tires falls into the category of a "high-trail" frame, consistent with its actual handling and is appropriate to the task of a heavy-tourer with lively low-speed handling and increasing stability with increasing speed. It behaves exactly as I (or anyone) would (should) expect of a somewhat high-trail frame. It is at home and happy unladen or loaded, with easy steering easy steering either way, and really reassuring high-speed handling.

But...what would happen if one wished for heavier, more stable steering at low speeds or the effects of lower trail, mitigating some of theSherpas's baked-in characteristics with a dose from low(er) trail bottle?

Glad you asked...SJS Cycles sells Sherpa forks in a choice of 4 rakes: 43mm, 46mm, 52mm (as came on my 560S Sherpa), and 59mm.

Playing with forks and Danneaux's wide tires...what happens if we change the original 650S' 52mm-offset fork for another?

If we hold constant for tire size (keeping the 26x2.0 Schwalbe Duremes I use) and run the numbers through the calcublender, here's the trail results provided by each of the optional forks on my 560S Sherpa:

Fork rake = Trail (26x2.0 Schwalbe Duremes @ 5bar for max. diameter measurement purposes; usual pressure of 3bar will yield a *slightly* reduced trail)
_________________
43mm = 72.48mm
46mm = 69.29mm
52mm = 62.93mm
59mm = 55.50mm

Clearly, substituting any fork with lesser rake would push this bike (my Sherpa 560S with the specified tires) too far into the high-trail category; the bike would be unpleasant or unsafe to ride for general use, with excessive wheel flop at rest, too-lively low-speed handling, and too-stable high-speed handling to be optimal.

Substituting the fork with 59mm of rake drops the handling into the neutral-trail range, and would alter the handling as noted in my comments for "Neutral Trail" above. Possibly just the ticket for dedicated randonneur use, or for carrying a heavy handlebar bag mounted low, but on balance not worth it for the loss of low-speed agility and loss of high-speed stability with full touring loads; the bike handles nicely as-designed by Andy Blance.

Playing with tires...what happens if we change to different widths for a given fork offset?

Since trail is in part a function of effective wheel diameter (outside diameter of an inflated tire of a given size mounted on a rim of a given size), let's see what happens to the trail on my 560S Sherpa if we change the width/height of the tires...

Wide tires...26x2.0 (Thorn loaded touring option)

We've already seen the trail for my bicycle with various forks and my 26x2.0 Schwalbe Duremes in the fork/trail discussion above, but I'll repeat the figures here for convenience:

Fork rake = Trail (26x2.0 Schwalbe Duremes @ 5bar for max. diameter measurement purposes; usual pressure of 3bar will yield a *slightly* reduced trail)
_________________
43mm = 72.48mm
46mm = 69.29mm
52mm = 62.93mm
59mm = 55.50mm

Medium tires...26x1.75 (Thorn default)

Let's see what happens to the trail on my 560S Sherpa with smaller 1.75" (44mm) tires[/u] (1.75" Panaracer Pasela tires are the default option for this model):

Fork rake = Trail (26x1.75/44mm Panaracer Pasela)
_________________
43mm = 70.35mm
46mm = 67.17mm
52mm = 60.80mm
59mm = 53.38mm

What this tells us is that merely by choosing a narrower tire (remember, bicycle tires normally have a 1.0 profile where profile/height is equal to width), we can reduce the trail with any fork, but the fork supplied with the bike is still the most well-balanced for general handling and loaded touring. If we were to substitute a fork with 59mm of rake and the smaller 26x1.75 tires, then we dip further into a true low-trail geometry (for perspective, some older Treks were commonly supplied with as little as 45mm of trail).

Narrow tires...26x1.50 (Thorn "good road" option)

Now, lets go a bit more extreme. Thorn offer a 26x1.5" Panaracer Pasela "only recommended for good road surfaces". Let's see what that does to trail at all options:

Fork rake = Trail (26x1.50/38mm Panaracer Pasela)
_________________
43mm = 68.23mm
46mm = 65.05mm
52mm = 58.68mm
59mm = 51.25mm

What this example tells us is the originally-supplied 52mm fork with the narrowest tires offered by Thorn ends up putting the bike smack-dab into the neutral-trail/neutral-handing zone. Switching to the 59mm fork with these narrow tires puts the bike into the upper-end of the low-trail range.

Really skinny tires...26x1.25

Let's take it one step farther, with a really narrow tire, like the 26x1.25" Panaracer Pasela:

Fork rake = Trail (26x1.25/32mm Panaracer Pasela)
_________________
43mm = 66.10mm
46mm = 62.92mm
52mm = 56.56mm
59mm = 49.13mm

Equivalent-diameter 650Bs rims and tires...

I discovered my fork and stay v-brake braze-ons are high enough to allow the brake pads on my Sherpa to accommodate 650B rims (and certain tires that don't exceed the overall diameter of my 26x2.0 Schwalbe Duremes). The closest 650B tire giving a similar OD (668mm vs my current 667mm) is the 650x38B Grand Bois Lierre. Let's take a look at the equivalent 650B setup:

Fork rake = Trail (650Bx38mm Grand Bois Lierre)
_________________
43mm = 72.65
46mm = 69.47
52mm = 63.11
59mm = 55.68

The trail results for a given fork rake are nearly identical as for my 26x2.0 Schwalbe Duremes; not surprising, since the effective OD is only 1mm greater than what I am now running. As a happy side effect, a switch to 650B rims with 650x38B Grand Bois Lierre tires wouldn't even require a fender adjustment and handling would be virtually identical to what I now enjoy.

So...what can we learn from all this?

1) If you aren't satisfied with your Sherpa's handling (if it is a 650S model; I don't have geometry for other sizes), then you might wish to replace the original fork with 52mm or rake/offset with one that has 59mm or rake/offset. For a given tire size, this will move you from high-trail geometry toward more neutral or even low-trail, depending on tire size. It just won't work to substitute a fork with less rake (waaay too much trail to be pleasant), so mark those off your list of possibilities.

2) For a given fork, a wider tire will yield a higher trail; a narrower tire will yield less trail with handling characteristics to match.

3) The original, Thorn-designed frame geometry is awfully hard to beat for all-'round use and loaded touring.

4) If you wish to change the handling of your Sherpa, I would suggest changes in tire width and/or pressure to accomplish your desired ends, rather than swapping forks. If yours is damaged, it is best to replace it with a like model.

5) Because wider tires are taller (and vice versa; narrower tires are also shorter), a more extreme change can be made by mixing
tire widths/heights on the front and rear tires. However, because width correlates to height, it would be unwise to make any F/R
difference very extreme because of differences in the comparative contact patches; handling could quickly become unpredictable and
would likely suffer with, say, a 1.25" tire in front and a 2.0" in the rear. Best to change tire widths/heights in matched or
nearly-matched pairs, F/R.

6) If you're used to running your Sherpa on 26x1.75" Panaracer Paselas and wish to go heavily-loaded touring, a change to 26x2.0 Schwalbe Duremes will not only provide more cushion and shock absorption (thanks to lower pressures), but will also optimize the handling by pushing the effective geometry (increased pneumatic trail) toward the high-trail end of the spectrum, giving quicker, lighter low-speed steering and greater stability at higher speeds (typically over 30mph/48kph).

7) If you're used to running your Sherpa on 26x2.0 tires, you can make it more stable and resistant to rider input at lower speeds and a bit less stable/more vague at high speeds by switching to narrower/lower tires (most suitable when riding unladen on good roads).

8 ) If you have a shimmy problem in/dependent of load, a switch to larger tires (wider/taller) should help, ceteris parabus (all other things being equal or held constant).

As a side note, when I asked Lisa Parsons what size tire was used to determine standover in the Thorn brochure, she told me it was the default 26x1.75 Panaracer Paselas. It seems reasonable to presume the bike was designed around these tires because they represent a midpoint between the 1.5 and 2.0 tires available in the Thorn options list. The 1.75" tire puts my Sherpa just a wee bit into the high-trail range, which is a really sweet spot for good handling and feel, with the option to gain more trail (high-trail geometry) simply by fitting wider/higher tires for heavy loads. It would be fair to say that starting with the base Sherpa geometry (at least with my 650S), the handling is almost self-optimizing according the tires suited for a given use/purpose. A little more than other bikes because of where it starts. Nice!

Handy links for more on trail, geometry, and handling...

Those interested in learning more about trail as a byproduct of frame geometry and how it affects bike handling might be interested in the following links:
Bicycle Trail Calculator (jimg)
http://yojimg.net/bike/web_tools/trailcalc_dual.php
Bicycle Trail Calculator (BikeRaceInfo)
http://bikeraceinfo.com/tech/trail.html
Tom Kellogg/Spectrum Cycles
http://www.spectrum-cycles.com/geometry.php#trail
Randonoodler: The High Spark of Low Trail Bikes
http://www.randonoodler.com/2012/02/high-spark-of-low-trail-bikes.html
An Introduction to Bicycle Geometry and Handling (from the guys over at Chunk)
http://www.dclxvi.org/chunk/tech/trail/
Jan Heine's Journey of Discovery
http://janheine.wordpress.com/2011/02/10/a-journey-of-discovery-part-4-front-end-geometry/
low trail bicycle geometry - Google Search
http://tinyurl.com/bpu2d8t

Best,

Dan.

[EDIT: Revised subheads for clarity, cleaned up some typos this edit]

[NZPeterG]

Thanks you make it harder for me on which bike frameset to got next, its looking to be a Thorn Sherpa MK3.
I'm a Motorcycle Engineer so love to low down on "Trail" etc.   

[Andre Jute]

Oh, that's nice work, Dan. Your examples are models of clarity and intrinsic interest, very pointed. As it happens, I have low-trail, neutral and high trail bikes, and all three can be made to work on my hills by a rider with enough experience of each, but I know which is the most pleasant and secure to ride at speed on bad road surfaces, and it's the "stodgy" high trail touring bike.

I must be growing old. Thirty years ago I would have said that a short trail bike separates the cyclists from the boys...

An element of bicycle design, which doesn't get enough attention, that you might tackle sometime in the same style, with benefit to all, is wheelbase.

Couple of typos you might wish to correct, where your mind was on the bike type number and the your fingers not the rim designation...

Andre Jute

[Danneaux]

Thanks, fellows, most kind of you.

Andre, thanks truly for pointing out the typos; I take pride in trying to avoid them (or editing them out later). This was a week with 20 hours' total sleep over the last five nights, so yes, the fingers got away from me.  I'll check my eyelids for holes this afternoon and then return to tackle them when fresh. Doing a lot here to make time for the Big Tour in June.

A wheelbase dissertation? Gotta be in three parts: Front-center, Rear-center, and how the proportional balance interacts with wheelbase as a whole. Plus the matter of cantilevered rider weighting with handlebars/torso positioning as a contributing variable. Up next when I can do it.

All the best,

I must be growing old....

Dan. ("you're not old; just discriminating by experience")

[JWestland]

Hi Dan,

Nice work very thorough!

[Vintagetourer]

Thanks Dan for this explanation. I've taken the liberty of linking your post to the page on my CGOAB Lake to Lake journal about my bike.
Regards Graham

[Pavel]

Dan; I've only now discovered this treatise and wanted to say a very late thank you. This has been on my mind a lot as of late because I've been vascilating on getting a new bike, despite being anamoured of my thorn bicycles in every other way - except the lively handling. I hate the low speed responsiveness so much that if it were not for the Rohloff, which I can't fit elegantly onto any other candidate bike, I would sell my beloved thorns and move on. Now you have given me fuel for thought and a good reason to postpone being hasty. Time to get experimenting. I run Kojak 1.25" on my RST and will put on, post haste, my 1.6 duremes (or whatever those tires in the closet are) and if the low speed handling deadens, then will see what the best fork choice low low trail handling is, and get a ordering.

The funny thing about choice nowadays is that we have an overwhelming degree of choices all of the same sort, often I was all set to buy a new Club Tour frame, but decided against it because I suspect that Andy's idea of "sweet handling" is always going to be some version of what I call "twitchy". I was about to drive six hours to go test out a different paradime, the Vello-Orange Campeur, to see if my desires can be satisfied that way, but I really, really don't want to give up on Mr. Rohloff if there is any other way. I've ride. About six Surly bikes in the same quest but it's the same set of non-choice. They all handle the same, and worse than the Thorns. Why oh why did Peugeot stop making bikes of the 70's kind?  Well now I understand all of this better, and have new hope for my RST or Nomad. Thanks.

I wonder, does SJS Cycles realize that you are one of the major features gained by buying a Thorn?

[Danneaux]

Pavel!

I'm over at the Coast clearing brush in the rain and on limited comms with my phone so this reply will be less elegant than usual.

I have now converted three non-Thorn bikes to sweet handling machines, thanks to the substitution of Thorn Sherpa forks for the originals. The result has been three former MTBS converted from marginal usefulness to fully pleasurable touring and randonneur machines. One was a Christmas gift for a new retiree who had not ridden his bike in years. He is so impressed with the handling provided by his Sherpa fork, he now wants a complete Thorn -- nothing else will do.

I picked up an old MTB at a reputable pawn shop to use as a gap filler in my own fleet and bought two Sherpa forks so I can tune the handling at will through a quick swap. One results in low trail (40mm of trail) geometry and is a delight to ride in randonneur mode with almost no wheel flop and does superbly with a heavy front load. The other fork provides neutral trail (57mm) with my 26x2.0 Innova Swiftor tires and does well with a balanced load apportioned F/R. The original fork provided high trail (~64mm) geometry but was so large in diameter and thick-walled as to be wholly noncompliant. The Sherpa forks have 531 Super Tourist blades and a lovely, tapered bend -- very comfortable compared to the original.

Both geometries with the forks on this old MTB are in marked contrast to my Nomad Mk2 which excels with a rear-heavy load and very lively low speed handling that becomes ever more stable at speed but is characterized by a lot of wheel flop at low speed. It remains my expedition bike of choice.

If you go this route, be mindful that changes in the distance between front axle and lower head race will alter head angl and so affect trail in that way as well as by offset. This is especially true with Nomad Mk2 forks, which are suspension corrected and longer than others. Be sure to confirm steerer length with SJS Cycles before ordering. Not all Thorn forks have the same tire clearance and different model forks have different length steerers. If you choose a fork with a long steerer (as I deliberately did with the Sherpa forks) and leave the steerer uncut (which I also did) you may not be able to fit a lower crown race using a conventonal race setting tool. If this is your intent, then be sure to purchase a separate lower crown race when you buy the fork and leave a special note to SJS Cycles on the order form, asking them to mount it for you. Orbit XL II lower races are available from SJS Cycles as well as Thorn forks and are a match for the headset Thorn fit by default across their line. An alternative to buying and having them fit the race can be obtained by splitting your present lower race with a hacksaw or Dremel so no crown race setting tool is needed. Orbit lower races used to come already split but no longer do.

Thorn forks are a rare bargain in the bicycle world and come all pre-powdercoated and with a full complement of braze-ons at a remarkably reasonable price.

A note of caution and a disclaimer: I am a hobbyist frame builder thoroughly familiar with how changes in frame geometry affect handling and dynamics of unladen and fully loaded bicycles, and I have the means to accurately determine actual frame geometry in my fixtures/jigs. I wrote the above article as a way to explain the concept of trail and how it can affect ride and handling. If you choose to make changes, you're very much on your own and while my predictions have proven out repeatedly in practice on frames I have built or modified, I can't endorse the same for others. If you wish to experiment at your own risk, fine, but I can't/don't speak for or represent Thorn in this matter.

All the best,

Dan.

[Cross-referenced with more information here. - Dan.: http://thorncyclesforum.co.uk/index.php?topic=12602.msg94949#msg94949  ]

[ourclarioncall]

Good stuff Dan, enjoyed that .