Where is that article that Dan wronte on this, again. I'm going to re-read it and think about buying a new fork. But which one? 
Pavel, my article can be found here:
http://thorncyclesforum.co.uk/index.php?topic=4245.msg19567#msg19567Okay, here's the redone reply.
Pavel, when I first inquired about buying my Sherpa Mk2 in 2011, I was told it was designed around 1.6in tires, the Nomad was designed around 2.0, in keeping with the mission of each bike. This makes sense, as tire size affects effective trail and therefore handling and the "jumps" between 26in tire sizes are generally larger (i.e. 1.25in, 1.5in, 1.75in. 2.0in, etc) than for 700C road-bike tires (23mm, 25mm, 28mm, 32mm, 35mm, 38mm, etc). With a few exceptions, most bike tires are bias-ply construction and have a 1:1 aspect profile, meaning they are essentially as tall as they are wide when measured from the rim edge hook (not the same as BSD or tire bead seat diameter). Wider tires will be taller tires and narrower tires will be lower and each will change the effective trail. Mixing tire sizes on a single bike can really make a noticeable difference in handling but at the cost of predictable handling due to the difference in tire contact patch size and shape.
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Last year, I faced a similar dilemma when I rehabbed a pawn-shop bike into a drop handlebar enduro all-road machine. The bike was an MTB-based commuter and had a fork that was too robust/non-compliant for me and the steerer was too short to get my back at my desired 45° angle with drop handlebars, so I perused SJS Cycles' Thorn fork offerings to see what I could find for a replacement.
I needed 1) a long uncut steerer for my needs and the Sherpa Mk2 fork had the longest of Thorn's offerings. 2) I wanted sturdy but compliant forks wth v-brake bosses and a 2in tire capacity with mudguards fitted and the Sherpa Mk2 fork again ticked the boxes. 3) I needed to determine the front-axle-to-lower-crown race seat distance to select the closest I could find to my original fork to minimize the difference in crown race height and therefore head tube angle and 4) I needed the correct offset to pair with the resulting head tube angle.
As it happened, the Sherpa Mk2 forks' front-axle-to-lower-crown race seat (AC) distance was lower than on my original fork, and this increased head (and seat tube) angle by 1 full degree -- I did a lot of measuring first-- so I had to keep that in mind when calculating the rake needed to get my desired trail.
I ended up buying two forks. The one I use most for rugged/gravel randonneur and all-'round use with front-biased cargo loading results in relatively low trail of 40mm. The other fork results in spot-on neutral trail of 57mm and can be fitted when I wish to tour with a load pretty much balanced front and rear or a bit rear-biased at levels far below what I load on my expedition-grade Nomad. For reference, the original fork provided 63mm of "high" trail.
I like the handling of the "low trail" 40mm fork (keep in mind "low trail" can sometimes be as low as 25mm or so, which I consider extremely low. Similarly, many MTB sus-forks paired with MTB frame geometry result in about 73mm of static/unloaded trail, which I consider to be very high, though it settles down under compression). With the 40mm trail fork, the bike is stable and doesn't shimmy on 100kmh descents and has the general characteristics I described here:
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.
The neutral trail fork also works well and has the general characteristics I decribed here:
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).
I like the handling with both forks very much and would call the bike a success. With either fork, it has the lowest trail of any bike in my fleet; they usually hover around 60mm-63mm of trail with the tires I have fitted to them. They also work well for their intended purposes and largely rear-biased cargo loading and I have no complaints about their handling. However, I do notice the difference in handling when switching back and forth between those with large differences in trail and it takes me awhile to fully adjust. After a few miles, everything feels "normal" again instead of "twitchy" or "sluggish" relatively speaking.
By the way, I did a similar fork replacement as birthday gift for a family to present a retiree. The long steerer provided a more upright posture for his back and he could ride comfortably for the first time in ages (the bike had previously been gathering dust in the garage). The resulting trail was within about 3mm of original, so handling change was pretty much imperceptible. All were happy and the result and I was delighted it got him back on a bicycle again. He loves it and is so impressed with the fork, he's thinking about buying a new Thorn.
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If you wish to change the handling on your Nomad by changing forks, then you're in somewhat uncharted territory and pretty much on your own for getting a result that is both desirable and safe.
My little article will give you a general guide to what you could expect from different trail, but the actual results may well be different based on numerous factors and the handling might not be sweet on your particular machine. I am an accomplished and successful hobbyist framebuilder with a full knowledge of how geometry affects handling and have fixtures to take accurate measurements. This is an important caveat as it can be sometimes be difficult to accurately measure existing fork rake or head angle without them. I've seen people mis-measure fork offset by as much as 3mm, which really matters in this case.
Currently, SJS Cycles lists over 100 different items for a "Thorn Fork" product search, yet the Nomad Mk2 forks are available in only one rake: 48mm, with a published axle to crown race seat distance (AC) of 430mm and a 390mm steerer tube length. That's it.
To change to a different fork in their lineup, things quickly get more complicated, as they vary by rake, AC distance, and steerer length as well as construction, blade wall thickness and effective blade length. A difference in AC distance will alter head angle and therefore affect trail even for the same rake/offset. Steerer tube length could affect your back angle. That's a lot to consider.
If you decide to go ahead with another fork on any bike, the steps are pretty much the same as I used for selecting forks for my pawn-shop bike rehab:
1) Measure your existing bike to get a baseline. You'll need to accurately measure head angle and AC distance and fork rake/offset and keep in mind the tire width/height you intend to use. A change in tires will affect the trail to some degree.
2) Run those numbers through a trail calculator including your preferred tire width/height. One of my favorites is from an old i-BOB colleague, JimG:
http://yojimg.net/bike/web_tools/trailcalc.php3) Using those numbers, decide on what trail or trail range will give you the general handling characteristics you prefer and select a fork with the characteristics needed to get you there, keeping in mind there may (will) be limitations. Check for:
a) Differences in the all-important AC distance. A taller one will result in a shallower head tube angle, a shorter AC distance will result in a steeper head tube angle. You'll need to compensate for the change in head angle with fork offset for a given tire size. A shorter fork will, for example, affect bottom bracket height and seat tube angle, possibly necessitating a change in saddle position/angle. If the bottom bracket is too low, you could strike a pedal while cornering. You'll need to measure the degree of lean before pedal strike occurs on the stock bike, then lower pressure in the front tire till the rim drops the required amount to check how soon it would occur with a fork having a lower AC measurement to see if it is acceptable risk.
b) The rake/fork offset may not give you the exact result you want, you may only get "close".
c) Steerer tube length will need to be long enough to get your back comfortable. While a steerer can always be cut down, it is difficult to raise it to the same degree or with the same ease and low cost.
d) Tire width. You will need a fork to accommodate the widest and tallest tires you intend to use -- with mudguards if that is what you prefer. Some forks may be sufficiently wide yet have clearance problems under the crown that get worse if you need to accommodate mudguards. If you change forks, your future tire-size options may be limited due to handling concerns compared to the original fork.
e) Suitability for purpose. If you have an expedition-grade bike, it might not make sense or be safe to fit a fork with a lower payload capacity to it. Also, the blade wall thickness and diameter may not be up to handling the demands of heavy loads. This can lead to an unsafe situation if you forget and then load the lighter-duty fork as you would a more robust model.
There's another caution for Nomad Mk2 owners: The rigid forks are suspension-corrected and so are markedly taller than the ones used on bikes that are not suspension-corrected. If you wish a different offset fork on your Nomad Mk2, the only alternative
might be the Mt. Tura fork with 52mm of offset. It has the same 430mm AC distance and a 400mm long steerer:
https://www.sjscycles.co.uk/forks/26-thorn-mt-tura-mk2-steel-fork-80-100-mm-suspension-corrected-matt-black/?geoc=USThe change would likely result in about 4mm less trail, moving you more toward neutral trail from your present trail. Out of respect to Thorn designer Andy Blance's preferences, I will not reveal my Nomad's head tube angle. Only you can decide if the change would be worth it to make the purchase and swap -- or indeed, if a lower-trail geometry is right for your needs.
As you might guess, you'll need to feed the corrected specs through the trail calculator to verify your choice, keeping all variables in mind. It is a fair amount of work and when all is said and done the results may not be what you want.
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Do you all think a low trail, perhaps with a 190 mm "tiller" stem or some other magic, would transform my Thorns to handle like a long wheelbase truck - or just the opposite?
In my experience, stem length should be used to adjust fit on the bike and not as a primary means to address handling woes. While a change in stem length with all other factors held constant will affect fore-aft weight distribution and therefore handling, it will cost in terms of comfort and efficiency.
When I fit a bike to myself and others, I start with saddle height in relation to the pedal spindle centers with the cranks vertical and saddle fore-aft adjustment in relation to the pedal spindle centers with the cranks horizontal (I prefer KOPS or Knee-Over-Pedal-Spindle myself) for my preferred length of cranks (170mm). Once that is settled, I adjust stem height and reach. I prefer the tops of my drop handlebars to be level or no more than 50mm below the top of my saddle, and I go for whatever stem results in a back angle of 45° with my hands on the brake hoods with my elbows slightly bent for shock absorption. This is my "Golden Mean" positioning and results in my weight being evenly distributed between my bottom and my hands, which I find comfortable*. Using these guidelines, all my bikes' "hard points" fit the same within 1mm though the geometry and seat tube/top tube lengths vary from bike to bike.
My bikes with longer top tubes (Nomad Mk2 size 590M and the pawn-shop bike) are fitted with shorter reach (60mm) stems to compensate. My other bikes happen to use stems with 80mm of reach. I have found no handling problems with the shorter-reach stems because the important distance for steering leverage is the moment arm (distance) between the brake hoods and the steerer centerine, which doesn't vary much on my bikes. The (randonneur) bikes with longer stems have handlebars that are narrower at the brake hoods (37cm, 45cm at the ends), while the bikes with shorter stems have much wider handlebars (42cm or 44cm measuring the same at the brake hoods as at the ends). The result is a wash or nearly so when it comes to leverage. I mostly steer by leaning when on the road, and steer by hand more when fully loaded on dirt roads or goat tracks at low speeds when it is inconvenient to lean-steer. The longer top tube-shorter stem thing is a relatively new thing for me; in the last six years of using this configuration, I have come to really like the feeling it gives of riding "inside" the bike instead of feeling I am riding "on" the bike. Indeed, more of my weight is inside the wheelbase of the bike (the longer top tube and shorter stem push the front wheel's axle centerline forward) and most likely contributes to this feeling (see photo below).
Long and short of it: I don't think a tiller-like stem will get you where you want in terms of handling, though low-trail might. What you've told me about your old Peugeot's handling is not quite enough for me to be certain.
All the best,
Dan.
*EDIT: Several people PM'd me and asked me to post a photo showing my preferred "Golden Mean" -- a back angle of 45° with my hands on the brake hoods with my elbows slightly bent for shock absorption. I draw the line through my hip and shoulder joints and through the palms of my hands on the brake hoods, elbows bent slightly for shock absorption. Photo below. Not intended to be prescriptive or proscriptive, this is just the cycling "fit" and positioning that works for me. Everyone is different. -- Dan.
