Author Topic: Sherpa 'Trail' and tyres; feeling unstable downhill  (Read 851 times)

Danneaux

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Re: Sherpa 'Trail' and tyres; feeling unstable downhill
« Reply #15 on: January 04, 2018, 10:58:00 PM »
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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#msg19567

Okay, 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:
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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:
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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.php

3) 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=US
The 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.
« Last Edit: January 08, 2018, 04:52:32 AM by Danneaux »

mickeg

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Re: Sherpa 'Trail' and tyres; feeling unstable downhill
« Reply #16 on: January 05, 2018, 12:28:26 AM »
My Sherpa and Nomad handle great.  But, I have not tried to ride no handed since I did that decades ago on an old single speed coaster brake bike that I used for newspaper delivery.  Nomad, I could get a very slight resonance in the handlebars at a specific speed when I was in Iceland with a heavy load, but I found that if I loosened up my grip on the bars that the resonance got weaker.  On both bikes I use drop bars, not sure how that would change handing or not.

I almost always have a handlebar bag on my bikes, that makes the steering a lot less responsive.  I had a Long Haul Trucker that had such a bad shimmy with a full camping load of four panniers that I eventually put the frame in the metal recycling bin.

I am too ignorant on frame geometry to comment on trail and how it impacts handling.  When I was trying to diagnose my shimmy problems on my Long Haul Trucker, I asked Thorn for information on the trail, fork rake, head tube angle, etc. on both of my Thorns.  They responded like I was asking them to give away their top secret business strategy which really perturbed me considering that virtually every other bike manufacturer openly shares that information.  I do not know if they have changed and are more willing to divulge that data or not, good luck trying to figure that stuff out.

A friend of mine has had slight shimmy problems with a bike that had rear panniers but no front panniers, it was worse the farther back his panniers were mounted.  He had large size shoes so he needed a lot of heel clearance so his panniers had to be pushed back a bit.  I have also found that shimmy is greater if I have my rear panniers back further so I try to have minimal heel clearance.

***

Sometimes I have typed my responses into a text file, then pasted that later into the website.  But lately my computer has been working pretty stable so not doing that right now.
« Last Edit: January 05, 2018, 12:33:11 AM by mickeg »

macspud

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Re: Sherpa 'Trail' and tyres; feeling unstable downhill
« Reply #17 on: January 05, 2018, 09:03:54 PM »
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=US

Dan, going by the Nomad Mk2 brochure and an SJS Customer Service answer at https://www.sjscycles.co.uk/forks/48-26-thorn-nomad-mk2-steel-fork-matt-black/ the AC distance is 420mm, the AC distance on the Thorn Nomad Disc Fork is 430mm https://www.sjscycles.co.uk/forks/48-26-650b-thorn-nomad-disc-fork-yellow-gloss/ so the Mt. Tura fork's 10mm longer AC distance should also be taken into acount.

pavel

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Re: Sherpa 'Trail' and tyres; feeling unstable downhill
« Reply #18 on: January 05, 2018, 09:16:57 PM »
Dan thanks for the thorough reply.  I was in fact thinking about the MT Tura fork for it's 4mm extra offset, but suspect that it likely would not be enough change to note a difference.  Do you think that's correct? 
My Nomad fork was cut down by mistake when the bike was ordered and though the semi-relaxed possition was close to ok, as I've aged and gotten out of shape I want to sit more upright than most of you would find prudent.  It's mostly the left arm that compels all of this.  It goes dead to feeling after a short time if I'm in about the position you show with the photo, which is what I consider "perfect" for touring and now for example I can't ride with only my left arm on the handlebar because I can't hold a straight line.  None of this is really the design of the bike, only the design of the bike for my shoddy ill kept body.  IN fact I've had to move the rohloff shifter to use upside down on the left side so I don't have to lift my right arm off the bike even for a moment as I reach to shift. :(  So I find myself looking for a very slow handling bike, and I'll take any negatives that come with it. 

I ride the RST mostly now however and am not sure if I ever get to go for another long tour which bike I'd take.  It likely would be the Southern Tier if I do go a wandering, and the RST would probably be the more likely steed.  Unfortunately I think that my 565M ( I think that's the size) already has the long offset.  There are three RST forks 43, 46 and 49.  I'm not certain but I think my size has the 49 already. Unfortunately there is no real info at SJS about the details of the RST forks, like axel to crown, but I'm thinking of perhaps replacing the fork with the 52 regular ST26 forks from a regular Raven. I would then consider the front brakes facing the "normal" way a secondary improvement.

I've ridden most of the Surly bikes as we have a local shop here that has a lot of stock built up but have not like a single one as much as the Thorns - not even close, so that option is out and of course I wonder if perhaps either the Velo-Orange Campeur or the Thorn Club Tour might not be the fabled bicycling promised land - and it sure would help with the neglected N+1 theory.  I mean I've got enough parts to build up two 700cc bikes sitting around.  But to step down from the luxury of the Rohloff and become a Derailleur driven plebe - well, there are limits to what I'm willing to suffer  :)

So the angst goes on.  At least no dollars have been harmed while indecision reigns. :D  Sadly, I've taken two twenty minute rides on recumbents thus far - and loved it. Does a Thorn badge change paternity? 


pavel

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Re: Sherpa 'Trail' and tyres; feeling unstable downhill
« Reply #19 on: January 05, 2018, 09:28:51 PM »
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=US

Dan, going by the Nomad Mk2 brochure and an SJS Customer Service answer at https://www.sjscycles.co.uk/forks/48-26-thorn-nomad-mk2-steel-fork-matt-black/ the AC distance is 420mm, the AC distance on the Thorn Nomad Disc Fork is 430mm https://www.sjscycles.co.uk/forks/48-26-650b-thorn-nomad-disc-fork-yellow-gloss/ so the Mt. Tura fork's 10mm longer AC distance should also be taken into acount.

How does the AC change affect the handling. Does anyone know?  Does it increase or decrease the trail.  I can't think on this medicine, and this is out of my purview on even my best days.  :)

Danneaux

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Re: Sherpa 'Trail' and tyres; feeling unstable downhill
« Reply #20 on: January 05, 2018, 10:26:23 PM »
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How does the AC change affect the handling.
A change in AC with other factors left the same will indeed change trail and therefore handling.
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Does anyone know?.
Yes.  ;) :)
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Does it increase or decrease the trail.
It can do either depending on whether the AC (axle centerline to lower crown-race-seat distance) increases or decreases.

For the same fork rake, a decrease in AC (shorter fork) will steepen the head tube angle and will decrease trail.
For the same fork rake, an increase in AC (taller fork) will slacken the head tube angle and will increase trail.
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I was in fact thinking about the MT Tura fork for it's 4mm extra offset, but suspect that it likely would not be enough change to note a difference.  Do you think that's correct?
Hmm. That's hard to say. I would be able to feel it, but I'm real sensitive to changes in bike geometry/handling. I don't think the difference would be "enough" for you to accomplish your desired handling goals.
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My Nomad fork was cut down by mistake when the bike was ordered and though the semi-relaxed possition was close to ok, as I've aged and gotten out of shape I want to sit more upright than most of you would find prudent.  It's mostly the left arm that compels all of this.  It goes dead to feeling after a short time if I'm in about the position you show with the photo, which is what I consider "perfect" for touring and now for example I can't ride with only my left arm on the handlebar because I can't hold a straight line.
My late father was very right-side dominant and this required some modifications to his bicycle to make it easy and comfortable for him to ride. For example, he lost the vision in his left eye and his left shoulder was completely shattered in a fall as a youth; it was uncertain for some time if he would keep it. He was left with a limited range of motion in it and could not steer reliably or pain-free with his left hand as a result.

It took me some time to come up with a solution that worked. It might not work for you, but I'll relate it anyway in case something there provides a sprout of an idea:
1) I rotated the handlebars slightly so the left side was closer...and shortened the stem so the reach to the further right side was still manageable. This allowed him to ride with both hands on the 'bars and -- amazingly -- in time, it served as a sort of physiotherapy so he could eventually steer with his left hand for brief periods and without pain. Despite the rotated handlebars, he could hold a straight line with no problem and regularly drafted just a few centimeters off my back wheel.

2) Because he could not turn his wrists forward or inward, I fitted randonneur-bend drop handlebars (shallow reach and drop, flared ends and an upward rise at the outer ends of the tops). See: http://img1.qbp.com/6SPsvm45/prodl/HB1024.jpg

3) To address problems with leverage when braking, I fitted Guidonnet-style brake levers:
https://activesport.co/WebRoot/Store5/Shops/80c85f8f-7a95-4b1c-9c30-e64b314f3f2e/5536/5D69/6088/2EE4/5D77/0A48/350B/39E7/1_ml.jpg
To provide a stop for his hands, I fitted Dia-Compe tandem dummy grips where brake hoods would normally go:
https://lh3.googleusercontent.com/proxy/B6M9Dmxwg0IoP2DX4WpOWq8fbObJYA9TFJkGPJeLaOaybMgyHxwz3iPyktBY9a2HFsPLxTIs9WocK18iyw-RHDRHSS36NbqFqTkT_RSdhQxrDyyMl7xTfW1Y3F92spW7XZA7UEHZA7FKfNihAOX1Sl1ZnWdzBP-jl34mdw=w5000-h5000
A variant resides on my tandem for the stoker's use:
https://i1.imged.com/dia-compe-tandem-stoker-levers-dummy-levers.jpg

Altogether, this helped a lot.

Both my sister and a Portuguese friend both have little twists in their pelvises that require their saddles not point straight ahead. Rotating the saddles about 5 to one side made a world of difference in comfort and endurance. Another friend has a shorter leg and I needed to come up with a "better" solution for him as well. I milled new sideplates for a pair of SunTour Superbe Pro track pedals so one pedal had a higher baseline, but this meant he was raising his leg too high at the top of the stroke. The final solution was to fit a shorter crankarm on the left side so he pedaled in smaller circles with that leg. It worked for him.

My point being...when not all is square with the rider, sometimes it is necessary to modify the bike in some unusual ways to fit. People change over time. Things happen. Cycling is a repetitive-motion activity. Get it wrong, and you'll incur repetitive-motion injuries.

I once had a fatal prognosis myself and came out the other side of Life's Adventures with some changes to accommodate, so I can relate.
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I ride the RST mostly now however...Unfortunately I think that my 565M ( I think that's the size) already has the long offset.  There are three RST forks 43, 46 and 49.  I'm not certain but I think my size has the 49 already. Unfortunately there is no real info at SJS about the details of the RST forks, like axel to crown,
For a given model, the AC distance is likely to be the same, but a call or email to SJS Cycles would surely resolve the uncertainty.
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I'm thinking of perhaps replacing the fork with the 52 regular ST26 forks from a regular Raven.
I can't advise you here and I would not proceed until you talk to SJS Cycles. The AC distance may well vary between your RST fork and the one for the current (regular) Raven and the result could be unpredictable. Andy Blance can best advise. Make that call.  :)

All the best,

Dan.
« Last Edit: January 05, 2018, 10:38:33 PM by Danneaux »

mickeg

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Re: Sherpa 'Trail' and tyres; feeling unstable downhill
« Reply #21 on: January 06, 2018, 12:39:27 AM »
Or, get some interrupter type brake levers to add.  I have them on most of my drop bar bikes.  Tektro and Cane Creek make them.  See photo where i have modern style bars, threadless stem, interrupter brake levers, modern brake levers on a 1960s vintage bike.  (The Mafac brakes are original to the bike.)

I also have my saddle twisted slightly, instead of being aligned directly ahead.  A line directly through the front and back of the saddle would pass about 30mm to the left of my stem bolt because of the twist that I use.  I had a back injury some years ago and I think that my back is asymmetric from that.