Sherpa forks in Raven Tour frame?

[Andyb1]

When riding my 537L framed Raven Tour for the first time I realised how high the BB is which results in a high saddle and possible instability when I stop as I am on tip toe.
The forks fitted are the standard twin plate type with an L1 measurement (axle to crown bearing) of 410mm.  Sherpa forks have L1 of 370mm, which I have calculated will drop the BB height by 16mm, making it the same as another bike I ride.

So has anyone else done this change?   If so did you use 46 or 52mm offset sherpa forks?

I am going to make the change as I have the parts and see how it feels; I expect the shorter forks will make the bike feel more responsive but hopefully not twitchy.

I have spoken with Thorn about it and they could neither confirm or deny that the change would work, which I can understand.  But it was not a ‘No, No’.

[PH]

I think you've covered it, the question is how it effects handling and there's a good bit of subjectivity in that.  It's probably that subjectivity, rather than any practical reason, which has Thorn sitting on the fence.  Some people have a preference  that has to be mm perfect, but I note Thorn have some current forks, for the same models, which vary between 385 and 410mm. 
When i set up an EBB bike I do so with the BB in the centre position and accept the fore/aft variation of +/- 6mm and the 3mm height variation, others change their saddle to correspond.  The third option, which I did for a while, is to use a half link to minimise BB movement.  I mention this because if BB height is critical, then getting the EBB to maximise it could be useful.
.

[mickeg]

I know almost nothing about frame geometry when it comes to handling characteristics, but the tiny bit I know tells me that your trail is a key number.

If you can determine what your current trail is and calculate what it would be with the change, then someone that knows more about geometry than I do could take a guess on what would happen to handling characteristics.

But in the big picture, if you are on tip toe when you stop, even if you can make a slight improvement with a different fork, if you stop on a dip where the ground under your wheels is higher than under where your feet would land, you could be in big trouble.

I assume you have the same bottom bracket eccentric that I have on my Nomad Mk II.  I have seen plenty of photos of bottom bracket eccentrics with the thin part on the bottom instead of top, if you did that and were careful not to overtighten the set screws that hold it in place, could that help?  I stretch rubber bands over the two set screws to prevent them from rotating and loosening.  Thorn advises against this due to possible distortion of the eccentric.

Thicker soles on shoes?

It is a major bummer to have buyers remorse, but that might be where you are headed.

[Danneaux]

Andy, you may wish to read this piece I wrote 13 years ago...
https://thorncyclesforum.co.uk/index.php?topic=4245.msg19567#msg19567
...and...
https://thorncyclesforum.co.uk/index.php?topic=12602.msg94949#msg94949

I've had great success using Thorn forks to replace the original forks supplied with other frame brands, both for myself and others, but that success depended on knowing in advance how the swap would affect handling, something I am familiar with as a longtime hobbyist frame builder. The key to success is to keep in mind the effect of the swap on trail and therefore handling. The effects are predicatable and depend on both fork rake and head angle. Rake has an effect on trail, of course, but that is only part of the equation. If the frame is already built, then a difference in the axle-to-crown (a-c) distance can change head angle and also therefore trail.

When I chose to replace the fork in my Enduro-Allroad bike, I purchased two Sherpa Mk2 forks so I could swap between and experience any differences firsthand. One results in "neutral" trail at 57mm, the other relatively "low trail" at 40mm. I prefer the handling of the low trail fork on this bike but have swapped between from time to time. These two forks share the same a-c distance but rake varies. In each case, the lower a-c compared to OEM steepened the head angle. For this bike, I had two initial goals:
1) I needed a more comfortable and compliant fork than the original, and...
2) I needed a taller steer tube to use with drop handlebars.
I found success in both goals. My hands on the brake hoods (with a short reach stem and compact drop handlebars) are within 1mm of where they were on the original grips with riser handlebars  (see attached pic) and as a bonus, handling also more closely matched my desires. In a couple other bikes, the primary goal was to safely extend steerer height for the rider while minimizing any change in handling. I do know a number of riders who have swapped-in Surly forks on their Thorns and Thorn have in the past marketed a Mt Tura rigid fork, intended for swaps on frames with suspension-corrected geometry.

As for the tippy-toe feeling, you have some options...

1) One is to adjust the BB eccentric as George suggests keeping the caveats in mind, or...
2) Wear shoes with thicker soles.
3) Another is to swap to a smaller tire size to lower the frame and therefore saddle height in relation to the ground plane. My treatise above shows the effect that can have on trail as well.
4) The cages/platforms/cleat engagement can vary between pedal-top and axle centerline. This difference alone might not be enought to matter, but combined with other things, can effectively reduce your saddle height in relation to the ground.
5) A no-cost option tahtta doesn't affect handling is to develop the habit of coming off the saddle at every stop, a matter of preference. I recall a post here from several decades ago where the rider grew up on too-large hand-me-down bicycles and came to regard coming off the saddle as normal, even preferable whenever stopping. A community discussion followed about preferred mounting and dismounting techniques. Some preferred swinging a leg over the saddle, others over the handlebars if a tall load was present on the rear rack. Others preferred to place a foot on the near-side pedal to provide a step and set the bike in motion as they swung a leg over the saddle. As for myself, I most often touch a toe for balance when stopped for a traffic light if the bike is unladen, but choose to slide forward and off the saddle for greatest stability of the bike is loaded for touring. It soon becomes habit and no change is required to bike, fork, or geometry.

The choice is up to you but the results can be predicted in advance. Where you have the parts on hand, I'd suggest you give a try and see if you like the results. If you do, then keep in mind there may be unintended/unforeseen effects with a load or at speed, i.e. a greater tendency to shimmy, so thorough testing is a good idea before you commit. Thorn's designer, Andy Blance, really sweat the details on their frame geometry, so OEM remains a good reference spec as you experiment.

Best, Dan.

[mickeg]

After reading Dans comments, I am wondering if I read this wrong.

The tip toe when stopped, I read that as stand over height above the top tube was too high.

But it appears that Dan read that as staying in the saddle and only having a tip toe for support when stopped.

The difference between saddle and top tube are huge differences, too little stand over height above top tube is a safety concern.  But not being able to have your feet on the ground while in the saddle is an inconvenience, no more.

Which is it?

[Danneaux]

Which is it?

That's a good question, George. I figured if I covered the other end, we'd nail it helpfully either way. 

Best, Dan.

[martinf]

As I use short 150 mm cranks, my saddle position is about 20 mm higher than with "normal" 170 mm cranks.

This means that I nearly always come off the saddle when I stop. 

So I don't really notice any difference in bottom bracket height, which varies quite widely across the bikes I usually ride.

Unless the bike is positioned oddly with the wheels on bumps and the saddle/bottom bracket area over a hollow I always have top tube clearance on all my bikes when I put my feet on the ground.

I have briefly ridden a bike where I didn't really have adequate top tube clearance - a loan bike on a home exchange to the Netherlands, which had a really big frame. It wasn't a big problem, but I can't remember what I did when I stopped. As I sometimes do this on my own bikes I probably leaned the bike to one side with one foot on a pedal and the other on the ground.

Top tube height doesn't yet bother me, as my usual mounting method is to push off on the left pedal and swing my right leg over the rear wheel rack and any luggage. This method doesn't work if I have a lot of stuff piled up on the rear rack, in that case I generally lean the bike over and step over the top tube before putting my foot on the left pedal.

[PH]

A couple of additional thoughts from me:
I've assumed it's saddle to ground that we're talking about, Thorn bikes have pretty low TT so it's be seriously the wrong size of bike for that to be an issue.
i don't understand those suggesting thicker shoes?  Wouldn't you have to raise the saddle to maintain the same position, in which case you might as well just lower it.
it is good practice on a loaded bike to come off the saddle when you stop, even so I'd still like to feel safe and stable while in the saddle for times it isn't possible.
Thorn don't publish head angles, but there isn't a lot of variation. 10mm fork length, with the same offset, alters it by about half a degree, measuring offset is easy (Measure, turn 180, measure again, half the difference)you can play with the numbers here:
 http://yojimg.net/bike/web_tools/trailcalc.php
Touring bikes are usually have train in the range of 50 - 70, you have the kit, you may as well try it, but I wouldn't like to stray too far out of that range.

[Andyb1]

Thanks for all the replies, I will read again plus the links.

The eccentric is fitted thick side at the bottom and I will think how a half link might change how it is positioned - but I may also go to 17T rear sprocket from 16T and that will alter how it sits.
No ‘buyer remorse’ - just a little project to get the bike exactly how I want it.

To be clear, I like to be able to safely touch the ground while sat on the saddle.  I can then easily restart riding again.
I will try sliding forwards at stops - but I do find that position can get a bit unstable on restarting, particularly if I need to make a turn or have luggage on the back.  For me the stand over height is not too important as I usually launch and demount with a leg backwards.

I have now fitted the Sherpa fork. Offset on that fork is 46mm (is this the same as trail?).  I must measure the offset of the Raven fork.

The BB height is down from 295mm to 280mm.   The lower height feels fine.

Visually the Raven now looks…..like a Sherpa with a much shorter steering tube (the tube in which the fork stem goes through).  I only had a short ride today but it felt OK - perhaps more responsive than with the original fork?
I will try to ride the bike tomorrow a bit more and maybe go to SJS again.

[mickeg]

...
To be clear, I like to be able to safely touch the ground while sat on the saddle.  I can then easily restart riding again.
I will try sliding forwards at stops - but I do find that position can get a bit unstable on restarting, particularly if I need to make a turn or have luggage on the back.  For me the stand over height is not too important as I usually launch and demount with a leg backwards.

I have now fitted the Sherpa fork. Offset on that fork is 46mm (is this the same as trail?).  I must measure the offset of the Raven fork.

The BB height is down from 295mm to 280mm.   The lower height feels fine.

Visually the Raven now looks…..like a Sherpa with a much shorter steering tube (the tube in which the fork stem goes through).  I only had a short ride today but it felt OK - perhaps more responsive than with the original fork?
...

Got it.  Not a top tube problem.

Since almost all bikes now have sloping top tubes and use longer seatposts than years ago, stand over height is almost never an issue any more.  My rando bike has a horizontal top tube, but the rest of my bikes have sloping.  Half a century ago bike shops often sized bikes by wanting just enough stand over height and maybe a tiny bit more for safety, but that was about it.  And a fist full of seatpost (roughly 100mm) between the frame clamp and the seatpost clamp was about right.  But now, often have more than twice that much seatpost exposed.

I think most touring bikes have low bottom brackets compared to some road bikes or cyclocross bikes because on touring bikes you never are leaning over for tight cornering.  And lower center of gravity for stability.  That said, my Nomad Mk II feels like it is very high compared to other touring bikes, I assumed that was by design since the Mk II is intended to also work well off road where pedal clearance on rough terrain can be an issue.  Attached photo, on this trail, many times the trail was so eroded that I could ground a pedal while pedaling.  For that reason, I wished my bottom bracket was a bit higher.

If you extend a line straight through the headset to the ground.  Then measure the distance back from that point to the center of the tire patch, that is trail.

Long trail favors holding a straight line quite well when riding, short trail will make a bike more responsive, but if you take your eyes off the road a short trail can result in you going off course easier.

That is roughly the extent of my knowledge about handling in bicycle design.  If I got that wrong, I am sure Dan will correct me.

If it feels ok to you when riding with this other fork, then I would not worry about it.  Go ahead and use it.

I always thought that the Raven and Sherpa were essentially the same bikes, Raven being Rohloff and Sherpa being derailleur.  So, I am a bit surprised to learn that they use different forks.

I have not ridden my Sherpa for about eight years, but it is a good bike so I have no desire to part with it.  I may use it on another tour again if the right circumstances arise where I would prefer a derailleur bike and would prefer one that is more robust than my titanium touring bike.

[PH]

I have now fitted the Sherpa fork. Offset on that fork is 46mm (is this the same as trail?).  I must measure the offset of the Raven fork.

Visually the Raven now looks…..like a Sherpa with a much shorter steering tube (the tube in which the fork stem goes through). 

Trail is as mickeg explains, if that's not clear there's a diagram in the link I posted above.  The steerer tube is the top part of the fork and the head tube is the part of the frame it goes through.  Offset (Or rake in US speak) and head tube angle are the elements that determine trail.  Offset is sometimes mistakenly thought of as the curve in the fork blades, but even straight blade forks have offset, it's just achieved in a different way at the steerer joint.
It's a good position to be in, too be able to swap stuff around and try it, that's a lot easier than trying to work out how much X effects Y before spending a fortune!

[Andyb1]

Good link PH.  What Thorn call Offset looks like rake?   I will try and get my frame terminology better!

So….a ride to SJS today and I bought a set of NOS ST26 forks with 46mm offset (rake).  The same as the Sherpa forks I had fitted to try out.  A later Raven is on sale on ebay and it also looks to have these forks fitted.  They should speed up the handling but I could not notice any difference compared to the original 40mm longer twin plate forks.

Anyway, the new forks have 3 x advantages:
- 100g lighter
- they have lawyers lips
- they are red
Nick at SJS assured me that red forks will make my bike faster, but as they are pulling the bike from the front I may not notice the difference!

(My best financial option would have been to continue using the forks from my Sherpa.  I would have saved £120 by not buying the new red forks but that would have meant that the Sherpa was broken for parts or scrapped, and it is too good a bike for that)

[Danneaux]

Andy, when all is said and done, what have the new red forks done for your ability to more easily touch the ground while on the saddle?

Have things improved in that regard?

Hoping so!

Dan. (...who likes the red-and-black combo )

[Andyb1]

Hi Dan,
Yes the red forks have made a significant improvement.  The BB height has been lowered by about 15mm, to 280mm from 295mm, which means that instead of being on tip toe when sat in the saddle my toes are now flat on the ground.  It gives a safety margin, useful when the bike is loaded.

I am lucky to live near SJS and that they had NOS forks at half price, and that they cut the steerer down for me.

The EBB is positioned with the crank axle at 12 o’clock, the highest possible place, so as the chain stretches the BB height will reduce a little.

As I said above, there is a later Raven on UK ebay with the same fork that I have fitted, so maybe later Ravens had the shorter fork as standard?
 

[Danneaux]

Yes the red forks have made a significant improvement.

Yay, Andy! So pleased it has worked for you and met your goals.

Now...safe, happy travels with just enough Adventure for terrific stories afterward but not so much as to cause serious inconvenience or grief.

Wishing you well, Dan.