Technical > General Technical

Sherpa 'Trail' and tyres; feeling unstable downhill

<< < (5/5)

Danneaux:

--- Quote ---How does the AC change affect the handling.
--- End quote ---
A change in AC with other factors left the same will indeed change trail and therefore handling.

--- Quote ---Does anyone know?.
--- End quote ---
Yes.  ;) :)

--- Quote ---Does it increase or decrease the trail.
--- End quote ---
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.

--- Quote ---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?
--- End quote ---
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.

--- Quote ---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.
--- End quote ---
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.
=====

--- Quote ---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,
--- End quote ---
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.
--- Quote --- I'm thinking of perhaps replacing the fork with the 52 regular ST26 forks from a regular Raven.
--- End quote ---
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.

mickeg:
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.

Navigation

[0] Message Index

[*] Previous page