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Sherpa 'Trail' and tyres; feeling unstable downhill

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pondweed:
I've read Dan's excellent treatise on High and Low trail on his sherpa. I had my first 10 then 20 miles on my 535L, and was very happy with its progress with the exception of... downhill. Despite me (5' 10") being within the sizing for the nearest I can see in the current catalogue, the 535L with straights feels short in body and I've got the saddle right back and the stem is a 125mm with the ?monkey bars rolled right round forward to get me a bit more stretched out. I realise I might not feel the same with longer distances, but its good experimentation...
I felt I wanted to get my bum back off the saddle descending, but I can see that that also transfers weight back off the bars, which may be not a good idea despite the perceived feeling of "safety" being further back on the bike.
Anyway, its presently on Schwalbe Marathon 1.25 tyres. I can't work out my 'trail' as I have no idea what the steerer angle of the frame is, as its not quoted. But could the smaller tyres be contributing to not feeling stable downhill?
Anyone with greater experience give me any sage advice on fiddling? (like 'shut up and just ride the thing')
Thanks

Danneaux:
Hi Pondweed!

Thanks for the kind words, I'm glad you found the article useful.

I've been down on power the last couple days after cleaning the rain gutters on my house Tuesday. While working, a bit of the dirt got inside a tiny crack in the skin of my elbow and by Thursday had caused a severe case of cellulitis. I'm on oral antibiotics now and hoping to avoid a trip to the hospital for IV antibiotics and to clean out a "septic" joint. When I can, I will comment in greater depth with some suggestions. Right now, extended keyboard typing is a bit much for me.

If you could post a fresh photo or two -- side view and looking down on the stem/handlebars -- of your bike as it is now setup, that would help enormously. I"m sure others will weigh in with useful suggestions of their own.

In the meantime, are you using a long-layback seatpost (a seatpost with the head/clamp assembly offset to the rear)? Alternatively, are you able to trial-fit some wider/fatter/taller tires to see what difference they make to the feel for you?

All the best,

Dan.

pondweed:
Thanks for the reply and hope you are getting better.
I must admit I haven't changed tyres yet, more because I thought I could change another set of wheels to suit, and baulked at thought of which other cassette to use to make it easy.
The fork is the standard ?43.
Helpful Thorn Sales have suggested the layback converter unit, but I wondered whether it might put unnecessary stresses on... and I thought there might be an integral seatpost that could integrate those features if I really need it.
I've put the handlebars up as far as possible on the steerer, and the bars cante'd round as far as possible... with some likely weight transference frontwards, I suppose.

geocycle:
I'm not quite sure what to suggest here. You are probably at the upper limit for the frame size. I'm a touch shorter and sometimes wonder if I should have gone up a size. You also have a skinny front tyre for a Sherpa which wouldn't help. The stem already looks quite long. The bars look quite wide which amplifies small movements especially if you are used to narrower set ups.

mickeg:
I have felt no handling problems downhill with either of my Thorns (Sherpa 610S and Nomad 590M).  But mine are larger sizes, so not sure how that changes the geometry.

All I can really say is that I wish you luck with your calculations.  I was trying to compare the geometry of several of my bikes a few years ago, I asked Thorn for some of the data like headtube angle, fork rake, etc., for my two Thorns.  Their response was as if I was asking for them to divulge state secrets or something like that.  I found it most unreasonable that almost everyone else freely publishes that data on the bike frames they sell but Thorn was insistent that it was closely guarded trade secrets.  They told me to just use a protractor and look at it, which I felt was quite absurd given how precise those measurements have to be when calculating trail.

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