Author Topic: Sherpa 'Trail' and tyres; feeling unstable downhill  (Read 212 times)

pondweed

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Sherpa 'Trail' and tyres; feeling unstable downhill
« on: September 15, 2017, 10:14:15 PM »
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

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Re: Sherpa 'Trail' and tyres; feeling unstable downhill
« Reply #1 on: September 15, 2017, 10:49:20 PM »
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

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Re: Sherpa 'Trail' and tyres; feeling unstable downhill
« Reply #2 on: September 16, 2017, 08:02:04 AM »
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.
« Last Edit: September 16, 2017, 08:19:55 AM by pondweed »

geocycle

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Re: Sherpa 'Trail' and tyres; feeling unstable downhill
« Reply #3 on: September 16, 2017, 11:28:31 AM »
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

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Re: Sherpa 'Trail' and tyres; feeling unstable downhill
« Reply #4 on: September 16, 2017, 12:55:12 PM »
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.

pondweed

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Re: Sherpa 'Trail' and tyres; feeling unstable downhill
« Reply #5 on: September 16, 2017, 04:47:03 PM »
Thanks all

yes, I was going to get out my child's protractor! I notice Dan has worked out his from his other post but wrong frame for me  :)

Sounds like I should get some new tyres first.

Be interested in widths of peoples' bars on the Sherpa. Don't think I have much option on these ones as the curve means I can't shunt things along.

StuntPilot

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Re: Sherpa 'Trail' and tyres; feeling unstable downhill
« Reply #6 on: September 16, 2017, 08:51:07 PM »
My Thorn Raven Tour also feels a bit twitchy downhill on rough terrain, but only when unloaded. Fully loaded with front and rear panniers its a whole new world of stability.

Might be worth experimenting with racks and bags on the bike to see the difference if indeed fully loaded touring is the aim. If for use without panniers, I agree that wider tyres are a good place to start.

The stem seems quite long as has been mentioned. A shorter stem may impart less leverage on the fork and its steering and greater stability. However you say that the 'straights seem short in body'. A short frame with straight or comfort bars took a little getting used to after drop (road) bike handlebars.

After a couple of long tours though and a lot of fiddling with seat position and Ergon grip position I have ended up with a pretty comfortable position. I aimed for equal weight on saddle, handlebars and pedals.

What about shoulder width in relation to your handlebar width? Perhaps swapping out to wider straight bars if you have wide shoulders?

I would leave the protractor with the dividers in their box for the moment and ride on for a while  ;)!
« Last Edit: September 16, 2017, 08:55:56 PM by StuntPilot »

pondweed

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Re: Sherpa 'Trail' and tyres; feeling unstable downhill
« Reply #7 on: September 16, 2017, 09:34:31 PM »
Thanks. Ok, so yes, I'll get panniers sorted and put my front bag on.. with some 'stuff' in..

I was surprised how well 1.25 Marathons did on the tow path out from Bristol to Portishead, so was wondering whether a larger size of those, or to go for the lighter Marathon Supreme. Dureme seem too heavy if I want to use it for everything?

Tyres... shall I go for 1.6 (not big enough rise?) or 2.0? Inevitably I'm going to be on road a lot but want to be able to do firetrail and mtb-lite stuff.

mickeg

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Re: Sherpa 'Trail' and tyres; feeling unstable downhill
« Reply #8 on: September 16, 2017, 11:37:57 PM »
Tires on my Sherpa for touring with camping gear, if mostly on gravel I use Dureme on front in 2.0 width (50mm).  But if mostly on pavement (tarmac) I use Marathon (with Greenguard) in 1.5 (40mm) width. 

But if riding unladen on gravel trails comparable to tow paths, the 40mm wide Marathons are adequate, I do not bother to switch to the wider Duremes.

The Duremes I use are the old discontinued ones for solo bike use.  The Duremes that are sold for tandem use are a much stiffer tire. 

I accidentally bought a tandem Dureme, I bought it mail order and I thought I was buying a solo bike tires.  I occasionally put it on my Nomad, but it is stiffer than I like.

martinf

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Re: Sherpa 'Trail' and tyres; feeling unstable downhill
« Reply #9 on: September 17, 2017, 08:50:53 AM »
Marathon Supreme are quite light, even in the fat 50 x 559 size. I have these on my two Raven Tour bikes. In my experience they cope OK with moderate off-road use on paths and tracks, and are very good on tarmac roads.

Marathon Supreme also last a reasonably long time, not yet worn out any, and still plenty of tread on the longest-serving pair with more than 9,000 kms on the front and 5,200 kms on the rear.

Downsides are the high price, and I reckon the light sidewalls will be more likely to suffer damage if you hit broken glass or sharp rocks.

Duremes (which I also have, now on a visitor bike) are slightly heavier, slightly slower, and seem sturdier, so maybe a better choice for expedition touring.

I recently changed the tyres on my old 650B utility bike, so have been able to do a direct comparison. I had been using 44 x 584 Marathon Standard for several years, these are a good sturdy tire but a bit on the heavy side. First of all I used 42 x 584 Marathon Supremes, these were significantly quicker, then I fitted 50 x 584 Marathon Supremes, not quite as quick as the 42 x 584 bit still quicker  than the 44 x 584 Marathon Standard. When riding off-road, the 50 x 584 Marathon Supremes are much better than either 42 x 584 Marathon Supremes or 44 x 584 Marathon Standard, and are also more comfortable.

The 42 x 584 Marathon Supremes went to my wife's bike, where she found them a significant improvement over the 44 x 584 Marathon Standard she had before. Clearances on her bike are insufficient for the 50 x 584 tyres.

pondweed

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Re: Sherpa 'Trail' and tyres; feeling unstable downhill
« Reply #10 on: September 20, 2017, 10:15:01 AM »
Great info thanks!
going to get the 1.6 Supremes and will flog my 1.25 Marathons. (Would have already had I not got to the checkout and decided an extra 5 quid postage for SJS demanded a look round the other suppliers :)