SF Cyclotouring

Ride reports and other ramblings from a San Francisco cyclist.

6/02/2008

More Low-Trail Experimenting...


Kogswell Test Rig
Originally uploaded by jimgskoop
To the dismay of my other bikes, lately I've been mostly riding my Kogswell 700x P/R. The adjacent photo shows how I've been outfitting the bike for the various test-rides I'm doing. I recently lowered the saddle 1cm (but then raised it ~5mm during the last ride), moved the saddle forward ~2cm, switched to a longer 120mm stem, and also tried 700x30 tires inflated to 75psi. The first set of tweaks are all aimed at dialing in my position on the bike; the tire swap was done to learn its effect on low-trail handling characteristics -- I had 700x35mm tires @ 55psi mounted previously, but I was advised to try 700x30mm tires @ 75psi since I wasn't feeling the low-trail love. I can say that the narrower tires do seem to reinforce the expected characteristics somewhat, although I definitely prefer the ride of the larger tires, especially during fast twisty descents.

After swapping the tires, I took the bike for a ride late Saturday afternoon, in cool and windy conditions. I rode across the Golden Gate Bridge and down into Sausalito, picking up the Mill Valley Bike Path as usual. The path is usually beseiged by winds in the afternoon, possibly due to the shifting tide and/or the fog rolling in -- one of those "Sheesh there's a headwind in both directions" occurrences that seem common in the SF Bay Area. Saturday was no exception, and as I pedaled along, I noticed that the bike felt "interesting" in that, as I got nudged left and right by the blustery breezes, the front end of the bike seemed to exibit a self-centering effect. It felt uncannily like the invert-a-broom-and-balance-it-on-the-palm-of-your-hand trick -- aka the upside-down pendulum. Pretty neat!

I left the bike path and climbed Camino Alto, one of my favorite hills. Due to the late hour, I turned around at the top and dropped back down towards Sausalito instead of continuing on towards Tiburon or Fairfax. The last time I rode down this hill on the Kogswell, the bike wore the 700x35mm Panaracer Pasela tires (at 55psi), and descending on that bike with those tires put a big grin on my face! For the first time ever, I felt hugely confident in the fast curves and rarely had to grab the brakes. Today was different, though, and the bike felt quite unstable. So much so, that as I dove into one curve, I though I'd overshot the lean and was about to go down, but in an instant I stopped panicking as I realized that no, the front wheel wasn't slipping out from underneath me, and the bike completed the turn and I emerged unscathed, but with a racing pulse!

Returning to Sausalito and back on the Mill Valley Bike Path, I remembered the many discussions on load distribution and how that may affect low-trail handling, so I stopped to shift some weight from my seat pack to my Rivendell Candy Bar handlebar bag. As I continued my ride, I noted that the front end indeed felt heavier, but I couldn't say whether the handling grew better or worse with this change (and that in itself may be a positive result!). Note that I've never done this test on any other bike, so the results here are inconclusive.

Later, I rode down to Fort Baker to check on the new resort hotel that's opened there. As I rode past, several times I looked back over my shoulder to check out the grounds and the rennovated buildings -- and, certain that the bike had wobbled and lost its line, I turned my head forwards again to find the bike pointed exactly where I'd left it. Pretty cool!

I left Fort Baker and climbed up towards the Golden Gate Bridge. By now, it was pretty late in the day and the wind blowing over the Golden Gate was very strong. As this air blows over the bridge, a curious swirling pattern is generated as the breeze is split by the bridge's two support towers -- and riding around them at this time is always challenging. I'm hoping that the Low-Trail Effect (henceforth: LTE) and its reported resistance to cross-winds will come into play here, but so far I've been disappointed. In fact, on this ride I actually got knocked off my bike by the blasting wind and had to stop, put a foot down, and then dismount and walk my bike around the pylon. Whether I'd have fared any better on another bike, I don't know...

So, conclusion-time: I think the skinnier tires amplified the LTE somewhat. Presumably the added pneumatic trail from the 35mm tires was cancelling the LTE, so the narrower/harder 30mm tires minimized that issue. In spite of this, I will definitely be going back to the 35mm tires -- they just feel so much better, especially on twisty downhill roads! I also definitely want to test the bike's other fork -- currently it's built with the 58mm offset fork (expected 42mm of trail), but I've also got a 67mm offset fork (expected 32mm of trail) -- that should give me a better dose of LTE with the 35mm tires. I need to ride one of my other (high-trail) bikes and try the heavy bag up front to see what effects that'll impart. Finally, on an unrelated note, I want to lower my handlebars 1-2cm since I think they're still too high, most noticeable during out-of-saddle climbing. I may also want to try a set of narrower Nitto B115s; I've a feeling those would be the perfect 'bar for this bike.


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2 Comments:

Blogger Fritz said...

I very much prefer hard tires over squishy on fast, windy descents. To me, larger low pressure tires seem to squirm. I have no idea what impact tire width has on trail, though.

The broom analogy is a good one.

2:20 PM  
Blogger Tarik Saleh said...

Jimg,

Try putting 15 pounds over the front wheel... I am still experimenting with mine, but it seems it rides pretty great with weight on the front. It looks like you are riding with neutral weighting to me. In that case I think a long low type bike miught do what you want better...

I get some weird handling effects on mine, but I suspect it is due to having the big squishy tire up front. I swear I am going to put 35mm paseallas on mine soon...

5:31 AM  

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