Long Distance Competition/Ultracycling, Randonneuring and Endurance Cycling - UO-8 front end geometry - too little trail?

Back to my UO-8 project:

I measured the front end geometry, but it's just approximate so far. It's clearly very low trail, but I wonder if the trail is too low. The head angle is between 72.5 degrees and 74 degrees, and the rake is 7.0 cm. I'm pretty sure of the rake since I measured it by fixturing the bare fork level on a level table and measuring distances to the table surface. The elevation of the steer axis is the average of the elevations of the top and bottom surfaces of the steer tube, and the elevation of the dropout center can be measured pretty well if a hub is clamped in. The offset is the difference between the steel axis elevation and the dropout center elevation. I think this is a pretty solid methodology, at least good to half a millimeter.

I still need to pull the headset cups and clean the frame in order to get a good head tube measuring surface for my digital carpenter's level, but several cruder measurements span the range above.

So with 700c x 28 tires the trail will range from 23 mm to 32 mm, and 25 mm to 35 mm with 27 x 1 1/4. This is only geometric trail, doesn't consider pneumatic.

This seems like it will be good with a front bag, but is it really too little trail? These numbers seem really low.

Back to my UO-8 project:

I measured the front end geometry, but it's just approximate so far. It's clearly very low trail, but I wonder if the trail is too low. The head angle is between 72.5 degrees and 74 degrees, and the rake is 7.0 cm. I'm pretty sure of the rake since I measured it by fixturing the bare fork level on a level table and measuring distances to the table surface. The elevation of the steer axis is the average of the elevations of the top and bottom surfaces of the steer tube, and the elevation of the dropout center can be measured pretty well if a hub is clamped in. The offset is the difference between the steel axis elevation and the dropout center elevation. I think this is a pretty solid methodology, at least good to half a millimeter.

I still need to pull the headset cups and clean the frame in order to get a good head tube measuring surface for my digital carpenter's level, but several cruder measurements span the range above.

So with 700c x 28 tires the trail will range from 23 mm to 32 mm, and 25 mm to 35 mm with 27 x 1 1/4. This is only geometric trail, doesn't consider pneumatic.

This seems like it will be good with a front bag, but is it really too little trail? These numbers seem really low.

I'd be surprised if the trail is even as low as the upper end of the range that you quote. Take a look at the bicycle geometry spreadsheet at:

http://home.comcast.net/~pinnah/dirtbag-bikes/geometry-project.html

There is a Peugot PR10 listed as having trail of 33.2, but looking at photos of the bike (e.g. http://ibikedb.net/bikes/3444-peugeot-pr10) I think it's pretty clear that the one in the spreadsheet it has been mismeasured (it doesn't have a 76 degree head angle, for instance).

There are a lot of bikes with trail starting as low as 38. Hardly any (three) with trail below that.

Maybe try re-measuring. Your head tube angle covers a wide range. Also ... are you measuring the rake perpendicular to the head tube angle (and the main line of the fork before it bends)?

I'd be surprised if the trail is even as low as the upper end of the range that you quote. Take a look at the bicycle geometry spreadsheet at:

http://home.comcast.net/~pinnah/dirtbag-bikes/geometry-project.html

There is a Peugot PR10 listed as having trail of 33.2, but looking at photos of the bike (e.g. http://ibikedb.net/bikes/3444-peugeot-pr10) I think it's pretty clear that the one in the spreadsheet it has been mismeasured (it doesn't have a 76 degree head angle, for instance).

There are a lot of bikes with trail starting as low as 38. Hardly any (three) with trail below that.

Maybe try re-measuring. Your head tube angle covers a wide range. Also ... are you measuring the rake perpendicular to the head tube angle (and the main line of the fork before it bends)?

No, no, UO-8s and teh P family bikes (PX-10, PR-10, et cetera) are very different, and Peugeot made their geometries less laid back starting in the late '70s. I have a PX-10 from 1968 or so, and it is more aggressive than this bike. Plus the Geometry sheet, as big as it is, is not exhaustive, it's the result of Dave looking at as many websites as he could and getting a lot of info contributed by cyclists. Most bikes on there, perhaps all, are rather newer than 1972. Dave's original intent was to capture the essence of the '80s sport-tour genre.

My wide range was based on measuring head angle. I'm very confident in the rake I measured. I did it with the fork removed and leveled on a level table, measuring distances from the fork down to the table, with a Starrett dial caliper. The fixturing was very stable.

But, I'm nearly done assembling teh bike with the config I'll try it in, and when that's settled I'll do a better head tube measurement. Another dynamic factor was I hadn't settled on 700c v. 27 inch. It is going to be 700c. I should have better data soon, perhaps tonight.

I am sure that roadfan's rake measurements are correct as I have seen the frame/fork in question and know Ken's profession. The only thing I have to ad is Kogswell's P/R offers three fork rakes with trail of 50, 40, and 30mm. The 30 is the "porteur" for heavy front loads. The 40 is light to moderate loads and 50 no loads. I think even the lowest trail would still perform just fine on normal non-loaded.

Bicycle Quarterly reviewed one with each fork and that review is http://www.kogswell.com/KogswellPR.pdf (scroll down)

Ok, I have the bike finished, and I fixtured it on the cement floor and have a good set of geometry measurements:

Seat tube 55.5 c-c
Top tube 57 cm
chainstay 45.6 cm
wheelbase 106.2 cm
standover 80.7 cm
front center 61.6 cm
seat lug setback 16.9 cm
seat tube angle 72.3 deg
head tube angle 72.8 deg
BB height 26.8 cm
tire width 28 mm
wheel radius 33.7 cm (700c x 28 with 15% squish)
BB drop 7.3 cm
head tube length 13.0 cm
fork offset 7.0 cm
trail 31 mm (assuming the 15% squish)
flop factor (don't have the equation for that!)

So this trail is about the same as the Porteur fork from Kogswell. I plan to see how well it works with a front bag and without. I want a bike to work well both ways, which of course is contradictory as far as I can tell. The head tube angle is real close to Jan Heine's ideal of 73 degrees, but there is a lot more rake than he likes, 70mm versus his goal of 55 mm. Obviously I'm not including pneumatic trail.

The build is Shimano 600 6207 derailleurs, crank and spindle, and hubs, 700c MA-40 rims with Conti Gators 28 mm, Sachs stem shifters, 39 cm aluminum SR handlebars, a faux Swallow saddle, SunTour calipers (the Mafacs did not come with the bike), mostly aluminum MKS quill pedals w. toeclips, original headset, SR or Sakae stem cut down, and the original steel seat tube.

[QUOTE=Road Fan;10049757]Ok, I have the bike finished, and I fixtured it on the cement floor and have a good set of geometry measurements:

.../QUOTE]

Sounds like it should be a fun bike to ride and should work well with a handlebar-bag-based randonneuring setup.

most of those bikes never saw a front bag, and some fairly inexperienced people put plenty of miles on them. I would think you wouldn't have any problems unless you are much pickier than you seem to be. Report back on the ride when you get it built.

most of those bikes never saw a front bag, and some fairly inexperienced people put plenty of miles on them. I would think you wouldn't have any problems unless you are much pickier than you seem to be. Report back on the ride when you get it built.

You mean most of the UO-8s? Yes, I don't seem to be picky, but I have found some niggling problems over the years. I am very picky about fitting and tuning.

I gave a preliminary ride report in the "UO-8 for long rides?" thread.