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Old 12-31-05, 06:28 PM   #1
Encinotrckrcr
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Fork Rake for track

I just broke my Corima Track fork. The fork is insanely expensive to replace! About $900 US , so I am looking for a new track fork. I would like a carbon fork with 1 inch unthreaded steel steerer.
What fork rake should I use for track???????????
I did some research. Now track bikes are supposed to be very quick handling bikes,so, that would probably mean a track fork should have more rake. More rake equals a faster handling bike according to Velonews's Zinn. Of course other things account for a track bikes faster handling aslo, but all other things being equal a fork with more rake is supposed to be faster. Reynolds track fork has 40 mm of rake which is less than road bike "standard" of 43. Why? Also I heard the Reynolds Track Fork is just a road fork without the hole drilled for a brake!
Then I was looking at some track bikes geometry. Look's Ten grand frame and fork has a head angle of only 72 degrees! and a Fork rake of 68! That is a head angle of a Touring bike! Why?
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Old 12-31-05, 07:50 PM   #2
Mike T.
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I'm going to sit back and watch the answers roll in on this one. I've been thinking and talking about track frames, their angles, rakes and trails with a few framebuilders and I've been reading as much as I can. My consensus it that there is NO consensus as to what is the "ideal" setup for track.

I've found head angles from 73 to 75 (my present '70s Colnago is 77), rakes from 34 to 55mm (mine is 30) and trails from 49 to 59mm (mine is 43).

Oh of course tracks' bankings range from the UK's Herne Hill to our track's 50 degrees but there are a heck of a lot of ready made frames out there which give no options. Even the high-$ Aussie BT has "we know what's best for you" geometry.

The custom builders that I'm talking to can't agree what's best for "my" track. I'm scared to part with my money.

At least with road bikes we're generically safe with a 73-74 degree head angle and a 43mm raked carbon fork but I wanna know what's best for (a steep) track AND WHY!
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Old 12-31-05, 10:45 PM   #3
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See dimension F in the diagram below? That is the first dimension you need to take off your old fork. Then you need to find out the fork rake on your old fork from Corima. Then you need to match those two dimensions, exactly if possible.
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Old 12-31-05, 11:07 PM   #4
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Wow - 72 deg, 68mm = trail 41

That's pretty low trail compared to 'normal' road bike trail - 73/45 = trail 59

I don't know what to tell you. I'm wondering myself.

In his framebuilding book, Talbot gives equations and shows a chart of rake for 27" wheels that shows "neutral steering" and "quick steering" with a "touring" value halfway between...where the smaller rake value (longer trail) is for the "quick steering" and the longer rakes (less trail) progressively go through "touring" to "neutral". In addition, he shows that from his "neutral" dimension, decreasing rake (increasing trail) creates progressively more oversteer and increasing rake (decreasing trail) results in more understeer. In the text he defines "neutral steering" as "neutral steering means the fork will neither rise nor fall as it is turned".

I'm making a new 'track' bike and I intend to offer three fork offsets: 35, 45 and 55 with a 75 degree head.

I figure folks can use what's best for their event/need.




Not that it means a lot, but we have a trail calculator here:

http://kogswell.com/trail.php

You'll need flash loaded to see the graph.
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Old 12-31-05, 11:13 PM   #5
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Actually fork rake works the opposite that you think it does. Less rake = faster steering response and more rake = less although there are many more factors in the equation than just fork rake. The overal figure that's important is called trail which for a road bike should fall between 50mm (fast) to 70mm (slow) and trail is a combination of head tube angle, rake and wheel size.

See this article for a good run down on bike geometry. Its talking about road bikes yet all the principles are the same. http://www.anvilbikes.com/story.php?catID=3

Simplisticaly if you want fast handling for a track bike you want a fork with less rake than a road fork so a fork with 40mm rake sounds right although it would be better to know what your head tube angle is and then calculate the resultant trail using a program available on the above link.

Regards, Anthony

EDIT: OK that didn't make sense did it because I made a boo boo. MORE rake would make the steering faster not less and that's my point but I went and contradicted myself. I beleive that track bikes have short rake forks because there designed for STEEP head tube angles and HTA and rake need to be considered in tandem which is kind of what I was trying to say.

Last edited by AnthonyG; 01-01-06 at 04:14 AM.
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Old 01-01-06, 12:21 AM   #6
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Quote:
Originally Posted by AnthonyG
Actually fork rake works the opposite that you think it does. Less rake = faster steering response and more rake = less although there are many more factors in the equation than just fork rake. The overal figure that's important is called trail which for a road bike should fall between 50mm (fast) to 70mm (slow) and trail is a combination of head tube angle, rake and wheel size.

See this article for a good run down on bike geometry. Its talking about road bikes yet all the principles are the same. http://www.anvilbikes.com/story.php?catID=3

Simplisticaly if you want fast handling for a track bike you want a fork with less rake than a road fork so a fork with 40mm rake sounds right although it would be better to know what your head tube angle is and then calculate the resultant trail using a program available on the above link.

Regards, Anthony
I've been testing three forks with varying offset for new model that we're developing.

The one thing I noticed about added fork offset (less trail) is that the bike has a lot less oversteer. I can change line in a corner with a lot less effort than I could using short-offset forks.

When I asked about this here on the BF I got almost no response.

I'm sure there's a reason why track racer use sub-40mm offsets. But I like to question conventional wisdom from time to time to see if it still works.

Seems like having a bike that doesn't fight you in the corners might be useful in some track events. Hence my decision to offer more than one offset.
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Old 01-01-06, 01:16 AM   #7
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Kogs,

If a track is designed on any mathematical model (rather than just "pile a couple berms and pave between 'em"), as most good tracks are, a target speed is picked, which determines the angle and curvature of the bankings. A bike will naturally track on the pole at that speed with essentially no steering required. At that speed, it doesn't matter what your front-end geometry looks like -- you'll have no steering problems on the pole line. This only applies to riding on the pole (or, on a really well-designed track, parallel to the pole at any point on the track); if you are riding anything other than a parallel line, you'll be back dependent on front end design.

Now, tracks have speeds picked for peculiar reasons. Trexlertown, for example, was intended as a community facility with a strong education and youth focus, so it was designed for a slower target speed (slower riders being the intended subject for design criteria). Consequently, a fast Cat I rider will have trouble staying on the pole because he'll be riding faster than the target speed. (This is why Trexlertown doesn't generate the kinds of times for the best riders that other tracks do.) This is where front-end geometry becomes important. If I ride at a faster-than-target speed on that track, I'll need a marginally more responsive front end so I'm not fighting the track; if I go too responsive, I'll tend to dip down. If it's a track with a very slow designed speed, I might need even more front-end responsiveness on the bike, although that would compromise my riding on some other tracks. Consequently, a capable track rider benefits from a front end designed to the specific geometry of the target track itself. A bike like the Look KG496 Athene is designed around Schuermann track parameters and current world-cup speeds (its front end was actually redesigned slightly from the KG396 because speeds had increased so significantly from when the 396 was designed -- that's the major contribution gained from the new model). And since most Olympics, World Championship, or World Cup events are scheduled onto tracks designed by Schuermann or following his principles (and designed for high speeds), one bike will work exceptionally well for all these tracks. A KG496 is not a great bike on a tight Belgian six-day track, which is part of why the European six-days are ridden mostly on aluminum frames which can be designed around the peculiar geometries of those tracks.

Practically speaking, a fast competitor can go to a slightly lower offset and take advantage of it when he/she is going faster than the track's target speed (which is the point of racing anyway), plus it'll generally help with maneuvering on the track. On this latter point, just watch a sprinter in trailing position going about 22-24 mph at the top of the track and then launching a drop down to the pole and an acceleration up to 35+ mph, all within the distance of the last banking. The centrifugal forces acting on that rider, and the very extreme abnormal weight distribution in those maneuvers make it very hard to steer that bike -- it'll feel like the headset is welded together -- so a very short offset and twitchy front end won't feel that way when you're actually racing. It's interesting watching new track riders on such bikes -- they are difficult to handle at low speed, but when maneuvering at speed they feel just like your typical road bike on a country road.

Last edited by 11.4; 01-02-06 at 02:50 AM.
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Old 01-01-06, 04:25 AM   #8
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Quote:
Originally Posted by Kogswell
I've been testing three forks with varying offset for new model that we're developing.

The one thing I noticed about added fork offset (less trail) is that the bike has a lot less oversteer. I can change line in a corner with a lot less effort than I could using short-offset forks.

When I asked about this here on the BF I got almost no response.

I'm sure there's a reason why track racer use sub-40mm offsets. But I like to question conventional wisdom from time to time to see if it still works.

Seems like having a bike that doesn't fight you in the corners might be useful in some track events. Hence my decision to offer more than one offset.
Yeah I stuffed up. See my corection above. I think that track bikes have steep HTA's for whatever reason so shorter rake forks match them steeper HTA.

Regards, Anthony
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Old 01-01-06, 06:45 AM   #9
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OK

Last edited by Kogswell; 01-02-06 at 10:36 AM.
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Old 01-01-06, 07:02 AM   #10
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Wow.

Thanks, 11.4. I appreciate the time it took to craft that answer.

Interesting about T-Town. I rode on the Hellyer Park track and it always felt a little less serious.

So I've been designing a frameset that is intended, among other things, to be used as a low-cost, serious bicycle for development work at a track. And entry point for folks who want to take up track racing or at least track riding. I know that there is no such thing as a universal track bike. But I'd like to make a trainer that would be as smart as a trainer can be.

I've been collecting input here and I'd like to hear what you have to say.

And what does 11.4 mean? You're best time on a flying 200?
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Old 01-02-06, 02:59 AM   #11
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Kogs,

That 11.4 is just my best time in a match sprint. I'd need to do better than that in a flying time.

Good luck with your frame. There's an opportunity for a real track frame, what with all the road fixie designs out there today. I don't think you need to plan optional trail designs -- just pick one since new riders won't know what to pick anyway, and the rest of the bike (and rider weight distribution, position, and maneuvering style) will affect steering needs anyway. Target a fairly short (but not radically short) trail such as you've described above and you'll have a fair bike for most tracks. A 6 cm bottom bracket drop will be sufficient for most US tracks and will help moderate any handling issues (a typical keirin frame still runs about 5.5 cm drop, and I've seen small-sized high-end carbon frames running at 5 cm).
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Old 01-02-06, 10:37 AM   #12
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Originally Posted by 11.4
Kogs,

That 11.4 is just my best time in a match sprint. I'd need to do better than that in a flying time.

Good luck with your frame. There's an opportunity for a real track frame, what with all the road fixie designs out there today. I don't think you need to plan optional trail designs -- just pick one since new riders won't know what to pick anyway, and the rest of the bike (and rider weight distribution, position, and maneuvering style) will affect steering needs anyway. Target a fairly short (but not radically short) trail such as you've described above and you'll have a fair bike for most tracks. A 6 cm bottom bracket drop will be sufficient for most US tracks and will help moderate any handling issues (a typical keirin frame still runs about 5.5 cm drop, and I've seen small-sized high-end carbon frames running at 5 cm).
Super.

Thanks, 11.4!
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