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-   -   New forks: advice on rake (http://www.bikeforums.net/fitting-your-bike/925787-new-forks-advice-rake.html)

cavasta 12-11-13 01:44 PM

New forks: advice on rake
 
I'm running a Trek 2.3 (2010 model) and need to replace the damaged carbon fiber forks. The bike is size 56cm, has a 73.5 degree headtube, forks with 40mm rake (offset), 58mm trail and 983mm wheelbase (all data from the Trek catalogue). It's a stable and self-assured ride though perhaps it's a little too stable sometimes. I've seen a lovely pair of Ritchey forks but the the rake is 45mm. I appreciate how this will change the way the bike rides and feels via its impact on the trail (longer rake = shorter trail = livelier handling). My question is will the leap from 40mm rake to 45mm rake provide just a bit too much liveliness? Would I be better going for 43mm or maybe even sticking with safe 40mm?

All the dimensions of all the 56cm Trek bikes in the Madones, 2 series and 1 series (in the 2010 range), are the same (as mine, above). The dimensions change according to the size of the bike frame (for example, 54cm have 73 degree headtube, 45mm rake, 56mm trail and 981mm wheelbase). I guess there's a pretty good reason why they choose these configurations?

I've found this tool quite useful in providing me with the data that a different rake will produce: http://yojimg.net/bike/web_tools/trailcalc.php

So, 45mm rake or 43mm? Any thoughts and advice will be greatly appreciated.

fietsbob 12-11-13 03:42 PM

Trail is the dimension on the ground, all else remaining the same , more rake is less trail.

NB: Fork tip to crown race seat difference changes things head tube angle,
, and tire width.
Jim only ads tire width change input.

Wheel diameter effects trail.. the intersection of the 2 lines is crossing above the ground .

a Change of just 2mm is likely not perceptible .. [depending on your obsession level]


OK I know you are talking Race bikes , but were the fork hauling a Load,
then the heavy weight stabilizes steering, then shorter trail is good.

My Folding bike has a 35mm trail.. load the cargo bag on and it's all Good.

unterhausen 12-12-13 12:01 AM

45 should be fine. I didn't see mention of the Axle-Crown (A-C) measurement, which is pretty important. You want to match that pretty well

cavasta 12-12-13 12:58 PM

Thanks for the replies. More information:

The Bontrager Race carbon forks that are currently fitted have an axle to crown measurement of 371mm; the Ritcheys are 368mm.
I'm running 700c x 23mm tyres.

I've no idea how any of this fits into the equation, especially the a-c measurement. Can anyone help me on that, particularly in relation to my bike's spec? Is 3mm within an acceptable tolerance?

fietsbob 12-12-13 04:54 PM

I have no idea about what is acceptable to You. It's your Bike.

HTA will be a bit steeper ( to calculate, think an arc of a radius with it's axis of the back axle)

:50: I'm Just an Old Tourist.

Road Fan 12-13-13 02:35 PM

Doing a fork "upgrade" as you renew it is all well and good, but I'd check for the price of a Trek fork to replace your old one. It should have all the same geometry, it other words, no decisions or worries. If you liked the way the Trek handled, you'll still like this one.

Consider price in your comparison. I would not assume the Trek fork is not available and affordable.

Regarding tolerance, these are essentially factory made. Reasonable efforts are made to make them all the same. If a few mm tolerance is acceptable in fork blade length, we'd see front wheels routinely cocked in the bike.

But: I have a steel Trek road bike of the classic days that came with a 52 mm fork rake. It resulted in about 52 mm trail. I did not like the handling, too darty when climbing up slowly - kinda scary! I had the bike de-rakedto get a trail around 59 mm and it felt a lot more consistent when climbing. Not as good as my nice Italian Mondonico, but better. I've since had a high-rake (low-trail) fork made for the same bike, and now with about 35 mm trail I like it a lot. But that fork is a custom job.

A few mm can make a difference.

cavasta 12-14-13 01:10 PM

Thanks for that. Good advice.
When I wrote about tolerance I was wondering if the 3mm difference between the Bontys and the Ritcheys would be enough to affect the bike's handling or if it wouldn't be an issue.

fietsbob 12-14-13 01:27 PM

Yes you did, but I cannot judge your perceptions from a Keyboard.

if you want the handling and steering geometry to remain the same , get all the numbers to match.


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