Effect of incr. Axle-Crown height & Changing rake ?

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Effect of incr. Axle-Crown height & Changing rake ?

Postby MichaelB » Fri Jul 02, 2010 3:46 pm

Given the thread from Cooperplace fitting a BB7 on Road bike (frame is custom XACD Ti frame) uses a fork that was built for the bike, and with my interest in achieving the same end for my "Winter Bike", I have some queries re the end effect of fitting either a 26" MTB fork or CX Fork that is capable of accepting a disc.

e.g. WoundUp Team X Disc, Trigon MTB Disc fork and Kinesis DC-19

Here is a pic of the typical issue (Kinesis DC-19 fork shown)

Image


The key differences seem to be the following ;
1. The MTB or CX forks have a longer Axle to Crown distance of around 410mm (I think the typical 700C road bike is between 365 and 374mm)
2. Rake is either 43 or 47mm (Typical road bike including mine is 45mm)

Sheldon's take is interesting - Fork Lengths by D Rinard, but this seems to deal more with minor changes, rather than ones that may bee seen with a CX/MTB style fork.

I guess the big question is, is there a ROAD DISC capable fork that isn't custom made ?


Here is a pic from a converted Planet X carbon frame on
http://www.alcester-racing-sevens.com/p ... ersion.htm .

They make no mention of any handling issues, so it is a matter of suck it an see ???
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by BNA » Fri Jul 02, 2010 4:32 pm

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Re: Effect of incr. Axle-Crown height & Changing rake ?

Postby zero » Fri Jul 02, 2010 4:32 pm

Plenty of road forks are 43 degree and the kinesis crosslight disc fork, which is cheap from crc is a 45 degree option anyway.

I guess you'll have to either search extensively, or select one of 43/400 (dc19) or 45/410 (crosslight), as the best replacement for your fork. Looks like your head angle would change by about 1.5 - 2 degrees, which would still likely see it at least 1 degree steeper than my MTB, which after all has to turn on single track, which is much tighter than anything I do on the road. I imagine you would be able to feel the difference though.
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Re: Effect of incr. Axle-Crown height & Changing rake ?

Postby drubie » Fri Jul 02, 2010 9:43 pm

I get the impression that it's not so much the rake of the fork, but the overall trail that makes the difference (i.e. distance between the centre of the axle and the centre line of a straight line taken down through the steerer when projected to the ground).

http://davesbikeblog.blogspot.com/2007/05/trail-fork-rake-and-little-bit-of.html

...so sit down with a pencil and paper and have a guess / get as close as you can get.

In practice - as long as the front wheel doesn't strike your frame (or your feet) just ride it!
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Re: Effect of incr. Axle-Crown height & Changing rake ?

Postby Nobody » Fri Jul 02, 2010 10:09 pm

I've read the change is approx a degree per inch difference, so a 395mm fork would be a bit over 1 degree. Reading this, I now understand why my answer here won't really apply to you. For now, like drubie said, just get something from what is available then worry about getting a closer match later when a better selection of road forks are available. The UCI have already changed their mind allowing disc brakes for CX. Who knows, maybe they'll allow disc brakes on road bikes in the future.
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Re: Effect of incr. Axle-Crown height & Changing rake ?

Postby Jean » Sat Jul 03, 2010 8:15 am

You could have a play around on BikeCAD (which is a free applet) by inputting your frame's geometry and that of the forks you are looking at. That will just give you some numbers which might give you an idea, with some research, about how handling might be affected.

In reality though I expect the the difference between the forks will be minimal. All those forks are designed to go on standard road bikes and whether you have a 43 or 45 degree rake and few mm change in length won't matter much in the real world. You might feel the change, but nothing will go mad.
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Re: Effect of incr. Axle-Crown height & Changing rake ?

Postby MichaelB » Sat Jul 03, 2010 10:16 am

Hi guys, will have a play with the options available and measure the trail and rake that I have, and then see what the end resulting head tube angle, rake and trail is to see what the end difference is.

Thanks for the input Nobody, will have a closer look at the options you suggested

Keep it coming
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Re: Effect of incr. Axle-Crown height & Changing rake ?

Postby MichaelB » Mon Jul 05, 2010 9:14 am

Latest update :

Measurements from bike :
Wheelbase : 1,000mm
Axle-Crown height : 370mm
Fork Rake : 45mm (need to double check)
Head Tube Angle : 73.0 degrees

Have contacted Wound Up, and they responded by asking what was the required Axle crown height, and that the rake wasn't an issue, so may be a positive outcome ?

Will give Bike CAD a go and see what the effect is.
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Re: Effect of incr. Axle-Crown height & Changing rake ?

Postby rustychisel » Mon Jul 05, 2010 1:04 pm

HT angle 73º, not unusual
43mm rake is pretty standard for a modern road bike fork, 45mm not unusual at all, especially for a slightly lager frame.

The only really change you'd notice and be annoyed by is if the fork crown length was so long that it propped the front of the bike up to the extent that the HT angle was effectively flattened out, say to 71º. At which point you find find the trail lengthened appreciably and the steering wallowed around like an old pig.
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Re: Effect of incr. Axle-Crown height & Changing rake ?

Postby cooperplace » Mon Jul 05, 2010 2:13 pm

MichaelB wrote:Latest update :

Measurements from bike :
Wheelbase : 1,000mm
Axle-Crown height : 370mm
Fork Rake : 45mm (need to double check)
Head Tube Angle : 73.0 degrees

Have contacted Wound Up, and they responded by asking what was the required Axle crown height, and that the rake wasn't an issue, so may be a positive outcome ?

Will give Bike CAD a go and see what the effect is.


Hi Michael,

just checked the Wound Up prices, not cheap. XACD was cheaper for a custom Ti fork. ANd it seems gorgeous to ride on. I keep on thinking, where is some bumpy road so I can find out how well it rides, but of course, I'm on the same old bits of bumpy road. it seems to give a smoother ride than CF. But it doesn't seem to flex at all. V happy with it.

Peter.
Please be nice to me, I'm not very bright.
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Re: Effect of incr. Axle-Crown height & Changing rake ?

Postby MichaelB » Mon Jul 05, 2010 2:40 pm

Gave BikeCAD a go, and found the following ;

Current :
Axle-Crown height : 370mm
Fork Rake : 45mm
Head Tube Angle : 73.0 degrees
Trail : 56mm (from BikeCAD)

Option 1 (Kinesis DC-19 fork or similar) :
Axle-Crown height : 410mm (+ 40mm)
Fork Rake : 45mm
Head Tube Angle : 70.8 degrees (-2.8 degrees)
Trail : 69.5mm (from BikeCAD) (+ 13.5mm)
Cost : approx A $190

Option 2 (Trigon 26" MTB fork or similar) :
Axle-Crown height : 400mm (+30mm)
Fork Rake : 45mm
Head Tube Angle : 71.4 degrees (-1.6 degrees)
Trail : 66.1mm (from BikeCAD) (+10.1mm)
Cost : approx A $230 via eBay


To maintain the same trail as the current setup, for the 410mm A/Cr height, I'd need 56mm of trail (HTA = 71.0 deg), of for the 400mm A/Cr height, I'd need 53mm of trail (HTA = 71.5 deg).

So, the general consensus is that the slacker HTA and increased trail would make the bike more stable, but harder to change direction.

Standby to standby

Update : 27/7 - Added Options 3 & 4 & costs

Option 3 (WoundUp Custom Carbon) :
Axle-Crown height : 370mm (same - custom dimension)
Fork Rake : 45mm
Head Tube Angle : 73.0 degrees (no change)
Trail : 56mm (from BikeCAD) ( no change)
Cost : approx A $530 via Aspire Velotech


Option 4 (Traitor cycles Steel) :
Axle-Crown height : 384mm (+14mm)
Fork Rake : 45mm
Head Tube Angle : 72.5 degrees (-0.5 degrees)
Trail : 59mm (from BikeCAD) ( + 3mm)
Cost : approx A $205 via direct order


Will update from BikeCad results
Last edited by MichaelB on Wed Jul 28, 2010 12:04 pm, edited 4 times in total.
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Re: Effect of incr. Axle-Crown height & Changing rake ?

Postby jules21 » Mon Jul 05, 2010 4:06 pm

drubie wrote:I get the impression that it's not so much the rake of the fork, but the overall trail that makes the difference (i.e. distance between the centre of the axle and the centre line of a straight line taken down through the steerer when projected to the ground).

yep. increasing trail (which will tend to happen as you increase rake) means more effort is required to overcome angular momentum (gyroscopic precession) of the front wheel. it's like a lever, the distance of which (rake) is the line between the steering axis (steering tube) and a parallel line that passes through the front axle. the longer the lever, the harder you have to push. but that also goes for other steering inputs, such as bumps in the road, so the more stable the bike is as well. in order to provide greater stability, MTBs have longer rake than roadies.
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Re: Effect of incr. Axle-Crown height & Changing rake ?

Postby MichaelB » Tue Jul 06, 2010 1:44 pm

jules21 wrote: ....increasing trail (which will tend to happen as you increase rake) ....


From what I have read, and seen on BikeCAD, this is not true. Trail increases when rake is reduced (all other things being kept the same).

I tend to agree with the rest of your post though.


I guess I'll wait for some better feedback re getting forks with much closer Axle to Crown height, as a couple of mm of rake is not much, but an extra 40+mm of crown height may be a bit much.

Standby to standby. Waiting on a reply from Woundup in the USA
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Re: Effect of incr. Axle-Crown height & Changing rake ?

Postby MichaelB » Tue Jul 27, 2010 4:14 pm

Seems that the Kona Aust importer/cannot/will not order spares in :roll: , so will have to try via the US maybe. (for the 2010 Kona Honky Inc)

Have been provided an alternate Steel fork from Traitor cycles with 384mm Axle-Corwn Height (only 10mm longer) and a rake of 45mm (same), for a shade over USD$200.

Image

But it weighs 950g :shock:

Will pursue the Kona fork a bit more before ordering the Traitor fork (and checking out geometry effects in BikeCad.

Update : BikeCad won't work here at my work - damn Java :twisted: Will try at home tonight. Will then update options table above
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Re: Effect of incr. Axle-Crown height & Changing rake ?

Postby jules21 » Tue Jul 27, 2010 4:25 pm

MichaelB wrote:
jules21 wrote: ....increasing trail (which will tend to happen as you increase rake) ....


From what I have read, and seen on BikeCAD, this is not true. Trail increases when rake is reduced (all other things being kept the same).

sorry, captain pedantic here, but i think rake does increase with trail :)
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Re: Effect of incr. Axle-Crown height & Changing rake ?

Postby MichaelB » Tue Jul 27, 2010 8:21 pm

jules21 wrote:
MichaelB wrote:
jules21 wrote: ....increasing trail (which will tend to happen as you increase rake) ....


From what I have read, and seen on BikeCAD, this is not true. Trail increases when rake is reduced (all other things being kept the same).

sorry, captain pedantic here, but i think rake does increase with trail :)


Ummm, no.

For a set of given dimensions, if you increase the rake on a fork, the trail decreases.

Taken directly from the wiki link above ;

Trail can be increased by increasing the wheel size, decreasing or slackening the head angle, or decreasing the fork rake or offset.

I know it seems odd, but that's suspension geometry for you :D

Either way, I need to match the axle crown length and rake to minimise the handling differences.
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Re: Effect of incr. Axle-Crown height & Changing rake ?

Postby jules21 » Wed Jul 28, 2010 9:42 am

oh nos.. you're right about the rake-trail thing. i needed to look harder at the diagram :)

good luck with finding the right fork. the additional 10mm of height in the Traitor fork wouldn't seem to have much impact on handling.
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Re: Effect of incr. Axle-Crown height & Changing rake ?

Postby MichaelB » Wed Jul 28, 2010 11:59 am

jules21 wrote:
good luck with finding the right fork. the additional 10mm of height in the Traitor fork wouldn't seem to have much impact on handling.


The effect of the Traitor fork is pretty small (see above details), so is a good option, but the weight is a bugger :?

Signing off

Cap'n Pedantic :lol:
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Re: Effect of incr. Axle-Crown height & Changing rake ?

Postby MichaelB » Thu Jul 29, 2010 2:39 pm

I'm still tossing up what fork to get (weight vs budget vs geometry changes), but have plunked for a hub ($35) from eBay, but looking at a DT Swiss/XT wheelset in 700C to use as a starting point, and then build up soomething at the end.

Decisions, decisions.....

Overall parts required :

BB7 Road kit - Wiggle (AT) $80
Wheelset - eBay (AT) $190 - $300
Fork - Traitor (AT) $205, Kinesis (AT) $190, XACD (AT) $240, or Woundup (AT) $530
Headset - Wiggle (AT) $30 ish

Adds up quick ....
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Re: Effect of incr. Axle-Crown height & Changing rake ?

Postby MichaelB » Tue Aug 10, 2010 8:34 am

Had an interesting conversation the other day with one of the guys at Dirtworks re the disc fork issue (chasing up price and availability on a Winwood Muddy Disc Cross fork).

He mentioned that in addition to the greater stability (i.e. not being able to change direction quickly), that there will be an effect that the bike will tend to understeer more that currently.

This got me thinking about this, and wondering how much of an effect that will be..... :?

BTW, the alloy Mosso fork arrived yesterday as well :)
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Re: Effect of incr. Axle-Crown height & Changing rake ?

Postby jules21 » Tue Aug 10, 2010 11:53 am

of course understeer doesn't mean a reduction in front end grip, it just means more steering effort is required. that's effectively where your stability comes from.

i wouldn't sweat, i'll bet you won't even notice much difference. i commute on my MTB - it has miles more trail than my roadie but it's no big deal.
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Re: Effect of incr. Axle-Crown height & Changing rake ?

Postby MichaelB » Tue Aug 10, 2010 12:52 pm

jules21 wrote:of course understeer doesn't mean a reduction in front end grip, it just means more steering effort is required. that's effectively where your stability comes from.

i wouldn't sweat, i'll bet you won't even notice much difference. i commute on my MTB - it has miles more trail than my roadie but it's no big deal.


Once I get used to it anyway....

Time to order some more bits 8)
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Re: Effect of incr. Axle-Crown height & Changing rake ?

Postby MichaelB » Wed Aug 11, 2010 8:34 am

Rims about to be ordered via LBS.

I'll use some Velocity A23's with a Magura labelled DT240s front hub, and a DA7700 rear hub.

Not cheap (AT) $100/rim, but supporting local, and is pretty similar to one delivered from OS anyway.

Spokes not yet fully decided, but likely to be double butted Sapim/DT's (2.0/1.8/2.0), but still to be decided.
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