Small frames-head angle and handling problems

Discuss light weight issues concerning road bikes & parts.
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xcnick
Posts: 142
Joined: Fri Jan 21, 2011 5:27 pm

by xcnick

Hoping other riders who are also not blessed with being average height can give some input! (Or anyone else in the know!)
Here’s my dilemma; I’m 169cm short legs fairly normal body so need frames with a low stack (520mm ish) but reasonable reach. Not easy to find. Small bikes often have slack head angles for toe clearance which seems to affect handling. My current bikes are-

BMC team machine 48cm has a head angle of 70.5deg, 369mm reach, 120mm stem. Mountain descents ok, twister stuff feels hard work.
Ritchey Logic 72deg head angle, 386 reach 100mm stem handles amazingly.

Both bikes are set up the same inc saddle, bars, shifters and I want to replace the BMC with something more like the geo of the Ritchey but I can’t find anything. Really my question is should I look to get the head angle as steep as possible for my size frame, or am I possibly overlooking something else in the geo of the BMC?

by Weenie


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euan
Posts: 1499
Joined: Sat Sep 05, 2009 11:20 am

by euan

The issue with the Team Machine is that the trail on that size is a bit wonky. They have slackened the headtube and used the same fork rake as the other sizes, if it had 10mm more rake it would handle more like the bike you expect.

The size up is only 10mm longer in reach, yeah its stack is 530mm, but if you have no spacers just get a -17 stem or something
"Step forward the climber and all those who worship at the altar of lightness" - R. Millar

kode54
Posts: 1213
Joined: Tue May 23, 2006 9:39 pm

by kode54

Doesn't the Storck bikes have similar geometries?
- AX Lightness Vial EVO D + DA9150 + Enve SES 4.5 carbon hubs
- Parlee Altum + DA9150 + Enve SES 4.5 carbon hubs
- Parlee ESX + DA9150 + THM SRM PM + Enve SES 6.7 CK hubs
- Guru Praemio R Titanium + DA9150 + Enve 3.4 CK hubs

Hexsense
Posts: 463
Joined: Wed Dec 30, 2015 12:41 am

by Hexsense

There is a thread here,
https://weightweenies.starbike.com/foru ... 3&t=152320

Basically, your BMC has an extremely high trail.
Trail increases with head tube angle and also fork offset.

So you should look for bike with
Steeper (more degree) head tube angle
or
More fork offset (more mm)
or both.

While both Steeper head angle and more fork offset both decrease trail and give you easier cornering. Steeper headtube brings wheel closer to your foot but more fork offset bring wheel away from your toe. If your toe overlap is a problem, look for bike with high fork offset mainly.

Some example
Cervelo S3 disc 51cm
head tube angle 72.2 degree, fork offset 53mm -> Trail= 54mm http://yojimg.net/bike/web_tools/trailc ... =622&tw=27
this is borderline twitchy due to very low trail numbers despite not super steep head tube angle, thanks to high fork offset.
Size 48 change head tube angle to 70.5degree while keeping the same high fork offset, yielding 65mm trail.

Specialized Tarmac Disc 52cm
head tube angle 72.5 degree, fork offset 47mm -> Trail= 59mm http://yojimg.net/bike/web_tools/trailc ... =622&tw=27
although this has steeper head tube angle than S3 Disc size 51, it handle slower, more in neutral range because of more modest (but still high) fork offset. 56-65mm trail is where it considered neutral. :thumbup:

Your BMC size 47 pair 70.5 degree head tube angle with only 41mm fork offset to create disasterous 78mm trail.
You know how it's ride. Cervelo S3 disc in size 48 which has the same headtube angle but with full 12mm more fork offset will beat it to death in twitchy cornering.
Last edited by Hexsense on Fri Jul 13, 2018 6:28 am, edited 1 time in total.

xcnick
Posts: 142
Joined: Fri Jan 21, 2011 5:27 pm

by xcnick

Oh I hadn’t seen the thread above! Thanks for the detailed replies.

I have understood basic geometry pretty well but not fork rake and trail. I’ve been researching and totally understand what you guys mean now. I made a mistake above and my Ritchey is actually 72.5 with fork rake of 45 gives a trail of 61.

So for a neutral handling bike I should be looking 60-65mm? The canyon aeroad looks like it would work for me, and that has an adjustable rake for dropout, but I can’t see any data for what the rake is.

xcnick
Posts: 142
Joined: Fri Jan 21, 2011 5:27 pm

by xcnick

Also I don’t see a reason for the toe overlap argument. In the real world this only occurs if you’re doing a u-turn in the road??

Hexsense
Posts: 463
Joined: Wed Dec 30, 2015 12:41 am

by Hexsense

trail value of 56-65 for me. Anything in this range.
Some people will disagree (especially if they use Colnago, which has long trail on every size).
Small size BMC Teammachine maybe bad, but at least their newest BMC Timemachine Road fixed it. The Timemachine Road has good trail number for small sizes.


For toe overlap:
yeah, pretty much U-turn or stop at stop sign and turn speed.
Except if you use long crank and having big foot with cleats set far back, then the problem is not limited to low speed turning.

BTW: i'm also 169cm tall, but with 78.5-79cm inseam. I also had a bike with 72mm trail in the past.
Currently my bike (Hongfu Avenger R8) has very similar or the same Geometry as Canyon Aeroad.
With stack=511 reach=378, HTA=72, fork offset=43. I'm using 165mm crank and 110mm stem. I only got mild toe overlap with my size 7 shoe and far back cleat position.
I might change my bike soon, to a bike that has pretty much the same front end values as your Ritchey (72.5 degree HTA, 45mm fork offset, reach=380mm).

by Weenie


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