Porteur Headtube Angle

[Evan Baird]

So I have this frame that I built around a Soma low trail fork, but I'm thinking about rebuilding it with a shallower rake fork since I mainly use it with a porteur rack. My question is this; has anybody spent much time on a purpose built (high trail) porteur, and if so what head tube angle feels best with heavier loads? I've read Jan's review of the GB city bike, and I've looked at al the classic french porteurs I could find, but so much of the discussion has been around lightweight rando bags that the details around porteurs are pretty muddy. My personal experience leads me to believe that steeper HTA will feel better with top loads, while the slacker HTA will feel better with panniers added, but I'd love to get other's feedback.

[Brad]

Go to JimG's trail calculator and look at possible flop numbers.  Flop gets real with a wide rack and a load.  Each steering correction will require you to lift up the load.  Of course, maybe you go fast always and never stop with porteur loads, in which case flop becomes way less relevant.  The geometry for Rando bikes and Porteur bikes was remarkably similar I beleive, at least that was theory behind the Kogswell P/R.   My recollection of Jan's testing is that low rider front panniers centered on the steering axle are pretty much trail and flop agnostic.   That explains how a certain type of "touring" bike evolved post 1978.

[Alex Wetmore]

There is a BQ issue where we played with different trail figures for porteur loads (30-50lbs).  General consensus among the testers was that ~40mm felt better than ~30mm of trail for that use.  It looks like that article was in BQ19:

https://www.compasscycle.com/shop/issues/bq-19/

I built my porteur with a slightly slacker HTA of around 72 degrees (vs 73 degrees on my other bikes) to get the wheel a little farther forward under the larger rack.  I stuck with 40mm of trail, so I had to use a bit more rake.  That bike has 700C wheels and wears 38mm Compass tires.  That works well for me.

alex

---

From: 65...@googlegroups.com <65...@googlegroups.com> on behalf of Evan Baird <vanst...@gmail.com>
Sent: Sunday, September 13, 2015 4:23 PM
To: 650b
Subject: [650B] Porteur Headtube Angle

 

So I have this frame that I built around a Soma low trail fork, but I'm thinking about rebuilding it with a shallower rake fork since I mainly use it with a porteur rack. My question is this; has anybody spent much time on a purpose built (high trail) porteur, and if so what head tube angle feels best with heavier loads? I've read Jan's review of the GB city bike, and I've looked at al the classic french porteurs I could find, but so much of the discussion has been around lightweight rando bags that the details around porteurs are pretty muddy. My personal experience leads me to believe that steeper HTA will feel better with top loads, while the slacker HTA will feel better with panniers added, but I'd love to get other's feedback.

--
You received this message because you are subscribed to the Google Groups "650b" group.
To unsubscribe from this group and stop receiving emails from it, send an email to 650b+uns...@googlegroups.com.
To post to this group, send email to 65...@googlegroups.com.
Visit this group at http://groups.google.com/group/650b.
For more options, visit https://groups.google.com/d/optout.

[Andrew Pollard]

Alex,

Do you think the headtube angle should be the same with upright bars like the Ahearne/Map or porteur bars vs drops? If you're more upright does the weight shift necessitate a change in geometry or trail?

Thanks

[Alex Wetmore]

My bike has Jitensha bars (Ahearne/Map bars are a near clone). I made the top tube 2cm longer to compensate for the shorter reach on these bars compared to drop bars while still keeping an upright riding position. My bike isn't that upright though, I don't like sitting bolt upright. I don't see any reason to change the HTA based on handlebar choice.

alex

________________________________________
From: 65...@googlegroups.com <65...@googlegroups.com> on behalf of Andrew Pollard <bapo...@gmail.com>
Sent: Monday, September 14, 2015 10:09 AM
To: 650b
Subject: Re: [650B] Porteur Headtube Angle

Alex,

Do you think the headtube angle should be the same with upright bars like the Ahearne/Map or porteur bars vs drops? If you're more upright does the weight shift necessitate a change in geometry or trail?

Thanks

[Andrew Pollard]

Thanks for the answer and your time Alex!

[Kieran Joyes]

Yeah thanks Alex! I'm finally embarking on a custom 700x38 rando-porteur, so good timing for your front-end geo figures.

KJ

[Alex Wetmore]

There are photos on my flickr if they are at all helpful.  Here are a few:

https://www.flickr.com/photos/88336152@N00/15773417514/in/datetaken-public/

Most are pre-paint photos and near this one:

https://www.flickr.com/photos/88336152@N00/13780264733/in/datetaken-public/

They bike is actually now an e-assist bike that I use for getting around town a little faster (mostly to make it easier to bike to after-work sailing when I have a narrow time window between leaving work and getting to the marina):

https://instagram.com/p/4sKUYimWPQ/?taken-by=alexphredorg

alex

---

From: 65...@googlegroups.com <65...@googlegroups.com> on behalf of Kieran Joyes <kjo...@gmail.com>
Sent: Monday, September 14, 2015 10:55 AM
To: 650b
Cc: bapo...@gmail.com

[Matthew Snyder]

I have a purpose-built Bilenky porteur with a 72 degree HTA and 650B wheels with 42mm tires.  The trail is low- to mid-40s (I can't remember the exact fork offset right now but it's 62 or 63mm IIRC).  Probably a bit more trail than the BQ guys generally like, but it works for me.  I used to routinely throw a lot of weight on the porteur rack for big grocery and beer runs.  Currently I've repurposed the bike (experimentally, anyway) as a camping bike, with Cazaderos, Albatross bars, a basket mounted to the rack, and a big frame bag.  It's not 100% ideal for that purpose, but I just did a five day off-road tour on it and think I'll keep it set up that way for awhile. 

Matthew Snyder

Seattle WA

[satanas]

That might be so, but my experiece suggests that low rider panniers are *not* agnostic to either: 1) fork offset (less is better IMO as there's less bump steer and "hinge effect"), or 2) weight behind the rear axle, where IME more offset => more chance of shimmy due to aforesaid "hinge effect." YMMV; I have no scientific proof for these observations, but experience with more than one offending bike suggests they are real.

Later,

Stephen

[Ryan Watson]

My Observations:

Putting small lowriders on my flexy low trail 650B Boulder made it LESS stable than bar bag alone. Not dramatic or unsafe but definitely noticeable. 

Putting the same lowriders in the same position on my slightly less flexy low trail Rambler made it MORE stable. 

I think both have 73 head angles. The trail with Hetres is 32 for the Boulder and 34 for the Rambler. 

I also put the same lowriders in the same position on my rSogn and didn't notice a difference. It's 73 HT, and 40 trail as I recall. 

Ryan

--

[Stephen Poole]

FWIW: I've used low mounted front panniers on a number of bikes, and they have improved stability in every instance. The bikes have included the following:

1. 531 ST 27x1 1/4" touring bike, small canvas bags (~20 litres)

2. 531 SL 700x28 audax bike, Specialized tailwind panniers, ~20 litres

3. Specialized Allez Epic (CF frame + alu forks), same bags as #2

4. Mantis X Frame inspired touring MTB with custom 531 racks. Front rack was mid-height (like Tubus Smarti), and bags as #1

5. Ritchey Logic WCS framed MTB with Logic forks, SWB, bags as #1 at first, then rears mounted on front later. With the latter bags, steering was rather heavy but the bike was even more stable

6. Soma GR with Tubus Tara rack and Deuter welded bags, 28 litres. The GR was noticeably *more* stable with the front panniers than without, especially with the out-of-spec original fork (31mm trail). The difference was less pronounced with the later, higher trail (45mm) fork, but still clearly apparent

Of the above, only the last was a low trail design; none of the others had any stability problems, loaded or otherwise, ever.

On the other hand, my experience is that rear bags usually reduce stability unless the bag and rack design is extremely rigid (as per #4), or the bags are very small and light and/or contained almost wholly within the wheelbase (as per #3).

I'm inclined to think that people's tastes re handling vary very dramatically, and that what's good for one person is quite likely to be really aggravating for another. Also, it seems like people mean different things when they are talking about stability or "improved low speed handling." (Stability for me means: "bike maintains a line easily without requiring any input to do so, and corners in a predictable fashion without surprises.") 

Without knowing where someone else is coming from, anything said here is just so much hot air (or agitated electrons). YMMV, and probably will.

Later,

Stephen