Author Topic: Moving bottom bracket fwd.  (Read 889 times)

sprogs

  • from your big sister, Steve.
Moving bottom bracket fwd.
« on: September 04, 2020, 05:06:54 pm »
A few years ago I built an fwd moving bottom bracket recumbent using an old Peugot mixte frame and some old upwrong tubing.
I rode it for a short while and it was comfy, but I seemed to have a lot of pedal steer.
Does anyone have experience of this type of bike ?
I would be interested in ANY info, especially relating to things like trail, head angle and bb position.
Ta.
Liz

Re: Moving bottom bracket fwd.
« Reply #1 on: September 04, 2020, 10:45:45 pm »
A few years ago I built an fwd moving bottom bracket recumbent using an old Peugot mixte frame and some old upwrong tubing.
I rode it for a short while and it was comfy, but I seemed to have a lot of pedal steer.
Does anyone have experience of this type of bike ?
I would be interested in ANY info, especially relating to things like trail, head angle and bb position.
Ta.
Liz

Re head angles there seems to be a french and a yank approach. The french approach is (or was, it might have changed in the couple of years since I looked) to have a very slack head angle, resulting in a machine that would virtually steer itself hands off (usually one of the design objectives). This is a bit like a conventional handi-trike but two-wheeled. The USAnian approach (which I think is also the brit approach although I have absolutely no experience or proof of that) is to use a head angle very close to an orthodox upwrong. I have seen the arguements for this somewhere on the Net (which I obviously don't agree with, being an assimilated frog) but I can't remember where.

If you can read french, browsing the homebuilder pages of the vélorizontal forum might give some ideas. At least you can look at the pictures. I will see if I can look up links to some of the threads. There is also somewhere a homebuilders data base with a lot of useful stuff; it may be someone's blog.

ElyDave

  • Royal and Ancient Polar Bear Society member 263583
Re: Moving bottom bracket fwd.
« Reply #2 on: September 05, 2020, 07:53:53 am »
One of the "mainstream" MBB FWD 'bents is Cruzbike.  I've been riding one for getting towards 3 years now.  There is a degree of pedal steer, but mostly on start up when you have a lot of pressure through one pedal without much forward motion.  Once you are going it's fairly easy to even it out with light counterpressure on the bars, and even use a bit of upper body on the hills.

I've only really had one negative experience of pedal steer, and that was when very tired on the last day of a hilly tour, on the last big downhill to where I was staying overnight.  At about 60kmh, I swapped the front foot coming into a bend and must have nudged the steering as I did so, resulting in a massive swoop across the road, then back again towards a dry stone wall.  The car behind me was being sensible, but I imagine he had a bit of a shock.
“Procrastination is the thief of time, collar him.” –Charles Dickens

sprogs

  • from your big sister, Steve.
Re: Moving bottom bracket fwd.
« Reply #3 on: September 05, 2020, 05:04:00 pm »
Velorizontal looks very interesting.
Unfortunately Je remember doux f.a.  from my french at school.
I was think about negative trail. (Python etc). I used to have a lowrider bike with the odd american style front suspension which had about 8inches of negative trail. Once I got used to it it was a joy to ride.
I once rode it about 8 or 9 miles including canal track and it was huge fun. Quite stable.
Anyone ever ridden a Python ?

Re: Moving bottom bracket fwd.
« Reply #4 on: September 05, 2020, 07:54:56 pm »
Velorizontal looks very interesting.
Unfortunately Je remember doux f.a.  from my french at school.
I was think about negative trail. (Python etc). I used to have a lowrider bike with the odd american style front suspension which had about 8inches of negative trail. Once I got used to it it was a joy to ride.
I once rode it about 8 or 9 miles including canal track and it was huge fun. Quite stable.
Anyone ever ridden a Python ?
And lived to tell the tale! I knew someone who built a Python trike, thought he had the angles correct and nearly killed himself.
Calling M. Wobbly John on the big red telephone!

Re: Moving bottom bracket fwd.
« Reply #5 on: September 05, 2020, 08:20:20 pm »
re velorizontal the starting point is this section http://velorizontal.1fr1.net/f10-le-coin-du-constructeur-amateur  After that it is a question of searching. I will try to sort out something useful (if I can and hoping that any old photos are still available, which is not guaranteed)

From memory the head angles were in the 55-60° but I have no idea for the trail and obviously the position of the bb is going to make a difference.

Edit I have just looked at the first page and what there is at the top is a series of stickies on all the different construction projects. What you are looking for is here http://velorizontal.1fr1.net/t23318-table-2-roues-traction-directe-td

For information an mbb is a traction directe (td), a fixed bracket fwd is a traction indirecte (ti) and a propulsion is the classic rwd configuration.

Note the velorizontal search function (rechercher in the box at the top) doesn't seem to work unless you are logged in (at least it wasn't working for me tonight) but all the photos seem to have URLs attached.

Re: Moving bottom bracket fwd.
« Reply #6 on: September 06, 2020, 09:54:15 am »
My Yank-tank and it's newly arrived replacment have a slightly more angled steerer than a conventional upwrong, but only by about 10deg I think. My basis is that the SKS longboard mudguard not only had to be mounted in front of the fork crown, but also required the application of 2x10mm M5 connector nuts to place it both far enough fwd to protect the BB, but also to get the mudflap far enough away from the tarmac.

Both have been very stable once learnt, and on the occasional return to sitting on the head of a pin (upwrong), I've now started finding them wobbly and uncertain as the steering axis is now so close to my head. My opinion is that very little is genuinely unridable provided you're prepared to learn it, and not insist it's the same as what you came from.
Cruzbike V2k, S40

ElyDave

  • Royal and Ancient Polar Bear Society member 263583
Re: Moving bottom bracket fwd.
« Reply #7 on: September 06, 2020, 10:59:12 am »
What size tyres did you end up with? When I had my fork replaced the clearance was much reduced and I've taken the race blades off for the summer as it was so much faff to manage the rubbing with 28mm conti GP4 seasons.
“Procrastination is the thief of time, collar him.” –Charles Dickens

sprogs

  • from your big sister, Steve.
Re: Moving bottom bracket fwd.
« Reply #8 on: September 07, 2020, 06:10:42 pm »
I went on Velorution and tried google translate.
Still laughing.
 "A weld farted. I'm going to strengthen with a ligature. I can do better on the torches.
Question: your proposals are welcome to make a guidondon."

Re: Moving bottom bracket fwd.
« Reply #9 on: September 07, 2020, 08:04:38 pm »
I went on Velorution and tried google translate.
Still laughing.
 "A weld farted. I'm going to strengthen with a ligature. I can do better on the torches.
Question: your proposals are welcome to make a guidondon."

As good as Hoffnung's "Letters from Tyrolean landladies"!
And Google think we should trust them with AI and autonomous autos  ;D
If you find something that interests you on velorizontal I can probably translate the most salient points (but not necessarily very quickly). Others may be better than me!

I have remembered (I think) the arguement for the two views of head angle. The yank view was that a slacker head angle could favour steering shimmy; the french view is that it makes for a bike with minimal pedal steer and that will ride hands off (if you have a better sense of balance than I do!)