DIY recumbent bike....?

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Brucey
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Re: DIY recumbent bike....?

Postby Brucey » 2 Jun 2015, 12:06pm

beardy wrote:In the event of an emergency stop on a bike that high you are going to be going over the front wheel fairly quickly. Is the plan to just uncleat and hit the ground running?


[XAP]Bob wrote:Not sure he'd go over much faster than a typical upwrong rider, and as you say, he'd be in a far better position to deal with the situation than the headfirst posture of the (un)safety bicycle...


I think my front/rear weight distribution is about the same as an upright rider with a ~5kg bar bag on, but my c.o.g. is a bit lower, so I think I'm better off than that.

With the brakes I have, I've yet to get enough weight transfer to start losing much weight on the rear wheel, but I have not really pushed it yet.

If I did go 'over the top' my feet would be on the ground when my body is still tilted backwards. A worrying thing about this prospect is probably the handlebars.

cheers
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AlaninWales
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Re: DIY recumbent bike....?

Postby AlaninWales » 2 Jun 2015, 12:31pm

How about moving the steering to under the seat? That would get the 'bars out of the way and get rid of the tiller effect?

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squeaker
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Re: DIY recumbent bike....?

Postby squeaker » 2 Jun 2015, 1:25pm

[XAP]Bob wrote:Crikey - not heard of people trying to countersteer a 'bent trike before - upright, yes - but everyone I've seen on a 'bent treats it as a completely new action.
This was literally within the first 10 yards of riding it, having spent the previous 3 hours on a low racer (had been riding 'bent bikes for a few years, just trying out something 'new'). My brain very quickly sorted itself out 8)
"42"

Tortoise
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Re: DIY recumbent bike....?

Postby Tortoise » 2 Jun 2015, 1:28pm

Brucey wrote:.... So I think the 'toppling' moment of inertia strongly affects steering feel, as does one's ability to shift body weight when steering normally.

[If I do move the riding position back at all, I'll either increase the tiller action to the steering or have to revise the steering geometry to include a slacker head angle].

Rationalising the way the machine feels when steered; if you put a steering input into the bars, one immediate effect is that the headstock (and therefore the weight of the rider) is displaced sideways. On an upright, I think that most riders hinge slightly at the waist, so only a fraction of the rider's weight is actually transferred, and the motion is damped. WIth a heavily loaded bike or one where the rider weight can't move or hinge in the same way (i.e. recumbent), the weight is transferred directly and/or the centring force in the steering is high. Overall I think this can generate higher than normal lateral forces in the wheels and perhaps also twisting forces in a recumbent bike frame.

So reducing the dead weight over the front wheel and/or reducing the trail value can reduce this effect, and these are the only things you can do on a recumbent machine....
Recumbent steering is odd. I don't think laying the head angle back is the way to go. Mike Burrows will advise a completely vertical head angle and reverse fork offset to give a little trail, for the optimum 'bent steering. See also the Ratcatcher SL This theory has been proven/demonstrated as viable up to 100 mph by BMW, using motorcycles.

The issue with 'bent steering feeling 'squirrelly' is I believe down to wheel flop i.e. the degree to which the front end is lowered when the steering is turned. The slacker the head angle the greater is the tendency for the steering to flop sideways, to the extent that many recumbents just cannot be ridden hands-free other than perhaps by especially experienced and talented riders. Example here. Weight distribution also affects this; a lighter front end will mitigate it to a degree.

It's certainly true that riding a 'bent takes a bit (a lot?) of acclimatisation. Rather like learning a delta upright trike, you have to unlearn muscle responses and learn replacement instincts. One of the valuable tricks I have learned is to remember to use your eyes and your head by which I mean that (a) if you look - actually stare - at the point you want the bike to go to; and (b) incline your head sideways a bit in the direction of desired travel, the thing will respond. The head movement is the weight factor. In the absence of having torso weight to shift as you do unconsciously on an upright, your bonce is pretty much all you've got to play with!

mig
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Re: DIY recumbent bike....?

Postby mig » 2 Jun 2015, 2:20pm

i do feel a little disappointed that this doesn't run a fixed gear :lol:

Brucey
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Re: DIY recumbent bike....?

Postby Brucey » 2 Jun 2015, 2:29pm

AlaninWales wrote:How about moving the steering to under the seat? That would get the 'bars out of the way and get rid of the tiller effect?


I did consider that, but I thought it would create a clash with this design; when you are stopped, you are stood upright with straight legs. Any underseat bars would have to be behind your, er, behind.

For all my concerns about the 'bars in a prang, where they are, I can haul of them, which is useful. It also mimics the envisaged riding position in a later machine.

cheers
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Brucey
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Re: DIY recumbent bike....?

Postby Brucey » 2 Jun 2015, 3:01pm

Tortoise wrote: Recumbent steering is odd. I don't think laying the head angle back is the way to go. Mike Burrows will advise a completely vertical head angle and reverse fork offset to give a little trail, for the optimum 'bent steering. See also the Ratcatcher SL This theory has been proven/demonstrated as viable up to 100 mph by BMW, using motorcycles.

The issue with 'bent steering feeling 'squirrelly' is I believe down to wheel flop i.e. the degree to which the front end is lowered when the steering is turned. The slacker the head angle the greater is the tendency for the steering to flop sideways, to the extent that many recumbents just cannot be ridden hands-free other than perhaps by especially experienced and talented riders.....


I have seen some high-racer style recumbents where the head angle is slackened in order to reduce the tiller action of the steering. Users of such report that they are not evil handling. I think the importance of flop is overstated on a machine where the rider isn't moving his bodyweight in a gross fashion, and the smaller the trail value, the less it happens anyway, for small movements of the steering especially.

Mike has also experimented with many different rake and trail values on a single machine and IIRC he concluded that you would have a controllable machine of some kind over a wide range of settings, but that the steering feel would vary. One thing that you can't easily quantify is the possibility of shimmy; any castor will shimmy, it is just a question of when and how badly. So up to a point you can select your steering on the basis of preference, and then reject those settings which give genuinely unacceptable things like shimmy.

My take on the steering is that there are two things going on; trail steer and lean steer. Each has a particular time constant, with the reaction to trail steer usually being much faster than that of lean steer. Thus on a normal bike you get used to a particular combination of responses, and (in the simplest terms) you use lean steer to ride no-hands, and trail steer sorts out any minor perturbations and gives basic stability. The amount of lean and therefore lean steer input available is large, and the rider can alter the time period of the trail steer response just by sitting differently on the bike (affecting weight coupling), and/or holding the handlebars differently.

Any time you increase the coupled weight over the front wheel, you alter the strength and possibly the time period of the trail steer response. On my recumbent machine the trail steer time period seems much shorter than normal until you sit upright, and then it changes (the weight is centred similarly but no longer coupled in the same way; that it changes at all makes nonsense of the importance of 'flop' IMHO ). By contrast the lean steer input available when recumbent is small, and the time period seems longer than normal. The result of all this is that the steering feels rather twitchy, but actually getting the blessed thing to lean-steer into a corner is kind of tricky.

In addition to everything else there is no real weight on the bars, and this affects both intentional and unintentional steering inputs. I have found that the steering behaves more 'normally' when I am pulling back on the bars, which may have a similar effect to leaning on the bars on an upright, because of the tiller action in both instances.

The normal things that you might do to the steering geometry to alter the lean-steer response can't be done without also affecting the trail-steer response, so it may be that there is no happy medium; that there are not more recumbents that steer nicely hands off is probably good evidence that this is the case.

cheers
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Tortoise
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Re: DIY recumbent bike....?

Postby Tortoise » 2 Jun 2015, 3:02pm

Brucey wrote:... a later machine.

Hmmm. Contemplates and speculates :?:

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Cunobelin
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Re: DIY recumbent bike....?

Postby Cunobelin » 2 Jun 2015, 7:09pm

beardy wrote:In the event of an emergency stop on a bike that high you are going to be going over the front wheel fairly quickly. Is the plan to just uncleat and hit the ground running?


A classic poser dismount

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[XAP]Bob
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Re: DIY recumbent bike....?

Postby [XAP]Bob » 2 Jun 2015, 7:28pm

Cunobelin wrote:
beardy wrote:In the event of an emergency stop on a bike that high you are going to be going over the front wheel fairly quickly. Is the plan to just uncleat and hit the ground running?


A classic poser dismount

Yes, great fun off the trike ;)
A shortcut has to be a challenge, otherwise it would just be the way. No situation is so dire that panic cannot make it worse.
There are two kinds of people in this world: those can extrapolate from incomplete data.

gloomyandy
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Re: DIY recumbent bike....?

Postby gloomyandy » 2 Jun 2015, 11:43pm

Have you been brave enough to try going down a steep hill on it yet?

Brucey
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Re: DIY recumbent bike....?

Postby Brucey » 2 Jun 2015, 11:47pm

I've thus far gone down a short (~15-20%) incline with a sharp turn in it; one that I'd earlier failed to ride up. So steepness per se is not a problem; speed however might yet turn out to be!

cheers
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UpWrong
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Re: DIY recumbent bike....?

Postby UpWrong » 3 Jun 2015, 12:02am

Well done. I wish I could implement my own designs to test out my preferences and prejudices. Chapeau!

As far as steering goes, I find the less trail the better on a recumbent bike. The smaller rear wheel on my Metabikes brings its trail down to close to zero and yet it's the most controllable bent I've owned for uphill climbing. Fast descents require a light touch and a relaxed back though to avoid unwanted steering input.

My brain can't handle much tiller effect on a bent either and yet logically the more tiller you have the less sensitive the steering is to handllebar movement.

Geoff.D
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Re: DIY recumbent bike....?

Postby Geoff.D » 3 Jun 2015, 8:47am

My 'bent was home made by a professional frame builder, just for his own enjoyment, in the early 90's. He'd copied the steering geometry from Crystal Bikes (Ross Engineering). It's very stable and predictable at all speeds.
He'd set it up with USS, with the bars pivoted directly below the seat There was a tie rod from the bars to a pivot at the side of the fork crown. The off-set of the pivot on the bar was variable, so that you could "dial in" different sensitivities. I found, particularly when I was learning low speed climbing techniques, that a 1:1 ratio suited me best. And I've just stuck with it, even though I could probably cope with a quicker response now.
Whether a slower response (less sensitive to handlebar movement) would help the learning curve I just don't know.

Brucey
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Re: DIY recumbent bike....?

Postby Brucey » 3 Jun 2015, 7:25pm

well I've had a chance to ride my chum's Bacchetta Corsa, and it makes an interesting comparison with my monster.

I'd read a review of the Bacchetta which lead me to suppose that it would be awkward in traffic because of its high seat. Now that is perhaps the case if you come from a background where a lower seat is to be expected, but I have to say I found the Corsa a really easy bike to ride by comparison to my own machine, so it is all relative.

I found no difficulty in getting my feet down when halted, and no problems with the feel of the steering once under way; in most respects it seems like it is a fairly benign machine. However had I started on it coming from an upright, I daresay I'd have found it a bit of handful; my chum is most used to a recumbent trike and even after 50 miles or so on this machine, he's still somewhat leery of it.

My chum's machine has the saddle about 4" lower and the seat more vertically inclined, so the rider's eyeline is within an inch or two of my own machine. However because the seat and the BB are lower, and the 'bars are different, the net result is a machine that may punch a slightly larger hole through the air than my own, and a riding position that (later...) perhaps won't fit inside such a small fairing.

Quibbles with the Bacchetta include

- the brakes come configured US/continental style, i.e. with the front brake on the left.
- the rear brake has a QR fitted; the front does not.
- the brake DP calipers are Bacchetta branded, but are not of very good quality; they work OK but the pivots have a substantial amount of free play in them which will surely only get worse in time
- there is enough clearance for a 30mm tyre at the rear, but anything much over a real ~26mm is going to rub under the fork crown
- the 23mm tyres supplied (Kendas) are light enough and grip well in the dry, but pinch-flat too easily on a recumbent, and tend to cut up very quickly in the wet if there are any flints around.
- heel strike on the front wheel is very often seen during low-speed manoeuvres, and is disconcerting/dangerous.
- the bike's setup when new was less than perfect.
- if you need to stop on an uphill section, you need to select a low gear beforehand, because it is not possible to change gear whilst stationary.
- the transmission is rather noisy (by comparison with my own machine)
- the wheels have rather too few spokes in for my tastes; although flexy as a consequence, this didn't seem to make the handling terrible or anything.
- the adjustable seat stays have a little free play in the joints
- the seat stays transmit road shocks directly into the seat frame; some springs in the stays might be a good idea.

But there are plenty of plus points too; I think it is overall a pretty nice machine; not too heavy, pretty comfortable and relatively easy to handle. The rear bag is neat, even if it isn't particularly large. In terms of ease of starting and stopping, the Bacchetta is slightly easier than (say) riding an upright bike with a high BB (where you need to slide out of the saddle when you need to put a foot down) and my own machine is somewhat more awkward than that, even when you are used to it.

Image00035.jpg
Bacchetta underway


You can see a side-by-side comparison of machines/riding position here;

Image00033.jpg
vaguely similar, but completely different...?


so it is interesting to compare and contrast. I suspect that, given a bit of time, the route I'm headed down will yield a faster machine, ultimately, but the Bacchetta is a pretty good OTP solution for those who want a recumbent but don't like being too low down in traffic.

cheers
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