Weight distribution and stability?

[tiago]

I recently bought a vision r40 (2001, OSS, 20/26") as my first recumbent. It has two possible seat positions along the frame and the boom can be adjusted to fit me in either one (I am 5"10. I first tried the position further back but after taking a fall and generally finding the bike unstable going over train rails, gravel patches, piles of wet leaves, turning over curbs, etc... I am experimenting with the forward position. Would you expect any obvious relationship between front/back weight distribution and stability? I have noticed that volae models differ greatly in weight distribution. The volae sport thas the seat between the wheels while the club's is almost over the rear wheel. What is the effect on behaviour?

I have also read that thicker tires would make it more stable. I have primo comet 20x1.35 and a Nimbus EX 26x1.5 that I run at 90-95% of the maximum pressuree. Any suggestions on low drag tires that would increase the stability and which one is more important to change (front or rear)?

Thanks,
Tiago

[Pockets]

Without you saying what you experiance level with SWB bents is its hard to answer. If you are new it is just a learning curve. The tire width seems OK. You do have to hit tracks at about 90 degrees. Figure out your X-seam and make the adjustments. With out more info its just a guessing game. The bike isn't designed for jumping curbs.

[tiago]

Experience:
I only have 1.5 months and 300 miles on the bike and this it is my 1st recumbent.

Details:
One of the most surprising situations when I felt the bike slipping under me was while turning into a driveway that has a shallow concrete drainline on the tarmac. I also took a fall going straight at 30 mph while crossing a small gravel patch at an intersection. I made the mistake of looking to the side to read a sign and by unvoluntarily slightly moving the handlebar I found myself traveling bikeless along the road. Damaged the seat fabric and a windbreaker :-(
There are several local roads crossed by oblique train tracks that I never had trouble cycling over at 20 mph with my road bike. So should I get used to *really* slow down to cross train tracks at 90 deg?

t.

[wink]

I do not know any thing about your bike but I do know with a EZ Sport AX I went to 195 tires street tread
an it rides a lot better.And I take it any where I take my DF but in the woods.I ride dirt roads all most every day on part of my ride with out any more trouble than on the DF.
If you go that way make sure the bigger tires will clear the frame.I have fenders on my bike and the 195`s
are only about and eighth of and inch off the tires at some points.I as you am new and just past 600 miles today on a 41 mile ride.

Wink

[tiago]

Thanks for the suggestion. Please enlighten me on these 195 tires. Did you upgrade from the 1.5 Kenda kwest?

[Recumbomatic]

With the seat too far back, there won't be enough weight over the front wheel for it to keep good traction. Moving the seat forward should help a lot. Fatter tires can help too, but 1.5" may be sufficient. BTW the Primo Comet has a reputation for terrible traction on wet roads.

Especially with the 20" wheel, you will want to cross any obstacles at an angle that is as close to 90 as possible. If there is any moisture on the road, be extremely careful when crossing metal.

[tiago]

Yes, that all makes sense. I will try a Kenda kwest or Schwalbe Marathon/racer in 20x1.5. It rains a fair bit here in Oregon.

t.

[Doug5150]

... I (am) generally finding the bike unstable going over train rails, gravel patches, piles of wet leaves, turning over curbs, etc... I am experimenting with the forward position. Would you expect any obvious relationship between front/back weight distribution and stability? ...
Wet leaves: nothing will help there. Bicyclists and motorcyclists alike fall in them all the time. Stay out of them, or coast straight through them slowly and carefully.

------

Steering sensitivity is common with SWB's--in particular the wandering off in odd directions when you are not always looking where you are going. It gets better with practice, but the feel is just not as good as uprights.
.....Lateral front-wheel instability in gravel is a major problem too.

Two things that will help are using a wider front tire, and dropping the front tire pressure. Many recumbents don't have 50/50 front/rear weight distribution, but people want the smallest rolling resistance so they automatically inflate both their tires to the sidewall max, and then wonder why the front tire slides out in gravel or in turns. Tires slide either because they are overloaded or overinflated--and if the front is sliding in turns but the rear isn't, and the rear is carrying more of the load than the front is, then the front is not overloaded--it's overinflated. Tires should be inflated proportional to the loads that they are carrying.
.....
You can figure out the bike's actual weight distribution by putting a bathroom scale under the front tire and sitting on the bike with your feet on the pedals, holding yourself up with a hand against a wall, and then figuring the front-end weight against the total bike + rider weight. Assuming that both front and rear tires are the same width, if 40% of the weight is on the front tire, then the front tire should only be inflated to no more than two-thirds the pressure of the rear tire. If 33% of the weight is on the front tire, then the front tire should only be inflated to no more than one-half the pressure of the rear.

Another opinion is that if the front tire is a smaller-diameter, it should always use a wider tire than the rear. The idea is that you want the same contact patch area for both tires--and the front's contact patch will be shorter because of the smaller wheel radius, so you need to mount a wider tire to make up for the difference. Very-narrow tires in particular handle badly in loose gravel, no matter what they're inflated to.

Alternately you can eyeball it by making sure that when you're sitting on the bike, both tires sag roughly the same amounts--but this is hard to judge yourself, you'll almost need an assistant to look while you're sitting on the bike.

If the front rim is very-narrow it would be an improvement to get a wider rim put on. It would probably improve BOTH tires--letting you run lower pressures, absorbing more road shock while still having good lateral handling. In the last ~10 years the US bike industry has gone hyper-stupid for narrow rims (3/4"-7/8") when most average riders would do much better with wider ones (1.25 to 1.5"). ...The upshot of trying to switch to wider tires is that they don't work well on very-narrow rims.

Ultimately I got rid of the SWB, and got a 20/26 LWB instead. The SWB was fine riding on clean pavement, but was a white-knuckle ride when I had to make "evasive maneuvers" through patches of gravel on the road and the jittery steering took more attention than I cared to spare. The LWB steering is still not as stable as a common MTB would be, but it is a big improvement over the SWB.

I only have two bikes now--the LWB and a RANS Fusion, and on both bikes as it happens about 33% of the weight is on the front tires--and the front tires are inflated to about half the pressures of the rears.
~

[tiago]

Thanks Doug, several good ideas there.

I used the bathroom scale method to evaluate effect of moving the seat between both positions on the rear/front weight distribution. This seems minimal, changing 2% around 55/45%.

On the other hand the tire pressure seems more relevant. I was using 95 psi on the front and 75 psi on the rear just because these are close the maximum recommended values and didn't give it any thought. By lowering the front tire to 65 psi I am already noticing better grip on the maligned turns into parking lot ramps and driveways.
I don't really understand the logic behind the empirical rule of 60/40 weight -> 2/3 the pressure. Namely, would increasing the pressure of the rear tire to 100 psi or using a thinner rear tire make the bike more stable, and if so why?

Thanks,
t.

[Doug5150]

...I don't really understand the logic behind the empirical rule of 60/40 weight -> 2/3 the pressure. ...
Assuming the tires are roughly the same width:
if the front/rear weight is 33%/66%, then the front is only carrying half the weight that the rear is, so the front should only be inflated at most to about half the pressure of the rear.
If the front/rear weight is 40%/60%, then the front tire is only carrying two-thirds of the weight that the rear is, so it should only be inflated at most to about two-thirds as much as the rear.

For the bike and tires you've got, putting the front at around 65 and the rear at its max 75 is about right (because the front is ~5/6 the width of the rear). If you had a 1.5" wide front tire, about 60 PSI would be right for it.

Namely, would increasing the pressure of the rear tire to 100 psi or using a thinner rear tire make the bike more stable, and if so why?
Neither of these will increase traction, going to a narrower tire or a higher pressure never increases traction. What it will do is lower rolling resistance, but only by a very-tiny amount.

The question of "if it would make the bike more stable in turns" is difficult to say. Most people with this issue tend to notice the front wheel sliding easily when turning through gravel--and while using a higher pressure or a narrower rear tire will help balance the front/rear traction, it does so only by reducing the rear traction--and neither raising tire pressures or using narrower tires will help improve traction in gravel at all. If you feel the urge to try running 1" wide tires you pretty-much just need to accept the fact that they need to be kept on clean pavement, because they will wander and slide badly in almost any kind of loose gravel.
~

[tiago]

Thanks for the explanation Doug. I now get the point on balancing the traction beteen both wheels.

Tiago

[sch]

Not all rims, brakes and sometimes forks will allow large changes in tire size such as going from
28-32mm to 45-55mm tires. Road type brakes are particularly unforgiving. Cantis have a much
wider range of adjustment. Rims can be a problem with larger changes in tire width, rim widths at
the bead ranging over 20-26 or so mm width. Bents tend to have fairly wide forks and rear
triangles but this should be looked at before buying 1.95 range tires on a bike with 1.35 now.
I generally agree with Doug's points otherwise, though I run my LWB Rotator Pursuit 25mm
Stelvios on the front at 100#. Generally speaking the more the trail of the steering axis the
less stable the bike will be. DF bikes have a relatively vertical steering axis compared to a lot
of bents. Bacchetta type bikes have a steering axis that is fairly close to DF axes but LWB
tend to be more laid back and less stable on uncertain footings such as leaves, wet paint, RR
tracks or gravel and sand.

[Giro]

I'll second Doug5105 & sch's comments.

Maintaining stability on a recumbent is also different from a diamond frame (DF). Your center of mass ("center of gravity") is lower and you can not move it around as you can on a DF. As a result, recumbents are stabilized more by steering than weight shifting. Compare riding a DF no hands either straight or turning compared to no hands on a recumbent.

That is why in winter, off-road, over city curbs etc. I'm on a DF, not my recumbent. Different horses for different courses.

[PaPa]

Maintaining stability on a recumbent is also different from a diamond frame (DF).I disagree...

Successful riding on both the upright bike and the recumbent, require steering inputs to remain balanced. Although the rider on the upright has marginally greater influence over balancing than the 'bent rider does, the same basic laws of physics apply equally to both platforms. If in doubt, simply secure the handle bars in a straight-ahead position, and try riding it.

Compare riding a DF no hands either straight or turning compared to no hands on a recumbent.There is no lateral CoM shift (relevant to the bike's forward plane) occuring on either platform during no-hands riding. The only reason no-hands riding is more difficult on most recumbents, is the lack of frame tilting control - which on an upright, normally requires the legs to be vertical (perpendicular to the bike's roll axis), and the feet firmly planted on the pedals.

[Giro]

I disagree...
...There is no lateral CoM shift (relevant to the bike's forward plane) occuring on either platform during no-hands riding. The only reason no-hands riding is more difficult on most recumbents, is the lack of frame tilting control - which on an upright, normally requires the legs to be vertical (perpendicular to the bike's roll axis), and the feet firmly planted on the pedals.

Thanks, you are quite correct! This explains why tilting my head left or right helps my brief attempts at no-hands riding my recumbent. I suppose grabbing the seat sides with my hands might let me tilt the bike even better, but this would not let me get my hands back on the handlebars quickly enough.

In view of the above, a DF's ability to accelerate your body up or down to hop over curbs (pull up on handlebars), bunny hop (best if clipped in), or "absorb" shocks may be more important than the tilting ability of a DF. I've tried unweighting my rear (not the wheel; me) on my non-shock absorber high racer. It helps some, but my legs (on the DF) can do much more force X distance vertically than my abdominal etc. muscles can on the recumbent. I'll never bunny hop the high racer.

[PaPa]

Would you expect any obvious relationship between front/back weight distribution and stability?Yes.

As the CoM is shifted rearward, steering begins to feel muted and less responsive. This forces the rider to input steering corrections faster and more deliberate for the same end results - the slower you ride, the worse it gets. Likewise, as the CoM is shifted forward, steering becomes proportionally more sensitive and 'quicker'. Too far forward and it's 'white knuckle' time.

[tiago]

While on the matter of stability and steering I wonder why is it that, on my recumbent, when I steer to one direction I feel the bike, and me, tilting to the opposite direction, forcing it out of balance. So to steer more effectively I tend to lean to the direction that I am about to steer to keep in balance. I guess that this is more noticeable at low speeds (<10mph) and once it becomes automatic it might be difficult to detect. More specifically my question is, does this vary across the gamut of bent geometries?

Cheers,
t.

[Giro]

That is a good question and a nice first year physics problem. A good drawing would be the easiest to understand. I suspect a book such as Bicycling Science (http://books.google.com/books?id=0JJo6DlF9iMC&dq=%22bicycling+science%22+wilson&pg=PP1&ots=Tq0EAMcqg8&sig=m6m5ecn2hl4ZlAgSc3juAN7JjI4&prev=http://www.google.com/search%3Fhl%3Den%26q%3D%2522bicycling%2Bscience%2522%2Bwilson%26btnG%3DGoogle%2BSearch&sa=X&oi=print&ct=title&cad=one-book-with-thumbnail) by David Gordon Wilson may have a good illustration and text. I will check in my copy.

Wikipedia has an illustrated article on bicycle dynamics LINK (http://en.wikipedia.org/wiki/Bicycle_and_motorcycle_dynamics) which has a section on turning. You may find this a bit difficult to follow depending upon how much geometry and algebra you know.