Climbing and bb height, low gears

[MrSteve]

My two Cruzbikes climb hills pretty much the same way, only different:
The Sofrider V1 has a short wheelbase and the seat is farther away from the road than the Vendetta's seat.
I always ride my Sofrider with the 18-degree seat I built installed and this seat is more reclined than the Vendetta's seat.
The Sofrider has three big rings/30 speeds and the Vendetta has a compact double/20 speeds.

My benchmark piece of road has an unclimbable piece of hilltop, when I'm on my Sofrider:
It makes no difference which seat, whether I sit forward and sit up, how light I am or how fit: the Sofrider will always either spin the front wheel and stop
or I'll run out of smooth power and the bike stops.
The combination of a less-stiff front triangle, taller Center of Gravity and Short Wheelbase all contribute.
When I climb this piece of hilltop on my Vendetta, well, it's a tough piece of road to climb, but I can do it on the V.
The Vendetta has a stiff front triangle, lower center of gravity and a longer wheelbase than my Sofrider does.

The Sofrider has lower gears, a triple ring and I have many, many miles of experience both on and off-road.
The Vendetta just climbs better... because, longer wheelbase & more favourable C.G. & stiff front triangle.

These are just facts about the relative climbing prowess of my stable of two MBB FWD Cruzbikes....

 

[Rick Youngblood]

No

Rick, Have you actually weighed front and rear of both to verify the difference ?
My Silvio with me on it is 92 lbs. front and 98 lbs. rear. About 49% F 51% R

Rick, no I haven't. What I'm trying to say is, the longer wheelbase of the Vendetta, has less wheel slippage than the Silvio due to the leverage being placed on the front wheel. The further out the real wheel, the more leverage will be placed on the front drive wheel. If you moved the rear wheel, say 12" more back, you would even have less wheel slippage because there is more leverage being placed on the front wheel. Like a teeter totter effect.

 

[super slim]

On my Silvio V1.0 45 degree seat back the weight was 51% front 49% rear

 

[Andrei Petrov]

I forgot that Silvio has a nearly 50/50 weight distribution. Might equalize tyre pressures around 65-70 psi. Interestingly, this equal weighting appears to be the source of traction limitation. Well, smoother pedalling it is then)

 

[ccf]

I worked out the math for how the geometry and mechanics of the bike affects its ability to climb, hoping that it might reveal something about how I could easily prevent wheel slip on the steep hills in Northern California. The math shows what others have pointed out in this thread, that if you increase the wheelbase simply by moving the rear wheel back farther and farther, then you will be able to climb steeper and steeper grades. The math also shows that if you lower the CG vertically while keeping everything else the same (e.g., by riding on smaller wheels), then you will be able to climb a steeper grade.

To determine the position of the CG of Silvio+myself, I weighed Silvio+myself three ways: front wheel on level ground, rear wheel on level ground, and rear wheel 10 inches above the front wheel. The readings were 90 lbs, 98 lbs, and 110 lbs, respectively. From these I determined the horizontal and vertical position of the CG of Silvio+myself. With those values, the math shows that if I could pedal perfect circles, I would be able to climb a grade of up to 27.2% with no slip under the following assumptions: friction coefficient = 1, Crr of rear wheel = 0.0039, CdA = 0.24, speed = 4mph.

Then I compared the Silvio with the Vendetta, making corrections for the differences in bike geometry and the change in the CG position that results from the lower seat angle. Under the same assumptions, the math says the Vendetta should be able to climb a grade of up to 32.0%. That’s consistent with what everyone who has ridden both bikes has observed; Vendetta can climb steeper hills than Silvio.

Then I looked at what would happen if I put smaller wheels on my Silvio. Unfortunately, it doesn’t have a big enough effect. If I could switch the 700c wheels for 20-inch wheels, it would only increase the max grade from 27.2% to 28.5%.

Just for fun, I ran a guestimate for a penny-farthing bike through the math. It shows that I should be able to climb up to a 23.8% grade on a penny-farthing bike, aided by the fact that much more of the weight is on the front wheel.

I don’t know about the rest of you, but my front wheel starts slipping long before the grade reaches 27.2%, even on clean, dry pavement. Looks like my biggest opportunity for improvement is for me to learn to spin smoother circles.

-Cliff

 

[ratz]

learn to spin smoother circles.

Being able to put out strong power at 50-60 rpms would also helps but there is a toll to pay on the knees if you don't slowly and carefully train for that correctly


Nice math work btw. Might be worth while to post the calculation details

 

[ccf]

At Ratz' request, here is the math. I've attached two files. One is a diagram showing the forces acting on the center of gravity and the two contact points with the road. The other is the math. I also have a spreadsheet that contains the math for the three cases (no rolling or aero drag, no aero drag, general case), and also a worksheet that will compute the position of the CG of a bike+rider if you put in the weight of each wheel when on level ground and the weight of the front wheel with the rear wheel elevated. But the forum won't let me post an Excel spreadsheet. If you'd like to have it, send me a note.

The general rule of thumb is that you lose half of the readers for every equation in a document. So I only expect 0.1% of you to read thru it all.

Coming back to the original question by Andrei about lowering the BB: if lowering the BB lowers the CG (seems likely to me) or moves the CG forward (maybe), then the math predicts that doing so will improve traction. You could take the three weight measurements with and without the lower BB to estimate how much it might improve traction.

-Cliff

 

[ratz]

Nice work.

Seems like the estimate of H could be fixed as the height of the wheels, and pick an average Crr for know reference point tires like a Conti II (I like that better than zero), the Cda for the V20, S30 are pretty well known enough to use constants, and suddenly that's a much simpler equation. Yes H would not then give you the true center of gravity but it would that's going to be pretty consistent offset for this application even though humans very, or maybe better said; it's easier to measure in a consistent manner for generalized questions.... Then what you have here makes a nice heuristic for questions like does the BB height affect traction; or at least a little more approachable that 0.1%

 

[super slim]

I worked out the math for how the geometry and mechanics of the bike affects its ability to climb, hoping that it might reveal something about how I could easily prevent wheel slip on the steep hills in Northern California. The math shows what others have pointed out in this thread, that if you increase the wheelbase simply by moving the rear wheel back farther and farther, then you will be able to climb steeper and steeper grades. The math also shows that if you lower the CG vertically while keeping everything else the same (e.g., by riding on smaller wheels), then you will be able to climb a steeper grade.

To determine the position of the CG of Silvio+myself, I weighed Silvio+myself three ways: front wheel on level ground, rear wheel on level ground, and rear wheel 10 inches above the front wheel. The readings were 90 lbs, 98 lbs, and 110 lbs, respectively. From these I determined the horizontal and vertical position of the CG of Silvio+myself. With those values, the math shows that if I could pedal perfect circles, I would be able to climb a grade of up to 27.2% with no slip under the following assumptions: friction coefficient = 1, Crr of rear wheel = 0.0039, CdA = 0.24, speed = 4mph.

Then I compared the Silvio with the Vendetta, making corrections for the differences in bike geometry and the change in the CG position that results from the lower seat angle. Under the same assumptions, the math says the Vendetta should be able to climb a grade of up to 32.0%. That’s consistent with what everyone who has ridden both bikes has observed; Vendetta can climb steeper hills than Silvio.

Then I looked at what would happen if I put smaller wheels on my Silvio. Unfortunately, it doesn’t have a big enough effect. If I could switch the 700c wheels for 20-inch wheels, it would only increase the max grade from 27.2% to 28.5%.

Just for fun, I ran a guestimate for a penny-farthing bike through the math. It shows that I should be able to climb up to a 23.8% grade on a penny-farthing bike, aided by the fact that much more of the weight is on the front wheel.

I don’t know about the rest of you, but my front wheel starts slipping long before the grade reaches 27.2%, even on clean, dry pavement. Looks like my biggest opportunity for improvement is for me to learn to spin smoother circles.

-Cliff

I think assuming a friction co-efficient of 1.0 is too high.

see the attached article where a dry bit road is 0.5 to 0.85 and a wet bit road is 0.2 to 0.75
http://rommet.com/remco/misc/friction.htm
What are the Max Gradients when using 0.2 or 0.5 cf?

You might also calculate the wattage required of the rider, as we are not all, a Larry or Jason, to achieve 27% grade at 4 mph.

I read somewhere on the internet, but can not find it now, that the centre of mass of a recumbent bicycle rider is the belly button, horizontally and vertically, so an Aero bellied rider, (NOT me!!!) C of G will be higher than a skinny one?
What was your calculated C of G heights?

The lowest point of the Silvio and Vendetta, seats (with out??? cushions) is 21.5" and 20.5", wheelbases 41.5", and 45.3", and with similar % wheel loads F/R 48% / 52%.

I THINK that the biggest difference between a Silvio V2.0 and Vendetta V20 (BIG Yellow), that Ratz, and Rick, both own, is the stiffness of the sliding boom with the Vendetta being 200% stronger than the Silvio!!

 

[ccf]

I think assuming a friction co-efficient of 1.0 is too high.

see the attached article where a dry bit road is 0.5 to 0.85 and a wet bit road is 0.2 to 0.75
http://rommet.com/remco/misc/friction.htm
What are the Max Gradients when using 0.2 or 0.5 cf?

I only see sliding friction coefficient for rubber on road in the article. The static friction coefficient will dictate the steepest hill that can be climbed.

I put the spreadsheet on Google Drive. Here is a link: https://drive.google.com/file/d/0B6iQH3osKDkeSzRtV2JhZXU4eFU/view?usp=sharing

You can change the friction coefficient (and other variables) to whatever you think is best and see what it tells you. Let me know if you have trouble with it.

-Cliff

 

[Balor]

The lower CG AND longer wheelbase, the better is climbing ability, because weight distribution does not shift as much as you climb higher and higher.

ALSO, it applies to shifts in weight bias when you mash the pedals. When you accelerate, you shift your weight bias rearward - in extreme cases, like in motorcycles, you can even do a wheelie. (Same goes for braking, and this is why you are much less likely to go 'over the bars' on a recumbent - much lower CG, and usually longer wheelbase. It all comes to equivalence principle, actually - gravity and inertia are the same.

So, smooth pedalling does not only help with not breaking traction with higher torques, but 'smoothes' the sine wave of acceleration/deceleration that unloads front wheel on FWD recumbents when we need it most.

 

[Balor]

Btw, lower BB height might actually help for very different reasons - an other recumbent I've had before my MBB had very high bottom bracket. My feet would go numb pretty quickly even when cycling on flats, and situation got worse uphill, when my feet got even higher than my head, gravity impeding blood circulation.

 

[Andrei Petrov]

Well, the past couple of weeks have been unusually dry here, and I can now reliably make it up a short traight 16% despite a gravel ridge in the middle, and halfway up another 16% until it makes a sharp left which throws me off balance: I am sitting up, and my arms are clenching the handlebars too tightly to make small adjustments. I'd like to think that I am pedalling a little smoother, but it's probably just the dry tarmac tbat's helping. Learning to relax the arms while sitting up will be my next drill.

I am still curious whether anyone can support or disprove my bb height theory, namely: the power stroke parallel to the ground does not help to "dig" the driven wheel down, as would be the case when the power stroke is closer to vertical (as an a df bike), thus contributing to esrly onset of wheel spin, other thing being equal.

 

[Andrei Petrov]

Here is another thought. Would we benefit from rear stay extenders on the Silvio to increase wheelbase? I am thinking something similar to the available chainstay extenders, but to elongate the rear triangle.

 

[MrSteve]

Mr. Petrov, Your question about power strokes and how they effect uphill traction is interesting.
The biggest factor in your question is where the power is delivered: Front or rear wheel.
Bottom bracket height has much more to do with personal preference and aerodynamics than it
does with power strokes.
Rear-wheel drive bicycles will always have the advantage over front-wheel drive up steep hills.
Work on your technique!

Would extending the wheelbase help? Of course, it would help the hillclimbing.
However, the Silvio is designed for the wheelbase it has:
Extending the wheelbase will change the handling characteristics of the bike.

The suspension of your Silvio soaks up road vibration and makes your bike
a superb grand tourer. It also soaks up some of your power, which makes your Silvio
a slower climber.

The Vendetta, with its longer wheelbase, better aerodynamics and lack of suspension, is the best FWD MBB climbing bicycle that Cruzbike makes.
Your Silvio is the finest grand Touring machine that Cruzbike makes.

 

[super slim]

Me We perfectly put!!

 

[Andrei Petrov]

Dear Mr Steve

I've never been so formal discussing bikes)))

I agree with your points, and improving one's technique, strength and general ability is always preferable in my view. I have been riding the Silvio for two years now and continue enjoying the progress along the learning curve. With that in mind, I am trying to make sense of certain limitations of the fwd mbb Silvio design that only became apparent to me once I started riding. I am not looking to swap for Vendetta or a rwd recumbent at the moment. I am not a fair weather rider, and ride throughout the year - wet, grit, mud and all. The climbing ability - as in loss of traction, not efficiency - is a real limitation for me, for now.

From where I stand, I could probably never scale a 25% climb on a Silvio despite my triple chainring and 11-36 cassette. That's OK, I use other bikes for those rides. But the risk of sliding out on a wet or imperfectly paved 10% climb with traffic is a real issue: I can not ride where I want to and need to ride.

I believe there are several riders here that have changed the bb height on their Silvios, and I am anxious to learn whether that made any noticeable difference to their climbing. This is one of the few variables I can work on with the bike set up. Changing the wheelbase, theoretically, is another. To the best of my understanding, the current Silvio wheelbase has been chosen in part to keep it 'standard' with df bikes. Perhaps for future design iterations of the Silvio platform the all-around, real-world versatility could get higher priority over standard wheelbase. That, and the ability to accommodate wider tires in 700c or 650B.

 

[MrSteve]

Dear Mr Steve


...
From where I stand, I could probably never scale a 25% climb on a Silvio despite my triple chainring and 11-36 cassette. That's OK, I use other bikes for those rides. But the risk of sliding out on a wet or imperfectly paved 10% climb with traffic is a real issue: I can not ride where I want to and need to ride.
...

Most of my experience with Cruzbike bikes was on my Sofrider.
The Sofrider (discontinued) I have has slick 650 tires, had an 11-36 cassette, has triple chainrings and front and rear suspension.
I've lost traction in loose sand, gravel and the worst was a slick patch of wet clay with algae that tossed me to the ground instantly.
The Sofrider came with a low BB which worked fine, but I felt that it was too low for me.
So I built a longer chainstay.
Now my Sofriders BB is high, about the same height as a Vendetta.
For me, the higher BB allows me to make more power, so with the high BB, the Sofrider climbs better than it used to when it was stock.
The higher bottom bracket makes a big difference for me.

Now, my Sofrider stays on the trainer.
The Vendetta is my road bike: Although the Sofrider is a better climber than any other local recumbent, the Vendetta is a much better climber than the Sofrider ever was.

If I was commuting like you do, I would be doing it on my Sofrider.
I'd mount big slicks and run them at low pressure for the best grip and comfort;
I'd also lock out the suspension.
Any suspension will absorb some of your power... experimenting with the Sofrider suspension proved it for me.
I can't ride a normal diamond frame bike, so I'm stuck with these recumbents.

Finally, if I commuted on a Silvio like you do, I would mount the largest tires that fit,
install a chainstay extender on the carbon fiber chainstay and see about disabling the suspension.

Hope this helps!
-Steve

 

[ratz]

Ok this is going to be a random hot mess, soo much to do...

and see about disabling the suspension

That's the first thing I would do, and it's easy with a $1 metal hose clamp.

For me, the higher BB allows me to make more power,

However I'm the opposite; If commuting on a Silvio, I'd get the BB down low and add some padding to the seat to get me up to 36-40 degrees. Much better power and balance that way for me personally.

But if commuting on what I have I would be using our Quirkr which are old Quests with the "long" chainstay and small fat wheels.....I have designs on a special S30 commuter project but more on that when the snow flies.


Something that everyone needs to remember when worrying about slipping on these bikes. On a DF when you corner you might lean at most 5 degrees, especially at commuting speeds. Most people lack the balance to do more, racers anything but a crit it's going to be 10 degrees limited by pedal strike on the ground. When we ride recumbents we take corners with at least 5 degrees usually 10 to 15 degree; racing it's 30 to 40 degrees. That much lean seriously compromises the grip and hold of the tire. While that's not related to climbing it's a reminder to think different, the bike is different. Up rights go over the bars head first; we wash out rear wheels with fish talling (see my rant on brol) and slip front wheels on slime. Both end with hips on the ground if you ride it down; and elbows and hands if you bail out. It's the nature of the beast study it learn and ride.

Andrei I commend you for sticking with it and thinking about it; not sure you can fix it completely for your roads, but you can maximize what you got through iteration.

 

[hoyden]

I am not looking to swap for Vendetta or a rwd recumbent at the moment.

I am staying with my S30 rather than a Vendetta and am considering selling my other two USS RWD recumbents because I don't ride them much anymore. S30 has limitations but my overall riding experience (fun, comfort, efficiency, performance) makes the other bents obsolete. I believe any performance delta S30 vs Vendetta would be incremental at best and limited by my physical capabilities (old lady on a bent). I also don't enjoy riding with the lower seat angle where I see out the bottom of my glasses. My normal ride doesn't involve grades that tax the S30's capabilities with respect to wheel slip or gearing. Thus the S30 with stock BB height delivers me the best bang for buck.