Thomas15

I guess a lot of it has to do with the kind of bicycle and the kind/location of bicycling?

cyccommute 

While you are correct that the wheel won't lock up and slide, what Retrogrouch is describing is wheel lockup. The wheel stops spinning and the rider is thrown forward over the bars. The wheel can't slide like the rear one can because the friction between the pavement and the tire is too great for us to cause the wheel to slide. The reason for this is that the bicycle and rider are part of a system that is lightweight and has a high center of gravity. Before the rider can slide the front tire, the center of gravity rotates around the hub (actually a bit higher but close enough) and the rider goes over the bars.

Cars, trucks, and even motorcycles all have low centers of gravity compared to the passenger and they can't rotate around the hub of their wheels. The rear lifts but it can't lift high enough to flip the vehicle forward. The result is that they can overcome the friction on the front wheel and the wheel slides.

The rider is the key to the problem in bicycles. As noted above, the rider is the reason the center of gravity is higher. In a normal riding position, there is a maximum amount of deceleration that a rider can develop before the system rotates around the center of gravity and the rider it thrown over the bars. The maximum is around 0.5g or about 16 feet/sec^2. Moving the center of gravity rearward and down (only a few inches in both directions) increases the maximum to about 0.9 feet/sec^2 before the CG spins around the hub. Moving back and down won't make it so that the front wheel will slide but it gives the rider more time before they go over the bars.

HTupolev

If you're going extremely slow and upright while grabbing a fistful of brake, it's possible to rotate the system forward and bring the front wheel to a stop by the time you're pitched off the bicycle. But I wouldn't describe this as "lockup", because the front wheel merely underwent normal braking behavior until it happened to come to a stop. The issue in this situation isn't that the front wheel stopped, it's that you rotated the bike+rider system forward until you fell off the front of it; the front wheel coming to a stop commonly happens in this sort of crash, but it's not a necessary condition.

fietsbob

Another Episode of a Pre Worry post

taylorgeo , you can make long black skids on pavement with a rear wheel 'locked up' with a coaster brake,
like you may have done as a child, if you wish..

or drag the back wheel in dirt . One would hope you can learn to feather the front brake.. to not Endo .. do wear a helmet..

a controlled front lockup is a Stoppie, it's like a front wheelie I'd work my way up to that with a lot of practice..





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cyccommute 

You have to have some momentum to be pitched over the bars from flat ground. The center of gravity has to move forward over the front hub which means lifting the bike and rider up. That’s not going to happen while going extremely slowly. You can get a pitch over at slow speed if the rear wheel is higher than the front wheel like on a steep descent but not on flat ground.

For a pitch over to happen the front wheel has to stop rotating. There’s no other way to get the rider over and out in front of the front wheel. That’s not “normal” braking behavior.

You can rotate the rider forward while the wheel is still rolling to do a nose wheelie but you need enough momentum to lift the rider upwards and you need the wheel to stop momentarily enough to do that lift and then a quick release of the stopped wheel so that it can roll again. But, generally, if you are going to go over the bars, the bike’s front wheel has to stop. It may only be momentarily but it will be at zero forward momentum with the center of gravity still having forward momentum.

cyccommute 

In that bike you have all the tools you need to learn how to deal with any pothole you’d encounter on a road way. Take the bike out and ride it like it is designed to be ridden...i.e. on dirt, rocks, and roots. Mountain biking will teach you more about bicycle handling in an afternoon than road riding will teach in a life time. Road bikes allow you to ride with bad habits while mountain biking will punish you for those very same bad habits. Yea, you’ll get some good scars out of the lesson but scars are tattoos with better stories.

Go! Have fun!

Gresp15C

That design might be part of the reason why so many people think that disc brakes work better than rim brakes. For ages, my spouse complained that the brakes didn't work on her hybrid. Finally I completely disassembled them, found the springs, and removed them. On the other hand, some amount of flex in the mechanism is what the enthusiasts call "modulation," but there's already plenty of flex in cheap brakes without adding more.

fietsbob

Yea there is a Modulator 'noodle' for V brakes as described 1st you compress the spring
then that moves the brake arms towards each other squeezing the rim.

some clever hacking may fit that in a cable disc brake scheme,

but shops liability insurance company may not have them comfortable in that adventure..






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HTupolev

We might be visualizing this on difference scales. I mean compared to someone who's tucked in on a fast road descent, where it takes a huge amount of braking force to get the rear wheel to rise. That's partly from posture, and also partly from the aerodynamics pushing back on the rider's body.

The physics of a pitch over do not require that the front wheel stop moving, they simply require that the rider be rotating forward relative to the bike.

The only thing making the core physics different between the wheel-rolling-forward case and wheel-stopped case is the amount of aerodynamic force pushing against the forward rotation of the rider's body relative to the bike.
There's also the practical matter of it being easier to over-actuate the brakes if the wheel does come to a stop, since static friction between pads and rim/rotor is stronger than sliding friction. (But this isn't a fundamental requirement of pitching forward.)

By "normal braking behavior", I mean that the brake itself doesn't have any fundamental change in behavior. If the wheel stops, it stopped because the brake decelerated it in a totally normal way until it came to a stop, not because any kind of mysterious "lockup" action occurred.

taylorgeo

Thanks for the reassurance! Some other members have recommended I not even ride a bike at this weight. And I totally respect where they're coming from.

However, you're confident this bike will hold my 350 lb. frame?

Found this in the Kona Manual, just wondering if I'm reading it correctly...

cyccommute 

It takes less momentum to pitch over the bars on an angle. Someone tucked on a fast road descent is going to have the center of gravity lower but it will be further forward. Pushed back off the saddle, the CG is both lower and further back. Thus the wheel is harder to lift.

But your scenario wasn’t about a fast descent but a slow level ground pitch-over. That’s highly unlikely because it does take some momentum to lift the rear wheel and move the CG forward.

Yes, they do require the front wheel to stop moving. Move your CG forward while the wheel is moving and there is zero chance of the rider going over the handlebars. If you put the CG far enough out in front of the wheel, you might be able to get the bike to tip forward but you’d have to be a long way out in front of the wheel and the wheel would have to be slowed significantly and quickly to initiate the pitch over.

My Dad used to be able to sit on the handlebars and pedal the bike while making it move forward. In that case, his CG would have been at or in front of the handlebars. As long as the wheel is moving, the rider is in no danger of falling in front of the wheel.



Aerodynamic force plays no role in pitch over. It’s a force but it is very weak.

That’s what I’m talking about. You can’t slide the front wheel because there isn’t enough force in a lightweight, high CG bicycle to cause the wheel to slide. The rider rotates instead.

The brake doesn’t have to do anything “mysterious” to lock up the wheel. Apply enough force to stop the wheel with a high CG vehicle and pitch over is inevitable. Moving the CG back and down gives you more wiggle room. Every mountain biker knows this in their soul. They practice it on every ride, probably dozens of times. The reason you see so many of them pushed back off the saddle is because they are trying to keep the rear wheel on the ground...usually on a descent. Strictly road bikers don’t do this because they don’t usually have to. When they do, they don’t understand the problem and mayhem usually ensues.

taylorgeo may or may not have a problem with braking. Knowing nothing about height, his CG may be low enough to make pitch over to be less likely. A greater mass is also harder to lift so he’d need more momentum to get the weight shift needed. On the other hand, the CG may be higher because of the extra mass (I’m trying to dance as delicately as I can around this, taylorgeo) is carried higher. The brakes will have to work harder in either case but with a higher CG, pitch over is going to be more likely. Steep descents are positively going to require more vigilance while braking.

Luckily...to at least put some shine on this, taylorgeo....it’s really easy to learn how to brake properly. Pitch over can occur but it doesn’t occur all that often.

cyccommute 

I think you’ll be fine. You are going to be rough on the equipment and you will likely have to replace things down the line. Wheels will be an issue. It looks like the wheels are 32 hole and are probably straight 2.0mm spokes. Start saving your money for a set of wheels that are built with 36 hole hubs and have them built with DT Swiss Alpine III spokes. They will last longer. They won’t be cheap but they will be more durable.

You might even work with your dealer to see if they can get a 36 hole wheel (not with the Alpine spokes) before you buy it. It will cost some money but you’ll get a better wheel that will last longer to start.

taylorgeo

What about these two bikes for my 5'8", 350 lb. frame? The Trek 820 & Jamis Trail XR. Under $400 bucks.

Both have steel frames, 26" wheels (36 spokes). The rims are single wall, though.

rosefarts

They're all kind of the same.

Can we assume that the 350 is a temporary weight? After some rides and healthy choices, it'll start coming off? Or are you pretty much set at that weight?

You'll be hard on anything that moves. I'd personally be inclined to go for the one without suspension, since at your size, suspension might bottom out alot and not help.

I think you definitely should test ride everything and build a relationship with your local shop.

fietsbob

In the larger picture wheels & the whole drive-train are 'consumables' in normal use you replace them..

Choose the Dealer you like first not the brand of bike, they can all get you a similar bike, service after the sale is a difference..





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subgrade

I'd say 32 spoke double walled wheel will be stronger than a 36 spoke single wall.

dabac

IMO, those were/are quite silly.
All they do is that for a short section of lever travel, the increase of the pinch force of the brake is determined by the spring rate rather than lever travel. Once the spring is fully compressed, you’re back to square one.
While I can understand what they were intended to do, I can’t quite see how they were meant to get there.
Perhaps they’d make the brakes safer for someone who uses lever travel as their feedback mechanism:”oo, need to brake a bit harder. Let’s pull the lever in by 1/4” more”.
I’ve never done that. Don’t know if anyone does. IME, people judge braking by what happens, not by amount of lever or pedal travel.
They’re still around. But I think they were more of an CYA feature than ever intended to be truly useful. Perhaps if they were available in several different spring rates to match rider weight etc

subgrade

I suppose they are made so that the spring doesn't get fully compressed, otherwise they'd serve no purpose indeed. As for their usefulness, I've ridden a bike with one, and the brake worked well enough and the damper spring was barely noticeable. That said, I didn't test it by doing white-knuckle panic braking which is what the device is for, I assume. I see lots of folks riding around with the front brake diconnected, apparently in fear of going OTB, so if such a device could encourage them to use the front brake, it can be useful.

dabac

Nah, installation instructions specifically state that the brake should be adjusted so that the ”brake modulator” is fully compressed BEFORE the lever bottoms out against the bar.
Anything else and you’re limiting the ultimate power of the brake, which seems like a questionable thing to do.
As I said, for a section of lever travel, the spring in the ”brake modulator” determine the rate the brake’s pinch force increases. Perhaps braking vs lever travel gets a tad more linear across the range of motion.
With all other things that influence rim brake response, the influence or a ”brake modulator” is indeed small.
All I’ve ever tested have compressed fully long before reaching white-knuckle territory. And assembly instructions say they should be fully compressed before the lever bottoms out.
Having seen a few OTBs in commuter traffic, I don’t think a weaker brake is the way forward. The ones I’ve seen have all started with the rider draping him/herself across the bar. At that point, it really doesn’t matter much if the rider is carrying enough speed to do the full 180 or merely flops over at an undetermined angle.
In order to save those who clamp down on the brake w/o bracing themselves against the bar you’d need some load-sensitive device built into either the bar, or the rear wheel. And then wired in to reduce brake force if those sensors go out-of-bounds.

cyccommute 

While these have more spokes, they are freewheel wheels which are prone to axle bending. The Kona has a freehub which is a much stronger mechanism. Neither broken spokes nor bent (or broken) axles are desirable but I’d take a broken spoke over a bent axle for repair. The Kona has a bit better parts spec as well. The Kona is a better choice all around.

taylorgeo

Hey, thanks so much for reviewing the specs on these bikes. Still educating myself, and had no idea about the freewheel issue. The last bike I had was a Hutch BMX in 1983! You've been a tremendous help, cause I'm probably going to buy this Kona blindly. It's extremely difficult getting your LBS on the phone right now, much less going there to try a bike out.

Kapusta

I think a good rule of thumb is to let off on the front brake if your rear wheel leaves the ground.

BlazingPedals

Drum brakes are used as drag brakes for tandems, in some recumbent trikes, and are very popular for velomobiles. Their advantage is that they are extremely low maintenance and have lots of thermal mass for heat dissipation.

I think having a heavier load makes it harder, not easier, to lock up a wheel.

cyccommute 

Yup. Mountain bike first principles.

taylorgeo

Appreciate the suggestion. It's funny cause I actually did purchased a bike two months ago, blindly. Which was a bad move, but luckily the LBS took it back for a full refund.

I bought the Specialized Roll for the upright seating position and their "Ground Control Positioning" that makes it easy to put a foot down when stopped. However, my knees were in my chest with the low saddle height, and I literally could not pedal the bike. So I raised the saddle so I could extend my knees better, then I ran into another problem: I could not touch the ground while on the saddle and had to straddle the top tube when coming to a stop – which did not work at all. I was squashed in the cockpit. I'm not sure how many people know this, but there is MUCH LESS ROOM between the front on the saddle and the stem on a Comfort Bike. The REACH on the Roll is 352mm. I think girthy people are better off with a Mountain Bike. For example, the Kona Lana'i has a 440mm REACH – which I'm assuming will give me a bit more room when I come to a stop and straddle the top tube.