Cyclocross - Ultimate City Bike

I was thinking of building a FAST city bike. I don't think a regular Al road frame will cut it. Jupping up and down curbing might be a little rough on it along with the occasional dirt trail though the park and other above average abuse. I could be wrong. But I was thinking a cross frame might be the way to go. Put non-studded tires with an agressive tread, and I want disc brakes. Not a big deal, many frames have disc mounts. A bike that interested me was the Schwinn Super Sport DBX, why custom build when a bike has what you want? But I was wondering, is the frame on this a little more rugged than the fastback or is it the same tubeing just different geometry? Any help is appreciated.

how about
http://www.konaworld.com/shopping_cart/FrontEnd/product_detail.aspx?productid=80&parentid=182

damn
anyhow it sounds like the Kona Dr. Dew.

http://kona.dk/bikeindex.php?bikeId=10023

Try that one.

indeed.
muchas gracias

Steel. Steel. Steel.

Just curious...why disc?

Because rims get wet in less than perfect conditions and disc dosen't tend to get wet. plus they're more powerfull

I am currently saving to upgrade from my Trek 7200 to a Cannondale Bad Boy. My 7 mile commute is mainly through city and after looking at several options, including them konas I think the cannondale looks like the best package for my needs, would suggest you check it out.

Steel. Steel. Steel.

Just curious...why disc?
Why are discs better than rim brakes?

The difference is friction. Friction is of course the force acting against the momentum. Friction under all circumstances will be greater in a disc system than a rim system. Not even ceramic rims and their pads can compare to the sustainable friction of a disc system. Not to mention the effects of inclement conditions on rim brakes.

Let's start by taking a look at the physics involved. There's a law of physics that states how an object in motion has a certain amount of energy due to its momentum. This energy is called kinetic energy. In order for this object in motion to stop or slow down, it must lose some or all of its kinetic energy. It does this by converting the kinetic energy to heat.

It's pretty simple. At your wheel you have a metal disc and a set of friction pads. The pads squeeze or push onto the metal. When this happens, you create friction. Friction generates heat, of course. Since the wheel is turning, then the kinetic energy of your momentum is converted to heat at this point and discharged harmlessly into the atmosphere (with a slight loss of pad material), and your bike slows down. The faster it is going, the more heat is needed to stop it. The more pressure you apply to the pads, the faster it can discharge the kinetic energy.

Adding a larger disc aids in the discharge of the heat generated the increased surface area allows heat to dissipate more quickly

Rim brakes work well, but they have a hard time shedding heat well enough to prevent fade when used really hard. Brake fade occurs when the brake overheats dramatically; braking power is vastly reduced.
Facts

Disc brakes handle heat load and dissipation better than calipers.
They don't transfer the heat generated directly to the rim, like calipers.
Disc rotors are MUCH cheaper to replace than an entire rim (as low as $15). As far as being able to lock a wheel: yes you can lock a wheel much easier with a disc than you can a caliper of any type, however if your brakes are PROPERLY setup, you also have greater modulation with less effort than any caliper system ever invented.

Do I have V brakes on my road only commuter? Yes. Due, in no small part, to the fact that both my frame and fork are not disc compatible. As I plan on eventually (after my shock upgrade on the trail bike) swapping out the fork on my commuter for a rigid model that has disc tabs I will not be without the added all conditions stopping power of discs for much longer. It is also possible that I may even just get a fork with V-brake bosses as the current setup is adequate for most everything I encounter while commuting in Florida, but it's funny how things can change.


I reposted this from an earlier discussion on the matter

[color=blue]Why are discs better than rim brakes?

Don't forget that if you crash and damage a wheel or break a spoke or two you can get home easier with a disc brake.

DanO

Marin Pt Reyes:
http://www.marinbikes.com/bicycles_2005/html/bikes/bike_specs/specs_pt_reyes.html

REI Novara Express:
http://www.rei.com/online/store/ProductDisplay?storeId=8000&catalogId=40000008000&productId=47841669&parent_category_rn=4500865&vcat=REI_SSHP_CYCLING_TOC

Hard to beat a large diameter, one pound aluminum rim as a heat sink. More thermal conductivity, more massive, and with more surface area traveling at a higher speed than any bicycle disc. Admittedly, discs do have advantages, but heat capacity isn't one of them.

May not be relevant for most riders/rides, but have seen tires blow out due to too much heat build-up from rim brakes when coming down mountain roads. Things get ugly really fast!
Agian, not a problem for most rides - just food for thought.

May not be relevant for most riders/rides, but have seen tires blow out due to too much heat build-up from rim brakes when coming down mountain roads. Things get ugly really fast!
Agian, not a problem for most rides - just food for thought.


That's true. Such a desent would also seriously overheat a bicycle disc brake, but at least you wouldn't have a blowout. I was thinking mountain tourers should install a simple blow off valve on the valve stem. Size it for 1.5x the working pressure, and problem solved. Plus, you get an audible warning when the rim gets too hot. Eventually, you would learn to brake in such a way as to avoid excessive heat buildup.

Another thing to keep in mind if you are riding in the winter is all the sand that gets stuck between your rim and pads, quickly detroying the sides of your rim. That's why I'm going with a Sutra next winter. And stopping will be nice as well.

Thanks for the physics lecture, Raiyn. You're right in that discs have more stopping power. Incidentally I was a physics major in college and understand what's going on. Anyway, I was curious not as to the objective reason for why one is superior to the other (there isn't any objective reason establishing one brake superior to the other; it depends upon your criteria), but rather why the person who started the thread wanted disc. What is that person's specific needs and desires. The necessity for the additional stopping power depends upon the rider in his/her context.

For starters, the increase in stopping power should be valued only in relation to issues of the rider's body mass and overall utility. Frankly, disc brakes sure look cool but the added power isn't generally necessary, especially for city faring, in most cases. Heck, disc braking might be too much for most conditions. What's more, maintenance is a heck of a lot trickier with disc than with cantis or calipers, and with a city bike I'd think it would be important to value ease & cost of maintenance. I'd rather tune a caliper than a disc. "Properly" is easier said than achieved. Further, those big disc tabs sure make fenders and racks damn difficult to add--two elements I find crucial to dedicated city biking (like when you grocery shop by bike in the rain). All that for discs. And the dish on the rear wheel in order to accomodate for the disc brakes makes that rear wheel weaker, less stable.

With that Kona, I think you end up trading the opportunity for a more sensible city frame (STEEL!) in exchange for a cheeeepnis jarring Al frame and some fancy looking brakes. That bike admittedly sure looks boss. But failure to adequately dissipate heat while braking isn't exactly a likely scenario for catastrophic brake *failure*, is it? Maybe racing down a steep San Francisco hill it might, especially if you weigh 300lbs... Otherwise, the fear of caliper or canti brake failure is not altogether approriate.

I have a city commuter with disc brakes and I just replaced the damn thing with a custom buildup with cantis. OK yeah, a little slower in the stops, sure, but I'm city riding for starters and not that big. I'll be ready for new wheels when the brakes wear the rims out. And I've got fenders and a rack on this bike, and even do my grocery shopping on it. And I've got drop bars & a bar end shifter (single speed front). Bike is a city dream....

Anyway, it all depends on what sort of riding the rider intends, where, etc. "city" means a lit of different things to different people.



Why are discs better than rim brakes?

The difference is friction. Friction is of course the force acting against the momentum. Friction under all circumstances will be greater in a disc system than a rim system. Not even ceramic rims and their pads can compare to the sustainable friction of a disc system. Not to mention the effects of inclement conditions on rim brakes.

Let's start by taking a look at the physics involved. There's a law of physics that states how an object in motion has a certain amount of energy due to its momentum. This energy is called kinetic energy. In order for this object in motion to stop or slow down, it must lose some or all of its kinetic energy. It does this by converting the kinetic energy to heat.

It's pretty simple. At your wheel you have a metal disc and a set of friction pads. The pads squeeze or push onto the metal. When this happens, you create friction. Friction generates heat, of course. Since the wheel is turning, then the kinetic energy of your momentum is converted to heat at this point and discharged harmlessly into the atmosphere (with a slight loss of pad material), and your bike slows down. The faster it is going, the more heat is needed to stop it. The more pressure you apply to the pads, the faster it can discharge the kinetic energy.

Adding a larger disc aids in the discharge of the heat generated the increased surface area allows heat to dissipate more quickly

Rim brakes work well, but they have a hard time shedding heat well enough to prevent fade when used really hard. Brake fade occurs when the brake overheats dramatically; braking power is vastly reduced.
Facts

Disc brakes handle heat load and dissipation better than calipers.
They don't transfer the heat generated directly to the rim, like calipers.
Disc rotors are MUCH cheaper to replace than an entire rim (as low as $15). As far as being able to lock a wheel: yes you can lock a wheel much easier with a disc than you can a caliper of any type, however if your brakes are PROPERLY setup, you also have greater modulation with less effort than any caliper system ever invented.

Do I have V brakes on my road only commuter? Yes. Due, in no small part, to the fact that both my frame and fork are not disc compatible. As I plan on eventually (after my shock upgrade on the trail bike) swapping out the fork on my commuter for a rigid model that has disc tabs I will not be without the added all conditions stopping power of discs for much longer. It is also possible that I may even just get a fork with V-brake bosses as the current setup is adequate for most everything I encounter while commuting in Florida, but it's funny how things can change.


I reposted this from an earlier discussion on the matter

Money no object I think I'd want one of these:

http://www.bikes.com/bikes/2005/images/05-rc-st.jpg

A Rocky Mountain ST RC, it's just over 2K CDN I think.

My 05 Kona JTS works good as a city bike... but I think a flat bar and disc brakes would be much better. In urban riding I want brakes that stop on a DIME and the more upright possition a flat bar offers gives better visibility.

... Incidentally I was a physics major in college ... As was I, and as is my elder son. :)

May not be relevant for most riders/rides, but have seen tires blow out due to too much heat build-up from rim brakes when coming down mountain roads. ... Years ago, I was warned about tubular tyre glue melting and tubular tyres rolling off rims on long descents, such as Tuna Canyon (Malibu area of Los Angeles CA). The threat of rim and/or tyre overheating was one reason tandems usually had supplemental rear brakes (originally hand-controlled drums), which could be engaged to check speed on long descents.