racing tire advantage

Discussion in 'Bicycle Repair' started by velzie, Dec 20, 2013.

  1. velzie

    velzie Member


    I do not race my bike nor plan to, but:

    Why do those who race use thick-walled 2"+ tires? I will be putting 700C rims/tires on a modified road bike after riding on a mountain bike with 1.95" tires. I believe this will help increase my top speed on a 66cc Skyhawk.

    I just don't know what the advantage is for racers. Wheel and tire wear? Comfort? Cornering?

    Thank you.

  2. grinningremlin

    grinningremlin Active Member

    Race tires have the least rolling resistance, and to me superior traction, given the right conditions.I have a 1984 Fuji Opus III, it runs tubulars and you feel like some death defying hyper-speed billy goat.There's hardly a half inch of rubber contacting the road but I can throw myself around nimble as you please, and going fast requires little initial force, I push off and I'm already flying.On an MAB it's just so much more force in every aspect, not to mention sometimes you're going so fast you can't avoid those potholes that would kill skinny rims pumped to 110psi, it's easier on you and the whole rig with fatter tires/wheels.That being said I'm considering motoring the Fuji myself.
  3. jaguar

    jaguar Well-Known Member

    the fatter the tire the more tread is in actual contact with the road.
    and it acts as more suspension.
  4. velzie

    velzie Member

    AH! The all-mighty Jaguar!! - I have been attentively studying your webpage and have many planned modifications to make over the winter. I look forward to a slightly faster and much more reliable machine.

    I guess my bike/tire combo is going to be more of a personal thing. I was looking for a particular reason why the racing-type mountain bike tire would be advantageous over the 700C. My mountain bike was a fine fit for the 66cc Skyhawk. With the 44 tooth sprocket I could climb the steepest hill around. I just think the lighter road bike will accelerate better and the larger diameter, thinner high pressure tires will help top speed a bit.

    GrinningGremlin - Thank you. Comfort, traction and rim damage are things i will have to asses after the initial build.
  5. grinningremlin

    grinningremlin Active Member

    Maybe I misunderstood the question.Skinny high PSI tires will most certainly make you speedier, a 700c has advantage on smooth pavement.If you run mainly pavement then there's no need for any knobby at all, if you hit the dirt you can get by with some enduro tire.I run Schwalbe Marathon Plus tires, they are basically a slick street tire for touring (super tough), they will slap right on your MTB rims and give you a faster/smoother (less rolling resistance) ride than a normal aggressive MTB tire.If you're still going 700c just get good ones as you'll be more prone to flats/rim damage .A member here (geebt48cc) has a few frame mount street racing bike builds (700c) here if I'm not mistaken.
    velzie likes this.
  6. FurryOnTheInside

    FurryOnTheInside Well-Known Member

    As it's a general bicycle question I guess I can chip in. :)
    As long as you use slick (no tread, road tyres) tyres and you ride on smooth surfaces like roads, your rolling resistance won't change much with a larger diameter tyre.. this is how Bromptons and other small wheel bicycles can still be practical.
    You may need to reduce your gearing in order to match the acceleration you had with 26" wheels, since the larger diameter of the 700C wheels is actually increasing your gearing.
    Top speed in a straight line is mainly limited by air resistance, which is only slightly reduced with a narrower tyre and rim. Rolling resistance (which wasn't much to start off with) becomes fairly insignificant as air resistance increases at high speed. Reducing tyre width excessively can be counterproductive for reasons already stated by GrinningGremlin.
    For more perspectives regarding tyre width and pressure on roadgoing bicycles carrying significant unsuspended weight, read touring bicycle forums.. but they generally agree with G.G.'s last reply "run Schwalbe Marathon Plus tyres". (They come in 26" AND 700C) :) Use Sheldon Brown's tyre pressure calculator as a guide too. :)
    Jaguar's comments are quite correct, of course, in the context of MAB racing as I've seen it portrayed on MBc. I prefer 24" x 2.3" slicks for downhill street BMX (racing myself, high speed, short straights, tight switchbacks, extreme danger, fun!) due to the low centre of gravity, low weight (from reduced circumference compared to 26"), wide contact area, even the advantage in leverage (low radius) when braking makes a difference.. well I am still alive anyway. :p
    velzie likes this.
  7. velzie

    velzie Member

    Furry: when you mention diameter, you mean like 26", 27" diameter and not tire width? It really doesnt matter: As i understand, rolling resistance has to do with how much of the tire is in contact with the ground. So a high PSI should lower the resistance for any slick tire. The tires on my mountain bike were quite slick, and I kept them at their maximum PSI.

    I guess the change over to my road frame with 700C tires might not be as advantageous as i though. The apparent gear-shift due to the larger tire may increase the normal operating speed, but may reduce the ability to climb hills. Thinner tires and rims are more likely to be damaged at higher speeds and will no provide much comfort. Most of all, my posture on both bikes will be similar, so the wind resistance should not change much. The only thing that still seems to be in favour of the switch is the weight difference between frames. It is about 10 lbs which still may help with acceleration.

    I was looking to build more of a on-road enduro-type motorized bike than a dirt bike. All of this is may also change since I plan to upgrade the motor for higher rpm.

    Thanks again for your advice Furry & Gremlin. Im going to try and find geebt48cc
  8. FurryOnTheInside

    FurryOnTheInside Well-Known Member

    Yes, when I mention diameter I mean so-called 26", 700c etc, on fairly smooth tarmac roads a small difference shouldn't really matter all that much.. but tyre choice is a little complex and is subject to several trade-offs between different variables/effects, and eventually comes down to personal choice.

    Rolling resistance is determined by a number of factors, not only the contact patch area (skateboard wheels will not roll faster than 622 (700c) x 32mm, for an extreme example)

    The wheel hub rotates faster with a smaller diameter wheel so on a very smooth surface this would be noticable at high speed on super-smooth surfaces like velodrome tracks, but the general preference in road cycling for larger diameters is more to do with gearing (tiny wheels require unfeasable chainwheel/sprocket sizes on pedal cycles), which is a trade off against rotating mass.

    "The apparent gear-shift due to the larger tire may increase the normal operating speed, but may reduce the ability to climb hills."

    absolutely correct! Calculate your increased circumference and you can calculate what size sprocket you need to get back to your previous gearing.
    "The only thing that still seems to be in favour of the switch is the weight difference between frames. It is about 10 lbs which still may help with acceleration." 10lbs is about half the dry weight of your motor kit which was a significant unsuspended mass; A reduction in unsuspended mass will help top speed as well as acceleration, braking and turning.

    Narrower tyres, if lighter, will reduce rotating mass and therefore help your acceleration, top speed and fuel efficiency (but need to be correctly inflated). Not all narrower tyres are lighter of course, nor are they all more easily damaged- yes if all things are equal (tread thickness, thread count, sidewall thickness, bead material..) but there's a lot of different tyre choices out there. Narrower tyres will cause less friction, so will lose less energy this way. Friction converts kinetic energy to heat. Some tyres are very narrow, but very well constructed, and very heavy (which makes a larger heat sink, if you need to reduce heat build up), some are lighter. Check these ones for example:

    Running the maximum PSI as labelled on tyres is not always an advantage, especially on rougher roads, due to the unsuspended weight or even with just the rider's weight. Over inflated tyres will make for a juddery ride, which actually means increased resistance due to the weight of the bicycle and rider being lifted over every tiny bump, as well as stress on bicycle parts and on your body (but your body is already being vibrated by your motor!).

    This table is for 700c (622mm ISO rims) on pedal bicycles (with no unsuspended cargo) but may help as a guide:

    Tire Width=20: Pressure(psi) = (0.33 * Rider Weight in lbs) + 63.33
    Tire Width=23: Pressure(psi) = (0.33 * Rider Weight in lbs) + 53.33
    Tire Width=25: Pressure(psi) = (0.33 * Rider Weight in lbs) + 43.33
    Tire Width=28: Pressure(psi) = (0.33 * Rider Weight in lbs) + 33.33

    Tire Width=32: Pressure(psi) = (0.17 * Rider Weight in lbs) + 41.67
    Tire Width=37: Pressure(psi) = (0.17 * Rider Weight in lbs) + 26.67

    Example: You are 150lbs running 28's

    Pressure (psi) = (0.33*150) +33.33 = 82.83psi (rear)
    Front Pressure = .9*Rear Pressure = .9*82.83psi = 74.55psi front

    Sorry this is so horribly complex! You will find your happy medium or sweet spot, which may not be the same choice as even the same weight rider on the same bike on the same road. Just have fun trying new stuff! :)
    velzie likes this.
  9. velzie

    velzie Member

    Thank you again. Those thickslice tires look amazing. I wont have a working bike until the spring since there is so much want to change with the bike and motor. This hobby has been around for quite a few years and there is much information to learn on these forums.

    Off topic but, I visited Wales in September of 2011. Some of my family grew up in Pwllheli. Beautiful place.
  10. velzie

    velzie Member

    road bike frame.jpg

    This is the frame I want to mount the motor on... The seat tube is about 25.5" and the top tube is about 3' from the ground. Those are 27" rims with Continental ULTRA SPORT 27 X 1 1/4" wire tire. I am about 6'3" so the frame is a good size.
  11. HeadSmess

    HeadSmess Well-Known Member

    now i can butt in with one of my overly lengthy lectures :)

    tyre design is pretty complex.

    to answer why a racer may prefer fatter tyres despite skinny having least resistance, i think that was covered...hitting potholes.

    now, you said thick walled. that means that they dont flex much. theres less deformation at the contact patch, which is added friction as the tyre revolves and the tyre is constantly deforming then springing back in place... much the same as pumping the pressure up high.

    thin walls and high pressures would seem to be the go, as the minimal rubber has minimal friction, and air, well, it doesnt mind deforming too much :)


    theres the roll over in corners, or the way the sidewall tends to bulge as you increase the side load on the bike... it can be really disconcerting coming through a corner and having the rear end feel "loose". think what its like when you have your tyre nearly flat! the front just tends to wash out...

    as the sidewall height increases, this effect gets worse...

    add more weight to the bike, it gets worse...

    tread pattern on a pushy isnt really a big issue. the contact patch is so small, hydroplaning due to slicks is virtually impossible, and on loose gravel...youre going to be slipping around anyway. big chunky stuff is good on mud and wet grass mind you :)

    the fatter the tyre, the larger the contact patch, BUT...keep the load the same, and the PSI loading on the contact point is much higher on the skinny... much harder to get it to let go. on the other hand, it sinks into anything soft... yep. big fat tyres on a car just make it easier to lose traction! but do sorta look cool...

    which then leads to the final thing i can think of.

    a big wide contact patch "scrubs". this is a function of the tread radius, or profile.

    the tyre is its own differential!

    think about it. the centre is the widest point, and travels the fastest. you lean over, and the outer point, or centre of the tread, has to travel slightly further than the inside point, thats somewhere between centre and sidewall...

    these four radii (two from the axle to contact points, two from turning circle centre to contact points) need to be spot on with the amount of lean, to corner really neatly :) so a lot of development goes into the suitable pressures,curvatures, rubber...on and on and on...

    thats what i HATE about can be upwards of a 700 dollar experiment on tyres... you know when you get good ones though. currently i like pirelli the best... not a chicken strip in sight, front or back!

    and, due to differences in frame geometry, what suits one wont suit another...

    consider a plain steel drum, 44 gallon if you will. it wants to just go in straight lines. whereas old timber barrels are curved, so you can lean em into the turns! (train wheels are also tapered in the same manner, for the bends, with flanges so they dont fall off {on that note, thats the one point on a train that ALWAYS travels backwards!})

    youll notice car wheels when rolled around want to go straight as well.

    watch a three axle truck trailer take a tight turn! theres a reason the third axle usually lifts off the ground when the things are unloaded. saves the wear on every turn. truck tyres cost a lot. but all the rear tyres cop a hiding, being doubled up and everything...something is always scrubbing, and at least one set of tyres is being slid sideways with doubled axles!

    :jester: class dismissed :jester:

    mainly, its a bit of personal preference. we all ride a bit differently :)
  12. velzie

    velzie Member

    Thanks for that, HeadSmess. I think I have reached my conclusion.

    Since the bike is no longer being powered by a human it seems like all the advantages of a road-type set-up diminish. The comfort, safety and reliability of thicker 26" rim tires when maintaining higher speeds seem to outweigh the slight advantages that road tires may provide. The output of the motor is not much affected by the tire weight as much as being under human power. The only advantage the road bike maintains is reduced weight overall.

    All in all I still feel like putting my 66cc Skyhawk on the road frame I posted earlier. The tires I have already are 700x35 but I want to put biggest tires I can fit onto that frame, possibly 700x42. It may be stubborn of me, the motor fits my mountain bike frame with 26" tires well, but I think that is what a hobby is all about - seeing how much personal 'ridiculousness' you can get away with.

    Thank you all for your input. I may put the rest of the build up in the proper section later.