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Front VS Rear wheel width

Discussion in 'Technical and Troubleshooting Torque' started by gegvasco, Nov 29, 2005.

  1. Was watching a race on the weekend and was thinking about the traction being generated by the front tyre when these guys were going flat chat around corners and it got me thinking about bike design. If more contact patch is important for traction during braking(hence setting up) then why is the front tyre so much narrower than the rear, especially on more powerful bikes. Surely a wider tyre would lay more rubber down.

    My newbie mind thought it may be because the rear tyre doesn't need to turn the bike so greater weight(ie. bigger tyre) gives more stability once moving whereas with the front tyre needing to turn, a wider tyre and more mass would make the front wheel too hard to turn once rotating at speed.

    Is this theory remotely close to the mark? Over to the gurus.
  2. I'll have a go :)

    It's more to do with the forces exerted under braking. In short, the answer is because that's all they (front tyres) need to be :)

    With modern bikes, under heavy braking 100% of the bike's weight is transferred to the front (if I remember correctly, that's about 1g of braking force, with modern bikes able to brake with a maximum force of 1.2g)

    The dampening affect of suspension activates so that only around 50% of that braking force is transferred to the tyre - the front forks absorbing the other ~50%.

    As the sidewalls of the front tyre flex out, and the contact patch doubles (again, if my memory is correct - under heavy braking front tyre contact patch doubles in size compared with normal non-braking riding), this area of rubber on the road, with it's tread pattern design to support braking forces and combined with the absorbtion of force from the front forks, leaves the front tyre more than capable of handling around 0.5g of braking force.

    Contrast that with the amount of force/torque produced from a powerful engine, the rear suspension nowhere near as strong as the front forks, and g force onto a motorcycles rear tyre now approaching 2g (again, from memory) and you see why the contact patch at the back of a bike has to be larger.

    To summarise, more g force is exerted by a motorcycle at the rear under acceleration, than at the front under braking. Thus, the tyre contact patch at the rear has to be larger than at the front.

    I'm happy to be corrected on any of this by someone who has a better grasp of physics than I do :)
  3. My physics teacher always claimed surface area had no effect on friction.


    Wider front tyre, if nothing else, would slow your steering fairly dramatically, and completely change the handling characteristics of your bike. I'm not sure it would really be a viable option if only for that reason.
  4. no that can't be. friction depends on surface area AND texture/quality of the two surfaces in contact. correct me if i'm wrong but that's just common sense. ie you want smooth dry road to be in as much contact with your sticky tires as possible for max. traction.

    i think smaller front tire has more to do with steering. easier to steer a smaller wheel than a big fat one
  5. friction coefficients are independant of area.

    Total friction FORCE is a product of the load on the tyre and the area presented to the ground and the friction coefficient, which can be regarded as the stickiness of the tyre/road interface.

    front tyres are smaller because of two reasons:

    1. sharper profile is easier to destabilise a bike in order to encourage it to turn. (you can already get loads of braking force out of it so that doesn't matter)

    2. in a turn, the front wheel has lateral loads only (ie across the tyre from side to side, and nothing front to back). A rear tyre is expected to do this plus an acceleration load to reduce overall weighting about 40% front, 60% rear. This is the reason you should never front brake is a corner and always be on the gas a little when in a corner.
  6. Oh, and this fails basic equilibrium physics

    yes, braking is a dynamic force exercise, but once you reach the bottom of suspension travel for the braking load (ie reaching equilibrium again, but at a different 'sag' if you like owing to higher load on the front wheel) all load has to go through the tyre AND the forks.

    The tyre does ALL the work in stopping the bike - you cannot get a force acting along the longitudinal axis of the bike from nowhere (or magically from the forks)


    the comments about area spreading under load for the front are correct, the rear suspension being 'weaker' than the front is not.

    To my knowledge, there has not been a racing or road bike/car designed to go around corners that can accelerate faster than it can stop. I doubt you can get 2g acceleration and only 1.2g braking loads out of a gp bike (but i could be wrong). The biggest problem with bikes is keeping the end of the bike not doing any work on the ground. The bigger contact patch of the rear is to do with load transfer - rear is expected to do about 60% and front the remainder.

    Keith code explains this very well in twist of the wrist btw
  7. LOL :LOL:

    In laymans terms as I see it, the bigger the contact patch the less pressure per square inch is applied, so the contact patch doesn't necessarily provide more grip per se. What it does help to do is suck up inconsistancies in the road surface. If you have a tiny tyre and hit a pebble on the road, that pebble may take up 1/2 of your contact patch. With a wider tyre, the same pebble may only take up 1/4 of your contact patch.

  8. Actually if bikes didn't make 200 ps (or even 100ps) then the back tyre would be similar in size and width to the front tyre.

    That has benifits in improved handling... but the high power output of the modern motorbike would 'spin up' the rear tyre with a 120/130 profile rear tyre.

    Of course GP bikes 'spin up' the rear anyway... but have a look at post classic (140ps CB750's with 2.5 inch tyres) if you really want to see spinning up... those things need big balls and quick reactions to ride well.

    Imagine how a 250ps bike would spin up a 130 profile tyre and how quickly it would be destroyed and you have most of the reason for gradually increasing tyre widths... it's gradually increasing power outputs!
  9. It's interesting to note that a few years ago, the R6 had a smaller profile front tyre, which they changed to a larger one (in 2004 I think?) the result was quicker steering and better handling. So I'd guess from that simply stating a smaller tyre means quicker steering isn't always true.... who knows
  10. the contact patch is determined only by the load on the tyre and the internal air pressure of the tyre. The comment that "the bigger the contact patch the less pressure per square inch is applied" is not directly applicable as the recirpocal of the first argument. ie, as an engineer, I can choose a tyre pressure and load to get a resultant area, but I can't fix an area to get a specific ground pressure if the other two are fixed already.

    'sucking up small surface inconsistancies' is also related to the rubber compliance (material characteristic) and tyre pressure - crossing pebbles may be a side effect but it is not a design consideration.

    It all boils down to loads - bigger heavier bikes have bigger tyres, but the smaller bikes can go around corners just as fast. The size difference between front and rear is more or less constant between all classes of machine (with exceptions of really small bikes like the posties or those with so little torque that you would find it hard to overload the rear tyre in a corner) and has to do with expected weight transfer on throttle going through a turn.
  11. I suspect that is more related to a handling limit issue... if the front end is 'pushing' because it's on the limit of traction then a wider front will improve handling.

    But again... it would only be pushing because of the high power output going through the rear end.

    Sweetest handling results from matched front and rears because the centre of one contact patch doesn't end up displaced away from the angle of the turn differently to the centre of the other contact patch.

    This won't be the fastest method... but it will be the most natural feeling.

    It depends upon if one is seeking the fastest way around a track or the nicest handling bike...
  12. This is true and I use this principlel all time in my work.

    What is often forgotten is that this principle counts on no other substance between the two mating surfaces.

    In the case of tyres there is always some contaminating substance, such as dust, oil or water.

    When this happen you are in an area that does not fall into a standard formula and surface area does become a factor.
  13. wasn't that from a 60 to 70% profile - the tyre size @ 120mm was unchanged.
  14. the original physics teacher is still correct if talking about friction coefficients, NOT friction alone - the principle is always correct, even in practice - in practice, you have a friction coefficient that was not correctly calculated - for coefficients, area is still an independant quantity.
  15. Rubbish, the width and shape of the tyre also determine the contact patch. A car tyre with 100 kg's load and 30 psi pressure has a dramatically different contact patch than the tyre from a racing push bike with the same load and pressure.

  16. if only some of you did some research before posting crap

    Force = Pressure x Area

    Force = Load (kg) x g (m/s^2) and for comparison can be directly dealt with in load only.

    ie Load ~ Pressure x area (~ = proportional)

    for the same pressure in the tyre, the more load you put on it, the greater it spreads out to put that load on the ground. The load taken directly by the sidewall of a tyre is negligible when compared to the load handled by internal pressure.

    When was the last time you ever saw a car tyre with 100kg on it and a racing bicycle tyre with less than 60-70psi in it?

    I stand by my engineering degree, phd and original comments. Ante up sir-b
  17. before you get all uppity, of course the shape of the patch changes between bicycle and car tyre because the geometry is different, but the AREA presented to the ground is a function only of internal air pressure and load on the wheel
  18. A bit of an exaggeration to suit your arguement here. A car tyre with it's valve removed. i.e. zero pressure and no ability to increase pressure, will hold it's own weight and a fair bit more, before collapsing. I'd suggest 25% of the load would be carried by the tyre it'self

    Also p=f/a is the solution to the intergral of the actual formual. I think it's possible that force varies over the area of the contact patch in the case of a curved motorcyle tyre.

    Of course I haven't convinced myself of this yet and whether it matters
  19. I'll defer to your edumication, Dr Bonox, as I am but a lowly peasant who lacks the anal nature to spend 4 hours researching something before having the audacity to talk sh!t on a forum.

    I will also choose to ignore the fact that you said "the contact patch is determined only by the load on the tyre and the internal air pressure of the tyre" which you know is untrue as is evident by your little "before you get all uppity" attempted save...

    Posting crap indeed.

  20. How does front and rear tyre width and the associated contact patches relate to the fact that more centrifigal force is being applied to the rear of the bike, which therefore requires more contact with the road to simply hang on?