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 Tyre choice for a Bantam.

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Mick Potter

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Number of posts : 125
Age : 60
Localisation : Cheltenham
Registration date : 2007-06-09

PostSubject: Tyre choice for a Bantam.   Tue Feb 19, 2013 3:08 am

Tyre choice is a personal thing because we all like our bikes to respond in different ways. Different manufacturer’s products perform in different ways. I do not have experience of all the different tyres that are available to fit a Bantam but I hope in the following scribbling to give you some insight into what is available and the difference between them.

1st, Some riders will go from upright to their maximum lean angle extremely quickly, literally throwing the bike on its side (from my experience about 10% of riders).2nd, The majority of riders roll the bike quickly onto its side until they become comfortable with the lean angle (about 75% of riders). 3rd, the riders who roll the bike on its side until it starts to slide (about 5% of riders). The rest are somewhere in between or just doing the best that they can. The advantage of condition 1 is that it is extremely difficult to be passed under breaking but the chances of over doing it on lean angle into the turn are high. In condition 3 entry into the corner tends to be slightly slower but apex speed is higher. One is not better than three or vice versa it depends on what you are happy with.

In the past riders were given racing tyres that were triangular in profile which was great for those in condition 1 who went straight to the average lean angle, and ok for those in condition 2 But anyone trying to go faster or who was not up to the lean angle was in deep s**t.

Modern tyres have profiles that are close to elliptical or parabolic in profile (irrespective of their aspect ratio) therefore as the bike is lent over the contact patch increases. While the old style of tyre suited a small minority of riders the new styles are better for the vast majority of riders.

Over year’s and what must be several hundred thousand miles of testing ALL of the manufactures (of both Bikes & Tyres) specify different wheel rim widths and tyre sizes front to rear on all modern bikes. In the good old days bikes had the same rubber front & rear because they didn’t know any better or have access to better information. Only the most basic of new commuter bikes have the same size tyres and this is due to price being the most important consideration. With my very limited experience in comparison to the (must be in the millions of) miles completed by the many factory test riders I have to say that the newer tyres are an improvement. If however for you the old style of tyres give a better feel because that’s what you are used to that’s fine as what gives you most confidence will make you faster (the majority isn’t always right).

The last paragraph may lead us in the direction of what tyre’s to choose but unfortunately we are constrained by what race tyres are available and also the dimensions of the Bantam itself. I have tried to get as much information as I can on race tyre’s that may (or may not) be suitable for use on a Bantam. Trying to get written information on race tyres is like trying to getting blood from a stone.

I know that some manufactures who are not on my following list of race tyre’s have or are producing suitable tyres. If you have any information please do post it so that new builders & currant racers can all make informed choices.

Irrespective of tubes/ tubeless, crossly/ radial ECT. Tyres come in 3 types, front fitment, rear fitment or universal. Front fitment tyres have a higher aspect ratio than rear tyres. That is the difference between the width compared to its height. As example a typical road front tyre is a 120-70/17. The 120 is its nominal width (in mm), the 70 equates to being 70% as high as it is wide and the 17(in inches) is the rim diameter (note the rim width is not specified). A typical road rear tyre to match the front tyre quoted will be a 180-50/ 17. Note the rear tyre is much wider but also the aspect ratio is far lower. Race tyres follow the same sort of differentials. Different bikes have different sizes but always the front is narrower with a higher or equal aspect. As the size of the bike/tyre is reduced then the difference is always reduced, but there is always a difference.

Universal tyres can be fitted to either end of the bike. Being neither front nor back they have a profile that is a compromise between the two. A universal tyre usually has two different arrows on its side wall. One will specify the direction of rotation for front fitment. The other will point in the opposite direction designating rear fitment.
Why can one tyre only be fitted to the front or to the rear while another can be fitted to either end of the bike? The clue is in how a tyre is constructed. All tyre carcases can run in any direction but when the rubber is laid onto the carcass it starts as a trailing edge thickening to the full depth of the tread. As the tread meets the start of the new tread the thickness is reduced and vulcanized to produce a (supposedly) seamless fit. The manufacturing process is the same for all types of tyre. The principle forces in the front tyre are in the opposite direction to the rear that is breaking at the front and acceleration at the rear. This is why a universal tyre can be used at either end of the bike. When under breaking at the front, the trailing edge of the tread seam is pushed back into the tyre. Reverse the direction and the trailing edge becomes a leading edge, result is a peeling back of the tread and a delamination of the tyre. Put the same tyre on the rear and the situation is reversed. A small number of universal fitment tyres have the same direction of rotation regardless of which end of the bike they are fitted to but I don’t know how there construction differs from the norm.

You will not be able to find any employee of any manufacturer that will agree with my following comments because of product liability.

There is nothing different to the construction of a front tyre, rear tyre or universal tyre. The only difference is shape of the carcass. A universal tyre will have an acceptable profile on its recommended rim size front and rear. The only reason that a front is designated front is because of its shape on its designated rim size. A front tyre on its designated rim is a perfect shape on the front of a bike but put that same tyre on a rim that is one size up if it is in the tyres allowed range then it will be closer to a rear tyre profile than a universal tyre is on its correct rim size. The same is true for a rear tyre on its correct rim in the rear of the bike it will be perfect, if put on the front on a rim one size smaller than recommended then it will be better than a universal tyre on its correct rim. The problems with putting tyres on incorrect rim sizes I have already mentioned in a previous post. In case you are unaware I will remind you. A tyre on a larger rim may mean you run out of tread before you reach maximum lean angle (but you can always lean of the side of the bike). A tyre on a smaller rim (at extremist) may mean that you will be grinding the wheel spindle before you reach the edge of the tyre. If you do fit a tyre to the incorrect end of the bike it is vital that the direction arrow faces in opposite direction to rotation.
The reason that I am mentioning putting tyres on incorrect rim sizes is because of the restrictive size of tyres that are available to fit onto a standard Bantam. A standard Bantam will allow a rear tyre of a maximum of 105mm width before it interferes with the drive chain line. If you wish to fit a rear tyre wider than 105mm then the chain will need to be moved to the left of the bike (i.e. moving the engine). I do know of at least one bike where the wheels were moved to the right of the centre line of the bike. This solution cannot be recommended in any circumstance as the handling will be seriously compromised.

More than one rider has told me that they use narrow tyres because wide tyres have more rolling resistance and this slows the bike on the straits. This concept is totally alien to anything in my experience as every day of my working life I push bikes around at work. I produce at most a ¼ horse power at my best. Yet no matter how big the tyre’s the effort to move them by hand is no more than a small bike with skinny tyres. If anything slows a bike when it is rolling it is most often incorrect tyre pressures. If there is any increase in rolling resistance caused by a slightly larger tyre it will be totally negated by greater corner speed available due to more rubber on the tarmac at full lean. Quicker corner speed=quicker speed at the start of the strait. Every ½ MPH at the start of the strait quicker than your rival will still be ½ MPH at the end of the strait. Every Quicker speed on the strait= quicker lap times who doesn’t want that.

If you have never experienced a slide or thrown your bike up the track because the tyres have been pushed beyond their limits of grip then what you have is not in need of improvement. If on the other hand you have then your choice of tyres may be stopping you from improving your lap times. It may be that your choice of tyres for your bike and the way you ride is the best it can be. But you won’t know that until some other tyres are tried and found to be inferior.

The following are all the race tyres that I have been able to find information on. Some are more suited to a Bantam than others. You will see that the tyre size doesn’t necessarily correspond with reality. Where information is missing it’s because that’s all I have been able to find. If you can fill in the missing information please do.
U=Universal fit, F=Front fit, R=Rear fit, Rim=Preferred size followed by minimum & maximum allowed sizes, W=Tyre width, D= Tyre diameter.


Dunlop.

KR108, 325-450/18. U. Rim 2.5”, -0.0,+0.0. W 113mm. D 627mm.
KR124, 350/18. U. Rim 2.15”, -0.0,+2.5”. W 102mm. D 638mm.
KR825, 2.75-3.75/18. U. Rim 2.15”, -1.85”,+0.0. W 95mm. D600mm.
KR364, 85-80/R17. F. Rim 2.15”, -0.0,+2.5”. W 85mm. D 570mm.
KR364, 115-50/R17. R. Rim 3.5”, -3.0”, +0.0. W 113mm. D 585mm.


Avon.

Classic Racing AM22. 110-80/18. F. Rim 2.15”, -2.15”,+3.0”. W 112mm. D 626mm.
Classic Racing AM22. 110-80/18. R. Rim 2.15”, -2.15”,+30.”. W 112mm. D 626mm.
Roadrider Race Tyre AM26. 90-90/18. F. Rim 2.15”, -1.85”,+2.5”. W 96.5mm. D 620mm.
Roadrider Race Tyre AM26. 90-90/18. R. Rim 2.15”, -1.85”,+2.5”. W 96.5mm. D 620mm.


Sava.

MC18 Race Soft, 90-80/17 46P TL. F. Rim 2.15”. W 99mm. D 586mm.
MC18 Race Soft, 110-80/17 57P TL. R. Rim 2.5”. W 120mm. D 620mm.
MC25 Race, 100-80/17 52S TL. F. Rim 2.5”. W 111mm. D604mm.
MC50 Race Medium. 100-80 52H TL. F.



Bridgestone.

These are described as wets but the tread pattern looks more like an intermediate rather than a full wet. Bridgestone buck the usual convention and quote the diameter rather than the aspect ratio in the sizing description of these tyres.
E03Z Battlax Racing. 90-580/R17. F. Rim 2.5”, -2.15”, +0.0. W 88mm. D 580mm.
E06Z Battlax Racing. 120-595/R17. R. Rim 3.5”,-2.75”, +0.0. W 115mm. D 595.


Heidenau.

M3R, 2.25/18 34H TT.
K45R, 2.75/18 42S TT.
K65R, 3.00/18 47H TT.


Of the above manufacturers Bridgestone is the only one that I am unaware of being used on a racing Bantam. I have raced on Dunlop’s, Avon’s and Pirelli’s so will leave it to others to comment on their performance but there can’t be much wrong with the Sava’s as they were on last year’s championship winning bike. The Avon’s I raced on was many years ago so they may now be better than I remember them. Of the different makes I have tried I have always found that Dunlop’s give both the best grip and predictability. Having used both Dunlop’s 17” and 18” rubber for me the 17” is superior in every aspect.

Getting the best out the tyres fitted to our bikes is something we all want to do but I wonder how many of us know how to. Most people set their tyres to the makers recommended pressure, but this is not always the best policy.

I will assume that everyone is using the same make of tyre front and rear. All race tyres are designed to run at a set temperature in order to give the best passable grip. Too hot or too cool and they won’t grip as well as they can. In top racing (Moto GP ECT) the tyres are preheated in tyre wormers so that the riders can get the best from them from the start. We don’t have that luxury. We start a race with cold tyres and have to wait for them to come up to temperature before we can push to the limit but come up to temperature they do and those who have the confidence to push hard from the start gain the most. If you ever watch top class racing on TV when the rider returns to the pit garage the first thing that happens is that a mechanic pushes a temperature probe into the tyres to check if they are running in the designed range. I don’t know about you but I don’t own a temperature probe and even if I did by the time I had parked the bike, fetched the probe from the van and switched it on, returned to the bike to get a reading it would be meaningless because the tyres will have cooled down. What I do use is some of the most sensitive temperature sensors know to man, sensors that we all have. I use my hands. The first thing I do after parking the bike is remove a glove and feel each tyre. Both tyres must be at the same temperature in order for them to give the same amount of grip. If one tyre is cooler than the other then the balance between the tyre pressures is wrong. Only experience will tell you if one is too hot or the other is to cold so for this example I will assume the front tyre is too cool. In order to bring the front tyre temperature up to that of the rear I will lower the pressure in the tyre by one or two psi. Once again only experience of the difference in the temperatures and the effect of lowering the pressures in your tyres will point you to how far to lower them. Remember that as the tyre has increased in temperature the pressure will have increased so it is vitally important that the pressure is rechecked and lowered from what it is now not from what it was before you went out on the track. Lowering the tyre pressure means that the tyre carcass flexes more, as it flexes it generates heat in the tyre. How hot a tyre becomes is dependent on several factors. Every track will need a different pressure balance because of the different amount of breaking and how long the straits are. How heavy you are will work the tyres more or less hard (I am light so I always run low pressures in order to get them up to temperature). If the ambient/track temperature is high then the pressure will need to be increased to prevent overheating. Ride harder than you normally do and you will need to raze the pressure to stop them from overheating. If it’s wet then that will cool the tyres and you will need to lower the pressures to counteract the effect. When it ‘s cold and wet or very wet it is impossible to generate enough heat into the tyres by lowering the pressures and that is when you need wet weather tyres. If the ambient temperature is very low it is possible to lower the pressure to a point where the amount of flex in the tyre makes the bike unstable without generating enough heat (I have been down to 20psi in the front), when you reach this point there is nothing to be gained in going lower.

As you can see there is no definitive number for your tyre pressures as every situation is different. When both tyres are not working at their correct temperature I can feel the difference. I can’t tell on the track weather they are to hot or cool but as soon as I get back to the pits and feel the tyres I know whether to raise or lower the pressures. Only one or two psi can make all the difference. If there a rise in temperature during the day I will often raise the pressures in the middle of the day and lower them towards the evening. You too will feel the difference if you ensure that your tyres are at their optimum temperature. Just knowing that your tyres are at the correct temperature will increase your confidence to push hard so improving lap times. Keeping notes on what you do and when is obviously important but you will soon get to know what pressures are a good starting point for each situation. Even I with all my experience usually make small adjustments after first practice. Just remember that you usually don’t go flat out in practice so slightly low on temperature afterwards is usually ok.

Just remember to keep it rubber side down.


Mick.

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john bass

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PostSubject: Definitely a difference...   Tue Feb 19, 2013 6:24 am

As you have said, there was definitely a difference, Mick: which I found when I eventually switched from Continental Sports (dunno who made them unless it was `Continental´ as the tyre manufacturing firm?) and Dunlop Triangulars on my Bantam in 1972. For 4 seasons I used the above Continental tyres and it was a scrutineer´s advice that had me change to triangulars!! The act of `throwing it down´ seemed more controllable with Triangulars and I am sure I was quicker by less braking and scrubbing away some of the speed in the bend.

My mentor reckoned there was nothing much to be gained by rushing thro´ a hairpin bend such as Druids at Brands but I found it great fun to pass there although I have an odd shaped right elbow to prove it that it was not always wise... My theory was much as you stated except that I called it, "keeping the speed on during the bend..." -- any of the bends....

Fantastic list of tyres you have provided -- well done!

Cheers!
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PostSubject: Re: Tyre choice for a Bantam.   Wed Feb 20, 2013 11:20 pm

Thankyou Mick ,
What a massive post , full of terrific information , anecdotes and a sprinkle of history ! Taking a broader view , any one racing in any class would gain a great deal of help from a close study of your series of articles . Yet more persuative reasons to reinforce the notion that , those Bantam Boys do know a thing or two !

And , to echo what John has already said , " fantastic and well done " !

Cheers Trevor
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mjpowell

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PostSubject: Re: Tyre choice for a Bantam.   Fri Feb 22, 2013 4:32 am

Top job Mick ! So much information for eveyone to digest and make
some tyre choices for this season? Many Thanks..

Alan can you lift Micks article and attach on the home page proper?

One point think you have Rob Duesbury's (Nick B old bike) and Nicks tyres confused.. Robs on Sava's and Nicks on a mixture of Avons and Dunlops.
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