GT4 and Brakes

Discussion in 'GT4 Tuning' started by Scaff, Mar 18, 2005.

  1. cusimar9

    cusimar9

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    Apologies as I can't be bothered to read through 9 pages of posts. I read through the first page and noticed a common misconception

    "To put it simply, your brakes (if working correctly) do not dictate how long it will take a car to stop, your tyres do."

    While this is correct at low speeds it is not at high speeds. If you slam your brakes on in any car, you'll never lock your wheels up above, say, 40mph, on dry tarmac.

    Bigger brakes allow the discs to cool quicker, bigger pads (or better pad materials) and bigger calipers will allow your car to stop quicker above (say) 40mph. As you pointed out at low speeds braking is dictated almost entirely by the friction between the tyres and the road.
     
  2. Scaff

    Scaff Moderator

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    As you say, you have not read through this thread and I am sorry to say that what you have said is not correct.

    Providing that the your braking system is fully functional and fit for the job, then increasing disc size, pad material, calipers, etc will not have any effect on stopping distance.

    The extract in my first post from the StopTech (manufacturers of competition braking equipment) white paper quite clearly says this.

    As for no being able to lock your tyres above 40 mph on dry tarmac, in the course of my job (which involves vehicle testing) I have done this on a number of occations, it is more than possiable.

    If a car has been modified beyond its original spec then it may benifit from improvements to the braking system, as the original system may no longer be up to the job, but if the braking system is fit for purpose then you will not improve your stopping distances in this way.

    The principal benefits of upgradingthe braking system are improved feel, modulation and fade resistance.

    I would strongly recomend that you read all 9 pages of the thread, as this would have addressed your point right away.
     
  3. cusimar9

    cusimar9

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    The speed I quoted is not the point, it is clearly different on all cars. When I drive my car on the track I'm telling you that keeping the same pressure on the brake the car slows massively in speed then on approach to a corner the ABS kicks in as the wheels begin to lock. My recollection of every time I've locked my wheels in different cars is that the car decelerates a certain amount and then the wheels lock once the car has slowed down.

    Could you elaborate on the 'improved feel, modulation and fade resistance' points you made? Are you trying to tell me that when Mr Porsche puts his huge 6 pot calipers on his cars it isn't to improve the braking power?

    I'm sorry but this is incorrect. Improving brakes massively improves your stopping power at high speeds. Once the wheels lock then fair enough they don't matter, but perhaps the argument should be that until your wheels lock, the braking can be further improved ;)
     
  4. Robstar

    Robstar

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    I'd like to add a point about motorcycle braking. Whilst this point of tyres are the most defining factor of breaking distances on cars, they are not on bikes. I have never managed, nor seen, a front wheel on a bike being locked unless it has ridden over a slippery surface. You will go over the handlebars before you lock a wheel on a dry surface. Disc sizes, caliper size, number of pistons, etc. all have a major factor on braking distances. On my race bike, I have AP Racing brakes and it will stop in a much shorter distance than standard road quality brakes. Again, carbon fibre brakes on MotoGP bikes are said to provide phenomenal braking power.
     
  5. Scaff

    Scaff Moderator

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    I think that you nay not has fully understood what I have said (and if its in the way I have put it across I apologise), so I will try again.

    If the braking system for a car is already sufficent to lock the tyres then fitting bigger brakes, etc will not reduce your stopping distance. The reason Porsche fits his larger brakes to his cars is that they require it to provide sufficent braking force and maintain it.

    Fit the brakes from a Mini and they would be able to lock the wheels, but they would very quickly overheat and you would find overall braking efficency reduced. In fact the pads would last about one use, also the brake fluid would almost certainly boil.

    Tyre slip occurs (under lateral deceleration/acceleration) at approx 8 to 16 % (the rate at which the tyre is moving in comparison to the vehicle) when this is exceeded the tyre will begin to lose grip and slid.

    As virtually any braking system (incluing drums if correctly adjusted) is capable of applying suffiecent braking force to slow the wheel by this rate is comparison to the car. Fitting bigger brakes does not change this, but its does allow you to maintain it for a race without overheating.

    You asked me to elaborate on feel, modulation and fade resistance, so I will.

    Feel, the drivers ability to tell how much braking power has been applied and how close to the threshold (as in threshold braking) you are. The better the feel the more control over the brakes you have; more powerful brakes tend to have a wider range of braking force and require less servo-assistance and as such will allow the driver to read the brakes better. If you have poor communication from the braking system you will never be able to reach and hold the braking threshold on a regular basis.

    Modulation, the ability to finely control the level of braking force being delivered, esential if the driver wishes to trail brake or left foot brake where the ability to hold a lower level of braking force consistently is essential.

    Brake Fade, as I have said almost any braking system is capable of exceeding the slip limit of a tyre, but for how many times. Repeated hard application of the brakes causes very high levels of heat, this heat is more evenly spread and dispursed by larger and better cooled brakes.

    Every brake component company, race technician and driver I have spoken to in my 10 years + in the motor industry has agreed with this and I have provided a link to a manufacturer of competition braking components that says the same. I'm sorry if you can't see what is being said here, but I would be surprised if you can find a direct quote from a manufacturer of braking components that states fitting part XYZ wiil stop you quicker.

    The following are taken directly from the Brembo website FAQ section, I'm sure we can both agree that Brembo know a thing or to about brakes.

    Brembo FAQs

    Where can I find test data on stopping distances?
    At the speeds that stopping distance is generally measured from (60 to 70mph), the test is primarily testing the tire's grip on the pavement. As delivered from the manufacturer, nearly all vehicles are able to engage the ABS or lock the wheels at these speeds. Therefore, an increase in braking power will do nothing to stop the vehicle in a shorter distance. For this reason, we do not record stopping distances at this time. The Brembo systems will show their greatest advantages when braking from higher speeds, or when tasked with repeated heavy braking. The increased braking torque provides for maximum deceleration at speed, and the ability to absorb and quickly dissipate the intense heat generated during repeated braking insures that the braking system will perform at the same high level each time.


    Why use larger discs?
    Braking generates heat, and the more heat the disc can absorb and dissipate, the greater the fade resistance of the system. Additionally, the use of a larger disc generally results in a larger effective radius, which increases brake torque.


    Is the biggest brake disc the best answer for my vehicle?
    Not necessarily. There are many factors to take into account when choosing a performance brake system. First and foremost, a brake system must be designed based on the vehicle parameters, and the type of use it will experience. Brake systems are designed to operate best within a prescribed temperature range. While modern high performance friction materials broaden this temperature range from relatively cold pad temperatures to the high temperatures experienced in performance driving situations, the use of a disc that is too large will limit disc temperatures to the extreme lower end of this range. Not only will the disc temperature not reach the optimum range, but it will also be heavier than necessary. Below you will find an explanation of how reducing the weight of the braking system further enhances performance.


    Now I think that if Brembo were able to reduce stopping distance through the use of uprated braking components they would shout it from the damn rooftops. Instead the closest you will find is this:
    The Brembo systems will show their greatest advantages when braking from higher speeds, or when tasked with repeated heavy braking, in other words in curcumstances when the standard brakes may not be sufficent for the job, and this is still not a direct claim to reduce stopping distances.

    As I have said (and provided links to two respected brake manufacturers) if your brakes can already lock the tyres then uprated brakes will not stop you quicker (but they will allow you to stop consistently).

    If your brakes are not sufficent for the job then fitting improved brakes will help (as the originals were not working as they should) but only up to a certain point, as once they are able to lock the tyres then the advantage of any further increase in size on stopping distance will be none.

    You may not want to believe me, but hopefully you will believe Brembo.
     
  6. Scaff

    Scaff Moderator

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    This has been discussed (admitidly very briefly) as the effects of weight transfer and how you brake on a bike are very different to a car.

    I would still maintain that the rubber is the single most important factor involved, but the other forces are very different, but then riding a bike and driving a car quickly are again very different.
     
  7. cusimar9

    cusimar9

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    This is what I said is it not? More powerful brakes will be more effective at braking from higher speeds?

    I agree with everything else you said and you seem to know what you're talking about.

    Imagine a test where you get 2 standard cars with ABS (which can lock their wheels at low speeds)

    Kit one out with Brembo 4 pots and Ferodo DS3000 pads (same size discs) and brake them both from 150 mph.

    Lets assume neither brakes fade, the fluid doesn't boil and they both trigger the ABS as they come to a stand still (why do you suppose the ABS doesn't kick in at 150mph, incidentally?)

    Are you trying to tell me that they would both stop at the same time?
     
  8. cusimar9

    cusimar9

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    A lot of people seem to have a problem regarding weight not being an issue with braking distance, amazing but its true :)

    If you take a wheel, and it takes 20N of force to move it sideways, its accurate to say that if you double its weight it will take double the force to move it.

    Essentially, regardless of the weight of the car, the car will have the same outright level of grip.

    The problems, as have been pointed out, are usually the high centre of gravity for heavy cars (ie 4x4's) and their soft springs. Also I dare say (though can't back up) a lighter car will have a more nimble turn in than a heavy one.

    Anyway... back to the subject of braking :dopey:

    A heavy car will take more energy to stop and so heat the brakes up further, possibly exceeding their optimum temperature range and fading/boiling the brake fluid.

    Don't drive heavy cars folks :tdown:
     
  9. Scaff

    Scaff Moderator

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    I'm not sure what I am failing to get across here so once again I will try, using your example.

    But first we need a clarification, if these two cars are both able to lock the tyres at low speed is irrelivent, as you are using an example of braking from 150mph.

    Now if the standard system is capable of locking the tyres at 150mph then fitting the Brebmo/Ferodo system will not stop the car any quicker.

    However if the standard system is not capable of locking the tyres at 150mph then an uprated system would be required, and would stop the can quicker, because the original system was not sufficent for the job.

    This is a point I have maintained on everyone of my posts, and its not me telling you, its physics.

    If your braking system is capable of lock the tyre at speed X, then increasing the size of dics, pad material etc, will not give the tyres a great slip percentage and will not stop the car quicker from speed X.

    I have tested as part of my job the majority of road cars on said in Europe and can say that with a very few exceptions all are capable of locking tyres at speeds of 90-100mph with the standard braking equipment fitted, as such bigger brakes would not stop these cars quicker at these speeds, better tyres would.

    The issue the standard braking systems would have is in appling this over a repeated series of tests (and this is an area I work on), standard braking systems are designed to allow maximum braking force to be applied for emergancy situations (i.e. a one off stop), not repeated application.

    Here is a real work example from last years Autocar 0-100-0 mph test

    Alfa Romeo 147GTA - 305mm front/276mm rear - 4.39secs (100-0)

    Renault Megane RS - 312mm front/300 rear - 4.45secs (100-0)

    The larger brakes of the Megane failed to stop the car quicker than the Alfa.

    Caterham R500 Evo - 254mm front/228mm rear - 3.6secs (100-0)

    Mosler MT900 - 330mm front/330mm rear - 4.4secs (100-0)


    It only takes a few hundredths of a seconds to apply full braking force so any benefit of appying that force quicker (which larger brakes may give you) is virtualy nil, so long as the brakes can lock the tyres at the required speed, fitting more powerful/larger brakes will not stop you quicker.
     
  10. cusimar9

    cusimar9

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    So you're saying that since my car can lock its wheels when braking hard at low speeds, by applying the same force to the brakes I should be able to lock the wheels at 140mph?

    I'm sorry but I don't believe that. All the cars I've driven, all the videos I've seen the brakes lock up at low speeds, not at high speeds. This is nothing to do with brake fade either.

    For instance, perfect example: http://www.ultimasports.co.uk/record.html he keeps the same force on the brake pedal and if anything eases off the brake as he comes to a stop, yet still manages to lock the wheels up.

    I understand the point you're trying to make perfectly well. But unless you can explain why its easier to lock the wheels up at low speeds we won't get anywhere in this debate.
     
  11. Scaff

    Scaff Moderator

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    Now I know we are talking at cross purposes here, please believe me I would never claim that because a car can lock up its tyres at 40 it could do it at 150mph. One (as you have said) requires far greater force than the other.

    I know the clip well, the cars owner is a member of the Pistonheads.com forum and its a great achievement.

    Now I'm sure we both agree than the braking force needed to lock the tyres at 100mph is greater than at 40mph. However no one would want to brake at maximum braking force as this would exceed the grip limit of the tyres and result in a loss of traction. Instead we need to break at a point just below than which the tyres can take, commonly know as threshold braking.

    Now if you brake to the threshold at 100mph (no lock-up just maximum traction deceleration), as the car slows and the force required to keep at the threashold point reduces you must carefully reduce your pressure on the brake.

    Keep to much on and you with exceed the threshold and the slip angle and lock up the tyre. Keep to little on and you will drop below the threshold and will not be braking at maximum capacity.

    It far more difficult to brake hard and right on the threshold, particulalrly to a standstill, very few people have experience of it and the vast majority will keep applying too much braking pressure as the car slows.

    The driver of the Ultima has said that he believes he could nail the braking better and posiably get a sub 10sec 0-100-0 run, its on the second page of the comments in the attached link (you are looking forr posts by G-man).

    http://www.pistonheads.com/gassing/topic.asp?p=1&f=20&t=119292&h=0
     
  12. goldbadge

    goldbadge

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    I would just also like to offer that weight, generated by aerodynamic downforce, will help with braking. As evidenced in F-1 cars, they have incredible braking abilities, in part due to the tremendous downforce pushing down on the tires that allows them to press so hard on the brake pedal without locking the brakes. Keep in mind though that they can only do this for a brief moment, and have to start easing off the brakes as they start to lose the aerodynamic downforce due to slowing down. This is how I brake in most of the cars in the game that have a hi top speed with wings that generate lots of down force. I've gotta think that PD modelled this into the physics engine.
    Thank you, "G"
     
  13. FIDO69

    FIDO69

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    118
    Interesting videos and posts.

    As I am still curious as to GT4's implementation of braking, so I went and did a few more tests.

    Went to the Highspeed oval test course with an Acura DN.

    I repeated the 100mph to 0 tests with and without 200kg ballast and there was no difference in braking DISTANCE (I respectfully submit that posters could avoid confusion and pointless arguments if they use scientific terminology and not words like "effective", "powerful", "easier", etc). 4.8ml on average.

    Then I shifted the weight forward (+50 ballast position) and the distance increased to 5.1ml. Shifting to -50 caused the distance to decrease to 4.4ml. This is good, seems that GT4 models COG effect on changing load balance between wheels during deceleration. :tup:

    Next I increased the Downforce settings to maximum (30/30) and the distance decreased 2% to 4.7ml. Decreasing downforce (0/0) caused the distance to increase to 5.0. (nominal test downforce was 17/15, just the setting I last raced with)

    Great! Downforce improves braking distance, just as we'd expect. :tup:

    Finally, I wanted to know if GT4 models brake fade. So I did repeated 0-150mph-0-150mph accel, decel tests. I did the cycles approximately 7 times consecutively with both "racing brakes" and "standard brakes". There was not one iota of difference. Average stopping distance from 150mph was 7.4ml.

    I have yet to find ANY effect of racing brakes in GT4. Has anyone else?

    Emotionally and physiologically, I "feel" that racing brakes improve my lap times. I "feel" that I am able to brake later and better, but I can find no scientific reason to say that they should. :confused:

    Perhaps I need to try braking while turning... this is a hard test to quantify... :odd:


    (BTW, the bottom line of this thread IMO is that sticky tires are the way to go. I tried 100mph-0 tests with Racing Soft tires (all previous tests were using Sports Medium) and the distance decreased to 3.5ml, a 27% reduction. Both racing and standard brakes were used, with no difference.)

    (edit: when I get the credits, I'm going to do the race brake test again with SUper super sticky tires. I hope that GT4 does model a maximum braking force difference between standard and racing brakes.)
     
  14. Scaff

    Scaff Moderator

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    Very true, aerodynamic downforce does indeed aid braking at high speed and the effect (and benefit) does decrease as the speed and therefore downforce reduce. Its one of the reasons you rarely see race cars (except those without aerodynamics) lock the tyres at speed.

    This is one I am hoping to test in GT4 soon, when I will get the time at the moment I do not know, but as soon as I do, I will get the results to everyone.

    By the way, if no one has said already, welcome to GT Planet goldbadge, hope you have a good time here and find plenty to interest you.

    Regards

    Scaff
     
  15. FIDO69

    FIDO69

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    Not true. Read my posts on page 8 of this thread.

    Mostly true... but not 100% true in RL. In GT4, 100% true.

    (Welcome to the debate!) :)
     
  16. cusimar9

    cusimar9

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    Aha!

    So is it fair to say that on cars running tyres with a suitable load index it is more or less accurate?
     
  17. FIDO69

    FIDO69

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    Yes, but the question is, what is a "normal" load index? Skip Barber's plot and write up implies that most race tires have a fairly steep sloping downward CF. However, I've looked a bit online, and can't find any real world plots... Makes me doubt a bit.

    [okay, I found one other place that tangentally talks abou t load dependence: http://www.edccorp.com/pdfs/WP2001-4.pdf. It seems that most CF vs. Slip angle plots are normalized for constant load. Interesting... back to work!]


    AH!... more theory: http://www.teamassociated.com/racerhub/techhelp/marc/Handling.2.html#27148
     
  18. cusimar9

    cusimar9

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    FIDO69... that second link you placed is very interesting. Showing how, on a certain car with certain tyres, the maximum cornering force comes from a load of 500lbs (for the front tyres) and 600lbs for the rear.

    It is plain to see in this case, that if a car were any heavier, it wouldn't be able to corner as well.

    However, its not the case that heavier cars would have the same suspension, camber and tyres. For another car the graph might peak at (say) 900lbs load.

    Its fair enough to say that if you make a car heavier it won't corner as well (thanks for pointing that out) but even a heavy car can be engineered to corner well. The question is can a heavy car be engineereed to corner AS WELL as a lighter car, or is it foiled from the start? The Porsche Cayenne suprised a lot of people when it first came out...

    Scaff... thanks for your last post, it explained a lot :tup:

    I appreciate the facts are very conclusive that most cars seem to take a very similar time to brake from 100mph... and that bigger/grippier tyres would probably cause the biggest improvement here.

    Of course the real point to uprated brakes (as you have pointed out) is their ability to sustain a high level of braking at high temperatures, resisting fade, and to improve modulation, feel and even initial bite (though this is often not the case!)
     
  19. Crayola

    Crayola

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    1,008
    We adressed this earlier on the thread, only I was on my mountain bike where it is almost impossible to lock the front wheel (I can do it on my tiles if they are wet, thats it.
     
  20. Crayola

    Crayola

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    Seriously, read the whole thread because what your saying is almost completely incorrect. Grip is almost entirely dependant on grip, a heavier car with better weight transfer characteristics will often outbrake a lighter car. (Range Rover Vs Elise for example) Also, force required to stop something depends on mass not weight, weight is itself a force.
     
  21. Crayola

    Crayola

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    1,008
    Actually coefficient of drag is almost completely independant of surface area, I found it hard to believe too, it just spreads the pressure out over a larger area.
     
  22. cusimar9

    cusimar9

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    :ouch: Not another crazy idea!

    Coefficient of friction quite clearly DOES depend on surface area. Namely bigger tyres will provide more grip ;)

    Why do you think Supercars have massive tyres to help put the power down?

    Don't be so pedantic :dopey:
     
  23. Scaff

    Scaff Moderator

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    Glad we got that one sorted, always a headache when you know what you want to say and just can't seem to get it across, hope I got there in the end.

    Just a very short one on the weight vs handling one, the Physics of Racing has some interesting bits on this, the link is as follows.

    Transient Cornering Forces

    Have a look, see what you think.
     
  24. Crayola

    Crayola

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    There is a big difference between size and weight and coefficient of friction doesnt depend on surface area, I thought that to but check:
    http://www.physlink.com/Education/AskExperts/ae200.cfm


    Friction is proportional to the normal force of the asphalt acting upon the car tires. This force is simply equal to the weight which is distributed to each tire when the car is on level ground. Force can be stated as Pressure X Area. For a wide tire, the area is large but the force per unit area is small and vice versa. The force of friction is therefore the same whether the tire is wide or not. However, asphalt is not a uniform surface. Even with steamrollers to flatten the asphalt, the surface is still somewhat irregular, especially over the with of a tire. Drag racers can therefore increase the probability or likelihood of making contact with the road by using a wider tire. In addition a secondary benefit is that the wider tire increased the support base and makes it hard to turn the car over in a turn or in a mishap.
     
  25. cusimar9

    cusimar9

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    Yeah I've tried to read all his articles one by one, but the maths is usually way over my head.

    What its basically saying is that it is PMI that causes the problem with handling?

    Well take the following scenario:

    I have a 306 Rallye FWD, weighing 1065kgs and has 180bhp. Weigh distribution is around 60/40 I think

    My mate has a mk3 Supra RWD, weighing 1520kgs, and has around 280bhp. Weight distribution is closer to 50/50 and I dare say his COG is lower than mine too.

    His car accelerates and stops fantastically well. Yet I think we both know around a track he'd have a hard time keeping up with me.

    Is it really just because his car is so heavy?
     
  26. hitman146

    hitman146

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    thx for the good information read through all of this.
     
  27. Crayola

    Crayola

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    I know I;ve learnt alot since I started reading this thread. We move at such a slow pace at school and focus on lame stuff like waves. Engineering studies is so much more intersting than physics but not as in depth.
     
  28. cusimar9

    cusimar9

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    I guess I understand that...

    If you have a contact patch of 1sq cm and a weight of 10 lbs (10 lbs per sq cm), if you double the contact patch to 2sq cm the mass is still 10 lbs and the force becomes 5lbs per sq cm, hence the friction is the same as 2 x 5lbs per sq cm = 10lbs

    That's fine for a perfectly flat surface. But as pointed out in that article, in this case a wider tyre will therefore cause more interaction with the road and therefore provide more grip.

    The bottom line is that wider tyres do provide more grip, regardless of the physics behind it ;)
     
  29. Crayola

    Crayola

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    No they dont. Smoother more reliable grip: yes, but in the same conditions (ie, not in real life) they dont get any more grip. I think the advantage of wider tires is more pronounced in cornering than in braking.
     
  30. cusimar9

    cusimar9

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    You try and fit 100mm diameter wheels to a lambo and see how far you get :)

    And we're not talking about lab conditions here we're talking about real life!