tyre->road coefficient of friction

And if there is, ie. your're braking while turning, then you asked for it yourselfe :-)

?Morten

Reply to
Morten
Loading thread data ...

Yes. Quite.

Reply to
The Natural Philosopher

So you brake in a perfectly straight line every time ? And on a perfectly flat road ?

There are some superb RWD handling cars with fully independent long travel suspension front and back but they are too easy on the limit to throw into the bushes under fierce braking so for most cars a front anti-roll bar keeps the front within known limits. Likewise a rear anti-roll bar and a good differential keeps the power aiming in a reasonably forward direction.

In the limit, i.e a F1 car, the best drivers used to change both the F/R brake bias and front anti-roll bar on the straights before a sharp corner to allow them to brake deeply into that corner whilst minimising lock-up. Mansell, Coulthard and of course Schumi were experts at it. I think anti-roll bar adjustments from the cockpit are banned now though.

Reply to
Mike

Agreed. But if the force line is above the axle line of the front wheels there is an adverse moment which comes into play which I assume was what was meant here.

Reply to
Mike

acceleration

Then don't call other's people opinions gibberish.

I made a simplification often made in automotive circles. I agree it wouldn't be obvious to all but should have been to most.

The two first mentions of aerodynamic forces I can find are in the 60s by Dan Gurney referring to "an increase in downforce" and Colin Chapman referring to an "aerodynamic gain", both referring to the wings added to the rears of their cars.

To my mind, this implies the term downforce already applied to those cars, i.e. the weight of the car. Also most school textbooks refer to the "force down" through a wheel which isn't too far away.

Reply to
Mike

I think you will find for many tyres those numbers are as near identical as makes no odds. Of course if one could keep the tyres literally at the point of locking without doing so then you would achieve ultimate grip but unless your name is Michael Schumacher forget it.

And ABS systems don't even get close. They are at least 10% away from ultimate grip but are deemed safer because they allow steering input to continue.

Reply to
Mike

As the tyre melts it grips more and will overcome the brakes. Look at any side-on footage from transport reseach groups and you can definitely see the locked wheel occasionally turning a degree or two before locking again.

Reply to
Mike

Without trying to sound pedantic, I would argue that if the brake was doing no work, the wheel would spin again. But I agree the work does drop off once locking has occured.

How early ? A pair of DS11s usually solved the problems on disc equipped ones.

Reply to
Mike

The message from "Mike" contains these words:

Gibberish it was and gibberish it remains.

Wierder and weirder. Recipe for complete confusion between vertical and horizontal forces.

Reply to
roger

And a prat you obviiusly choose to remain. The End.

Reply to
Mike

No - the work done by the brake is zero with a wheel locked. The work is done by the pressure applied to it.

Early Jags didn't have disc brakes - they had poor drum ones. Which is why they were one of the first to adopt them. But were still an option until about '60 on some models.

Reply to
Dave Plowman (News)

overheats

coefficient

identical as

I think I won't find that because it isn't true. You are welcome to post any sources of data that support your views though. Here is a detailed study of static and sliding tyre friction with actual data measured on a Renault Megane.

formatting link
From the chart towards the end. Mu peaked at 1.2 with 20% longitudinal slip and fell to 0.85 with 100% longitudinal slip i.e. brakes fully locked up. That's a 30% drop in adhesion with locked wheels.

The percentage slip at which peak adhesion occurs varies with tyre construction and road surface but the general principles are well known. Don Alexander in 1991 in his book "Performance Handling" shows tyre company data with peak adhesion at about 10% to 12% slip but otherwise similarly shaped curves to the data above and considerable drop in adhesion as slip percentage increases.

Reply to
Dave Baker

It's not a question of pedantry but one of basic physics. Work = Force x Distance. Once the brake disc has stopped rotating there is no work being done because there is no motion relative to the force being applied. There is a torque being applied to keep the wheel locked against the counter torque generated by the tyre/road interface but no work being done and no energy being absorbed.

When you tighten a bolt up it applies a force to the structure it holds in place but that force exists ad infinitum with no further energy input. When you hang a coat on a coat hanger there is a force but again no work because nothing is moving. You only get work when there is motion.

Reply to
Dave Baker

You were the one who posted the phrase so presumably you know what was meant by it or were you copying something someone else had written?

As for force lines and adverse moments god only knows what you now mean. I presume you mean the centre of gravity being above the wheel centre but this doesn't introduce some unusual factor into weight transfer. There will be more of it as the CoG rises for a given wheelbase but it still won't "overshoot" anything.

Reply to
Dave Baker

direction.

Let's go back to the original quote.

"As for RWDs the high front roll stiffness is to keep both front wheels on the ground during braking."

So you are now saying you meant braking during cornering? In that case a high front roll stiffness would achieve the exact opposite of what you claim. When roll stiffness is increased at one end of the car the weight transfer at that end is increased and the inside wheel at that end has a greater tendency to lift off the ground. The whole point about RWD cars having high front roll stiffness is to keep the rear wheels on the ground not the front ones. That's why RWD cars lift a front wheel in corners and FWD cars with a high rear roll stiffness lift the rear one.

The aim is to minimise weight transfer at the driving end of the car or you end up with only the outside tyre trying to both provide traction and also handle all the cornering forces. Roll stiffness and roll couple distribution is about modifying cornering behaviour not braking and even if it were about braking it works the opposite way to what you think.

Reply to
Dave Baker

The message from "Mike" contains these words:

Whatever your opinion of me the fact remains that what you posted was not correct. Your explanation of why you used such wording was unconvincing and I don't for a moment believe that your usage is common usage in the automotive world except perhaps in the realms of car salesmanship. I am surprised that you should consider this thread at an end when your other fantasies are taking such a pounding but that is your choice. I too have a choice - welcome to my killfile.

plonk

Reply to
Roger

Dave

As I expect you're the only one still interested, I'll reply in one message.

Regarding tyre adhesion. First thanks for this link

formatting link
background. I was involved in tarmac rallying in the late 70s, early to mid 80s. At that time the sport evolved towards a point and squirt technique, peaking with the GroupB cars. The best tyres then, or at least the Pirellis we used, exhibited much less drop off with locking, perhaps 5% or so, and so provided the tyres did not flat spot or overheat this was not discouraged. Now this has probably got nothing to do with road tyres but as I am sure my current Impreza on it's road tyres could easily beat all of those cars back then even with less power, I made the incorrect assumption that modern tyres would exhibit similar behaviour. I am surprised at how much a drop off (30%) there is though but I assume this is done to increase tyre life. Anyway on this point I accept I was wrong.

Regarding the locked wheels and work done by the brakes, this was coupled to my assertion that there isn't usually such a thing as a fully locked wheel (except obviously on ice or other low CoF surface). Under locked conditions the wheel does still spin in small 'jumps' for want of a better word. Whether this is due to the tyre melting and increasing grip or whatever hasn't been agreed here but film of lockups on tarmac surfaces regularly shows this effect. When this happens some work is being done by the brakes, albeit much less than during the initial braking.

Finally (I think) the roll stiffness. I totally agree with what you said about a high roll stiffness at one end of the car causes a wheel at that end to lift off the ground under cornering and I don't believe I said otherwise. What I tried to say, obviously not at all well, is that despite this when a RWD car is under braking into a corner or on a bumpy surface, a high front roll stiffness makes the car more predictable under braking, not that the braking is improved. As a comment to your further point about having a high front roll stiffness to keep the rear wheels on the ground, there are many other ways of doing this, as demonstrated by cars such as the Stratos (and I am sure more modern ones) which relied on unconnected independent long travel rear suspensions. But remove the front anti-roll bar completely and the car would become kart like and often head off for the next hedge. However on smoother surfaces, some drivers did this to good effect. I believe one of the finest examples of this was Ari Vatanen in the Rothmans Mk2 Escort (the one with fully independent suspension) in I think it was

1981.

I trust this clarifies my thoughts on the subject, which obviously have wandered well off the original subject. Obviously you or others may think differently about these points, but if it was an easy subject we would have had the perfect car a long time ago. I for one am still waiting for it.

Best Regards

Mike

Reply to
Mike

It will. Cf what happens on a bicycle if you slam the front brakes on. If the weight transfer effectively exceeds 100% there is an upward force on the back wheel and your nose ends up in the gutter :-)

Reply to
The Natural Philosopher

You ncrease rll stiffness to essentially make that part of te car slide first - thats dure o more load being yransferred to teh outside wheel on teh end of teh car with teh greater rolls stiffness.

Understeering off with massice roll stiffness at teh front, or a tendency to oversteer, but with better control sometimes if the front is too soft.

However all car set up is a bloomin compromise, as you can't get the CG dwn at ground level, and there is always a tendency to tarde offg corenering performance for traction as well.

Incidentally, I do and always have braked deep into corners when trying to go fast. I note that senna and mansell do teh same. It encourages in RWD cars that have rather understeerimg characteristics, a nice balance, which can then transfer into decent exit balance under power on the rear wheels.

Its not very useable on FWD cars though, as one tends to simply yank the steering and then use the right pedal to control the line.

Reply to
The Natural Philosopher

The message from The Natural Philosopher contains these words:

That is not correct. :-)

Using the front brake creates a couple that produces an upward force at the back wheel with any degree of braking. When the weight transfer reaches 100% (it can't exceed it) the normal reaction at the real wheel contact patch is reduced to nil and any increase in braking will cause rotation. I doubt if it is a very common occurance on level ground as ISTM that even with a coefficent of friction as high as unity the angle between the contact patch on the front wheel and the conbined C of G would need to be 45 degrees or more to the horizontal. Add a downslope of course and it becomes much much easier to achieve. Having said that the only instances of going over the handlebars that I can recall from my childhood involved hitting something or someone.

Reply to
Roger

HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.