Safe Speed Forums

Hmmm? Another project, get a flexi pipe and see how much stick it can take.

hmmm ? you've confused me now... please quote which bits you're in disagreement with ?
i was careful to say "handbrake turn _and_ apply the brakes".
ABS won't do anything without the driver putting pressure in the brakes.

Oh No!!!!! I'm getting worse, my only excuse is these posts are late at night and I am getting thought a large amount of Liebfraumilch before it goes off. Yes I can see exactly what you said, and my general experience is the handbrake can be applied to lock-up, then touth the brake pedal and the ABS starts pulsating.

Rather than re-read the posts (and make more mistake), I'll briefly state wher I find myself at the moment. My conclusion is ESP is detecting 'bent' cars due to a conflict. Providing ESP/ABS doen't react to front wheels on axle stand and rear wheels stationery then I can continue, and lock the wheels over and activate ESP warning.

The first pit fall is the stationery rear wheels, but that will either show or not. The second is the Audi A4 I can use may not be so clever - so abscence of results is no proof. Have I missed anything?

I seem to have forgotten what it is you're trying to prove ?

(quite why is another question altogether!)

I feel the urge to disagree!

We can both agree that an ordinary "open" diff just spins the wheel with the least resistance. As soon as one wheel starts to loose grip, you're stuffed. the diff will progressively transfer everything to that wheel. Easy to see on axle stands, you can stop a wheel with your finger and once it's stopped, you can virtually stand on the throttle and the stopped wheel won't see any significant increase in torque. I think we're agreed that far...?

When you apply the footbrake, it will apply a resistive torque equally (if the brakes are working correctly) to both wheels.

Going back to your numbers, let's say we're on a slippy surface with 100Nm (torque is measured in Newton metres, FORCE is measured in Newtons!) going to (in my case) the front wheels. One wheel looses grip and starts to spin. We now have 100Nm going to the spinning wheel and 0Nm going to the one with the grip. I then gently apply the brakes, and that puts a resistive torque of (say) 20Nm on both discs. The stationary wheel now has a bit of resistance (as does the spinning wheel). Nothing much changes because the wheel with the least resistance is still the wheel with the least resistance (it's just that they both have a bit more from the brakes as well). Is that what you were saying?

Now, as I increase the braking effort, however, there will come a point when the torque that the engine has to overcome from the road surface is a vanishingly small percentage of the total torque needed to turn the wheels (because the vast majority of it is, in fact, going into overcoming the torque being applied (equally) by the brakes). I think that's why it works. Remember that these are REALLY slippy conditions (otherwise I wouldn't have been in that mess in the first place)! So the amount of brake effort you need to make the diff "think" that both wheels are encountering similar amounts of resistance compared to the resistive force that the road surface is providing at the contact patch) is VERY low. (if it wasn't, the back wheels would just lock for a start - which would be somewhat counter-productive)! Putting the numbers back in, let's say that we need 20Nm to break traction, and I'm applying a braking torque of (I dunno, 500)? On top of that (and I've no idea if this bit's true, but it makes a bit of sense to me!) the braking torque applied to the spinning wheel is actually very fractionally greater than that to the static wheel because the disc on the spinning wheel warms up so (a) the pads work a bit better and (b) the disc expands slightly between them - neither of which can happen on the static wheel. (Again, I accept that if that supposition is true, the effect will be very small)!

I do, of course, agree completely that this is a VERY poor alternative to proper "fiddle" brakes( as on a tractor) or, indeed, traction control, which has the luxury of being able to apply the brake to only one wheel. Naturally, it's also FAR inferior to a diff lock! I do still maintain, however, that in circumstances such as those, it can provide some limited help!

As far as the G-clamp thing goes, yes, it's perfectly possible and unless the clamping thing has ridiculously sharp jaws, it doesn't bother the brake hose (well, not an ordianry one anyway - you can't get away with it on braided hoses)! I've done it with "Mole" (I'm famous, see!) grips in the past and there are also proper brake pipe clamps that you can buy which are made from round bar. In this situation, however, it wouldn't have been any use because the wheel with the least resistance is constantly changing as the car moves and each tyre gets on to a different patch of road. The "spinning" wheel just swaps from side to side constantly, depending on which bit is slippiest.

Mole: Yep, got vids of me grabbing a spinning wheel in axle stands with bare hands, makes the other wheel spin twice as fast.

Yes my mistake torque is Nm, motive force is N as is resistance. Should have said Motive force and now edited, thanx.

Turning to your paragraph starting “Going back to your numbers”, I generally prefer maximum acceleration to measure traction in m/s2 (sorry can’t do ‘little’ 2), but that’s of no issue here.

Whilst the foot brake does apply resistance to both wheels, the path of least resistance is still the ‘spinning wheel’ and hence is spins. Quite correctly the resistance is transferred, but only until the resistance of the ‘spinning wheel’ is greater than the ‘tractive’ stationery wheel, will there be any forward motion.

If you have a trolley jack, then jack a front wheel up with the jack in line with the car. Start her up, in gear and obviously nothing will happen - unless you have Brake Intervention Traction Control which will apply the brake to the spinning wheel only. You can overcome TC by disconnecting the muti-connect on the ABS unit – it will send your dash into a spasm but fine when reconnected, or keep you foot on the brake, as this will disable TC, but this mean you will need to crank and start the engine in first gear if you aren’t confident at getting a gear without the clutch. Not a problem if wheel is right off the air as there will be no resistance to damage the engine.

You may like to purchase the Bosch Automotive Handbook published by the Society of Automotive Engineers, the 6th Edition (I believe is the latest ISBN 0-7680-1513-8) explains this issue on page 862. But don’t take my word for it have a go yourself.

If you don’t have a trolley jack place the car on bits of wood with sand in between them so it can drive off. The spinning wheel doesn’t have to be air-borne, just enough weight taken off it to loose traction.

I haven’t googled this, but my experience of these types of technical issues is that there is a lot of erroneous material out there, often from respectable looking web-sites.

The key is that the foot brake can never give the spinning wheel MORE resistance. If that were the case, differential lock would never have been invented, it would have been left to brakes and gearing.

The resistive force you need to apply at the spinning brake is as high as the resistance of the car, and it keeps increasing the more you brake the tractive wheel, so the spinning wheel can never catch up.

Fair point about the rear wheels, although they have less braking resistance than the front, I anticipate that they would in practice ‘kill-off’ the torque, so their flexi-pipe would also need to be clamped. It really all depends on the resistance, off-road, steep incline, (1500kg at 10mph on a soft field going up an incline of 5 degrees will have an overall resistance of approx 6400N and a with max engine torque of 160Nm, a typical car will be able to create a maximum of 8,800N of forward motion) rear brakes are going to be a problem, in fact 2x6400N i.e. 12800N is required and hence the car can not transfer torque via the diff as it is limited by the combination of its torque and gearing. Whereas on level tarmac, spinning wheel on ice, then the overall resistance is 209N so I guess that the rear wheels could be “dragged along” if the traction of the good wheel is sufficient.

Of course your right about the practicalities of hose clamping, I never really though of that. Yes you would be in and out of the car all the time on a severe road as the ice patched swapped from left to right. I guess it limited in practice.

Just to be certain, I have just put my car on a trolley jack (its non-TC), wheel was only slipping (not air borne) and spun the wheel (handbrake alarm had a fit so released it!) applied the brakes. Had throttle to the floor and all the brakes where ever going to do is stall the engine.

But have a go yourself. I hope you will find this sort of experimenting interesting. It may seem basic, but in fact it is exactly what an expert would do albeit they may have access to a rolling road.

Good luck, it might all sound silly, but the best way is to experiment and understand how your car functions. Seeing is believing.

P.S. A danger of clamping brakes, certainly if you do three to cut out the rear, is if on an iced road, the one wheel with any braking may stay up on ice as you move forward and you will be somewhat helpless!

I have also pondered if a flexi-pipe or at some other point a manual valve could be fitted to make life easier than clamping although I expect this may fall foul of the MOT test.

I think you'll find it's on the 7th edition now - it's been very substantially revised in the 7th edition - that's why there are quite a lot of 6th editions floating round Amazon and such places! (and, as you'll hopefully gather, YES, I'm quite familiar with it)!


As I have said in my previous post, we can both agree on this in theory. In practice (for reasons that I've tried to outline above), it seems to make a bit of difference!


I'd love to see those sums! I'm intrigued by the great conviction with which you pronounce the resistive force on ice to be 209N, I must say! Remember, however, that in the instance I'm refering to, I was trying to back a 2 tonne car up a 1-in-5 slope!


And up to a couple of times per second too! Yes, it's completely impractical!


I did. Read the post. If I hadn't, I'd still have been there spinning one wheel until Spring came! Quite odd really, I wouldn't have though it would make as much difference as it did but I was desperate and wasn't exactly spoilt for options, so I gave it a go!


Indeed it was!

It would almost certainly contravene the C&U Regs. The drag racing boys like to fit "line locks" to their rear brakes so they can do good burnouts. They work the same way. I have to put "fail valves" in the braking system when I'm doing the type approval braking tests so that I can simulate failure in one or both braking circuits. I guess I could have four - one for each wheel, but I'd never be able to operate them fast enough in the situation I'm talking about!

Mole:
First of all, I have had a technician with me today and seen proper flexi-pipe callipers. They are rounded so as not to damage the pipes and he would reckon that the flatness of a G-cramp might cause damage.

I think in your real-life scenario, the wheel scrapped its way out by ‘digging’ its way to traction. Obviously you have made your mind up and you wish to disagree with the SAE – that’s fine. But if you don’t want try the trolley-jack test, then there is little more I can do.

As for 209N resistance, here’s the calculation:
(please note I can’t do little raised 2s and 3s but I am sure you will known when its referring to squared and cubed)

Aerodynamic drag = 0.0386.Q.Cd.A(v+vo)2

Where:
Q is air density at 200m asl, 1.202 kg/m3
Cd is drag coefficient 0.39
A is largest cross sectional area of vehicle 3.13803m2
v is vehicle speed (10mph) 16.1km/h
vo is headwind 0 km/h

Do the maths and that is 14.71N
(note, I have noticed that I has mistyped 11mph instead of 10mph on my PC program which gave me 3N more).

Ok so far?

Then Rolling Resistance = f.G

Where:
f is the coefficient of rolling resistance for Asphalt, 0.013
G is weight (mass of 1500kg), 14715N

Do the maths and that’s another 191N

Is gets easier.

There is no climbing resistance as it is on the flat, so add the two resistances and you get 206N for Running Resistance.

So ok I was 1mph out! At least I’m honest, could have easily tweeked a coefficient to suit my cause, such as say f was 0.0132 and indeed that could be f as in practice f varies upon circumstances.

So your 2t vehicle on a 1 in 5 (11 degrees) at say 2 mph would be facing a rolling resistance of 3999N.

Discussing wheel clamping with a technician this morning, it would seem it’s a 2-man job. Driver applies brakes, passenger clamps spinning wheel. Brakes are released and driven forward hopefully with the spinning wheel braked by retained pressure and clamp released as soon as you are out of the mess – no pulsation is required.

But please do the trolley jack test – it won’t take 2 mins.

if only there was some kind of automated system to do it for you

Ed m. yes we have been discussing Brake intervention electronic Traction Control and how that is not as effiecient as a mechanical diff-lock.
At the moment we are debating if just applying the brake pedal has the "diff-lock" effect and my calculations have come into question so I have laid them out for scrutiny.

It's funny this traction control on cars.

My new tractor has every conceivable electrical gizmo to make my life easier. I can select a direction start my machine up and oil will flow at the right rate to the right spool valve for the right time, monitoring devices will measure the distance I've travelled and calulate the area I have worked along with my average work rate and total and average fuel used. The engine and transmission will talk to each other in certain modes and will sense changes in load and adjust the forward speed accordingly. Yet, as the name suggests, traction is the name of the game and my "traction control" consists of a switch for 4wd and on for the diff lock (which I can automate). If the wheels are spinning it's because there isn't enough traction. Courses of action are reduce the draft load. Buy bigger tyres. Add weight. Concede that it is too wet and go home.

I get 711nm of torque and have no traction control, a diesel hatchback has a piffling 251 pound feet and has traction control. Come on car manufacturers, just admit you can't get the power to the ground....

Sorry, I think I gave the wrong impression! What made me laugh wasn't the fact that I didn't believe your 209N, so much as the fact that you quoted such a precise "two hundred (AND NINE)"! If you'd said that you only need a force in the order of a few hundred Newtons to keep a typica 1.5 tonne car moving on a flat road at 10MPH, I wouldn't have batted an eyelid! As it is, you've done something that I always used to tell my students off for. You've put blind faith in your computer programme to give you an answer which, when you think about it, can only ever be a VERY crude approximation! We recently had a placement student working with us and I wanted to know the volume of a small tank of fluid. It was roughly rectangular in section and I gave him the dimensions and left him to it. The tank was about the size of a shoe box. When I came back he told me it was 0.085342 (or somesuch ridiculous number) litres (even gave me several decimal places!) I told him not to be daft and he looked positively wounded and explained that he'd done it on the computer (implying, I guess, that it MUST be right)! I asked him how many pints of beer he thought he might fit in a shoebox and he (much more reasonably) told me that he'd get a couple in there at least. I then pointed out that in that case, the volume wasn't likely to be 0.0ANYTHING of a litre!

Getting back to this, I would hope that you would agree, that your answer is going to depend on a variety of factors that are not included in your equation. Temperature doesn't appear in there. Neither does suspension alignment, driveline layout, or tyre design. (unless you've not mentioned them). You quote (to a depresingly large number of decimal places!) a "coefficient of rolling resistance". Where does this come from? Is it REALLY the same for a car on cold, hard, skinny crossplies as it is for the same car on hot, wide, sticky slicks? Is it the same for a car running toe-out, toe-in, or paralell suspension settings? Is it the same for a 2WD and a 4WD car?

When you think about it, that clearly can't be the case! 4x4 manufacturers would never bother fitting free-wheeling hubs to save fuel if it didn't affect rolling resistance! Tyre manufacturers wouldn't develop energy-saving low rolling resistance tyres either! You mention that "f" is going to vary. I agree, but would go so far as to suggest that it can vary so much as to make a calculation like that almost meaningless!


As for my MPV trying to haul it's lardy butt up a frozen 1-in-5, I hope you've not bothered factoring in the air resistance at 2MPH! Anyway. I'd suggest that the rolling resistance won't be any different on a slope but that's just semantics. I think the rolling resistance will be whatever it is (and on ice that really means the square root of stuff-all)! and the component of the car's weight acting down the slope is by far the main force that must be overcome. (which, interestingly works out at about 3.8kN for a 20% slope by my reckoning, so we're not a million miles apart)! Just out of interest, what "coefficient of rolling resistance" does your programme give for ice?



It's even easier for me than that! I'm lucky enough to have a ramp in the garage with a jacking beam on it!

...unfortunately, however, that doesn't help much. It's quite hard to drive the car with your foot on the brakes AND at the same time, get out and try to stop one of the wheels turning! (and if my feelings are correct, it might also hurt quite a lot)!

Incidentally, please understand that I DO agree with you (and have done right through this thread!) that an ordinary "open" diff will only allow the engine to turn the wheel with the least resistance. What I AM saying is that in this case, applying the brakes and driving the car against the brakes seemed to help. If you don't believe it possible, I'd welcome any alternative theory you might care to put forard to explain it!

Actually, while we're on the subject of calculations, you also said:

"...and a with max engine torque of 160Nm, a typical car will be able to create a maximum of 8,800N of forward motion)"

I'd be interested to see how that number is derived too - seems quite a lot, intuitively!

As for accuracy, my PC is 100% accurate, it’s the user that is at fault!

Sorry but when I use generalisation of numbers people jump down my throat saying I’m not accurate and denounce me a charlatan. It is very convenient for me to tap the numbers in and I would not manually calculate such numbers for blogging (or anywhere else). I also prefer to rely on the calculations as sometimes the physical facts can be surprising and not always what we assume despite our experience.

I don’t think not my not noticing 1mph out compares to your student and his shoe box and pints of beer. As I pointed out the difference of 3N could be made under a miniscule difference in traction undetectable by humans. Could you detect a difference of 0.0002 in the coefficient of rolling resitance? Now you can see why I chose to use accurate figures when people pounce on the most ridiculously small error that has no bearing on the issue.

Still if I’ve made you laugh, at least I’ve spread a little happiness in the world before I die. Lol

But to answer what I believe are serious questions but if you are joking other people might like to know:

Tempreture, suspension, driveline, tyres are factors that influence the coefficient of friction so are included in the calculations and would hence vary according to each vehicle.

You may be veering towards how such calculations are used in the real world. Say a car gets wrecked with bald tyres, but the defendant says the accident was unavoidable caused by a third party regardless of his bald tyres. Then using known factors, the accident can be simulated mathematically to see what would have happened if he had bald tyres[*edit sould have said good tyres]. Whilst the defendant will always be culpable for the bald tyres, he may not be for a fatality if tests show it was unavoidable.

I have used an industry standard coefficient of friction for the relevant condition for the average car. Individual coefficient can be calculated but don’t vary significantly. Weight is the greatest influence on traction.

2wd/4wd does not effect the coefficient of friction. It’s the same regardless of whether or not they are driven. However for maximum acceleration/traction calculations, the weight distribution over the driven wheels is factored in the calculation.

Free wheeling hubs reduce wear of drive train when 4wd is not required and do reduce resistance, hence fuel, but these resistances are minimal.

Whilst f does vary, as different surfaces vary, f is accurate to the surface in case and so it is used with great accuracy. If ever you watch the aftermath of a serious car collision, you may see a police car locking its brakes up (ABS disconnected). This evidence is gathered so that the exact coefficient of friction can be subsequently calculated and be used by any parties.

As for your MPV, I can’t deny it I did add the 2mph. It’s a force of habit. Its “approximately” 1N by the way. But if I didn’t people would say my calculations were estimations – I can’t do right for wrong here!

You rightly suggest that the Rolling Resistance wouldn’t be any different on a gradient, however there would be Climbing Resistance which would be significant which must be added in to make the total Running Resistance.

Rolling resistance for Asphalt and ice are the same – they are both as hard. However I can see it’s the coefficient of friction you are seeking:
Coefficient of Static Friction of dry road is 0.85 and an iced road is 0.1

Re the Trolley Jack Test. You claimed that you could create a “diff-lock torque diversion effect” by applying foot brake when driving. Nothing to do with getting in and out – so please try it. I have spent a great deal of time and effort tying to make this thread informative, perhaps you could at least to the same and take 2 mins to do the Trolley Jack Test.

My explanation why both brakes won't divert torque is this. The Force takes the path of least resistance. It is easier to spin a wheel against ice than propel 2t of vehicle forward. Whilst applying the brake will make the spinning wheel ‘not so easy to turn’ it will also make the stationery wheel equally more difficult, leaving the spinning wheel still the path of least resistance.

The Trolley Jack Test would show this and I strongly recommend you do this.

There are potentially very specialise techniques of left foot braking that I use that can help traction but are not the issue here and see no point in going there, when everything is rubbished. However when you’ve done the Trolley Test Jack Test, let me know and I will gladly explore vehicle dynamics further.

Re; Your second calculation request. I hope you appreciate that have taken a lot of time so far and can’t justify making a manual calculation when you can’t find 2 mins for the Trolley Jack Test. However I understand there is an elite technical Safe Speed forum where people like me are excluded. Perhaps they might do a calculation for us or you may want to have a go at it yourself. I even did the Trolley Jack Test at 1am, pissed, on my drive, to make sure that I had not overlooked anything – heavens knows what the neighbours think!

Mole: Whilst I said 8800N and your intuition said much less, would you care to put a figure or range on that? Because I fear if I say its actually 8799N some may claim you victorious.

Perhaps you could tell me what your intuition says the figure would be for your 2t MPV and let me know the make and model and I can hopefully calculate it while you are doing the trolley test - seems fair to me?

Victorious?! What's with all this "adversarial" stuff?! I thought that was something blokes were generally guilty of!

Anyway, rest assured that if we end up agreeing on 8799 (or even 8000!) I'll still think you were right!

You mention that this relates to a "typical" car that has a peak torque of 160Nm. I'm making the assumption that this is the torque available at the flywheel? I'm also making the assumption that the "typical" car is a 2 wheel drive...

If so, then we have 160Nm going through the clutch and into the gearbox. The car's going to exert it's maximum tractive effort in 1st or reverse as this is the lowest gear - so let's say that the ratio is about 4:1. That means we have 4 x 160Nm = 640Nm going to the final drive. Again, let's assume the final drive is something like 3.5:1 so we have 3.5 x 640Nm = 2240Nm, coming out of the final drive. NOTE that I've allowed NOTHING for transmission losses - which makes what I'm talking about an absolute "worst case" for the purposes of my argument! If you even remotely agree with the logic, we can argue the toss about a figure for those later!

That 2240Nm is going to go down the half shafts to the tyre contact patches. Let's further assume that it is distributed equally between the two (because as I've already been told, if it's NOT equally distributed, the diff will make ALL of it go to the wheel with the least resistance! ) (Again, I make the further assumption that the "typical" car has an open diff)!

For my last assumption, I'm guessing that the wheels of this typical car will have a rolling radius of about 300mm. As a torque is a force times a distance, we have a torque of 2240/2Nm = 1120Nm acting through a distance of 0.3m so we have a force of 1120Nm / 0.3m = 3733N at each contact patch - giving a total available tractive force of 7466N. (and even that seems like a lot)!

Naturally, depending on the road surface, not all of this might be usable. If we assume a decent set of tyres and a good, dry road, then as long as the driven axle weighs more than about 800kg, the wheels shouldn't spin.

So there you have it. I don't think the car in your example could exert a tractive force greater than about 7.5KN and if we make some allowance for transmission losses, I think we'd be below 7kN.

As for the 807, well I don't know the gear ratios but it has a rather healthier 300Nm of torque (or at least that's what Peugeot claim)! The weight won't have anything to do with the price of fish as long as it's heavy enough not to spin its wheels when it hits peak torque in 1st on a dry road. (And I can assure you it doesn't do that)!

No agenda - would just be curious to see where your computer disagrees!

Will repsond to the other one later - my tea beckons...

I’m referring to maximum engine torque, the figure quoted in the brochures.

2wd/4wd is not an issue for generating motive force. It’s an issue when it’s better to spread it between 4 wheels rather than 2. My jeep creates the same motive force in 2wd as 4wd, but obviously gets better grip in 4wd. But if the maximum acceleration can’t exceed the traction available on a road, then it will accelerate just as fast in 2wd as 4wd. (but if I engage low range 4wd gears that will increase acceleration at the expense of top speed but knacker the transmission on a road!)

Your principles are right re the trail of 160Nm to the wheels, but here’s the calculation:

Motive Force = ((M.i)/r)n

M is engine torque, 160Nm
i is Gear Ratio 17 (note this includes not only the gear box ratio for first gear but the intermediate reduction ratio – often brochure publish pointless gear box only ratios, show that the marketing department haven’t a clue!)
r is the dynamic radius of the tyre, 0.29m
n is the drivetrain efficiency 0.93

Do the maths and that 8723N

As you pointed out it’s just a gearing ratio thing, right down to the rolling diameter of the wheel.

This set up would give a maximum acceleration of 2.42 m/s2 (1400kg car). But for some reason the chavs round here fit bigger wheels rubbing the wheel arches – they should put smaller wheels on!!
If they fitted front wheels 10cm smaller (rolling diameter of 0.24m) this car would accelerate at 3.71 m/s2. If they could get down to 0.21m rolling radius they would double their acceleration - may look silly, but would be faster! (if you don’t mind losing some top end that is, but all these chavs do is race from up and down the high street from chip shop to chip shop)

I’ve banged your figures in the PC using 100% efficiency, diameter of 300mm and a gear ratio of 14 (your combo of 3.5x4) and yes we agree, well almost PC says 7467 you say 7466! Lol

My guess is your ratios come from a higher torqued engine or lighter car.

Hope you enjoyed your tea.

Blimey! What's the car with a 17:1 overall first gear ratio?! Sounds like a Landrover on Mini wheels! I thought I was going overboard on the low side with a 4:1 1st gear and a 3.5:1 diff!

Anyway, if that's "typical", then I must concede defeat, but at least there's nothing different in the what thay me and your computer are looking at this - just down to selecting numbers. The ratios I chose weren't off anything in particular. The only handbook readily to hand was my wife's 2 litre petrol 156 so I went for something imaginary and much lower geared. My own car (not the MPV!) has a 3.5:1 first gear and a 3.1:1 diff. It's rolling radius (dynamic radius as you call it!) is probably not far off 300mm (being an oldie on 15" wheels but with much higher profile tyres than more "modern" machinery)! It does, however, have a generous 260Nm of torque!

If I might speak up on behalf of the chavs though...

...most of them won't go too far up on overall wheel diameter because they go up on rim size but down in tyre profile so as to make everyone think they're driving a touring car rather than a clapped-out Corsa or Saxo! Also, the acceleration figures you give in ms^-2 are only transient for a fraction of a second as the engine hits peak torque in each gear. Obviously, dropping gearing (by whatever means) is going to help acceleration but only if the box can shift quick enough. If they dropped the tyre diameter but it meant that the all important 0-60 needed another gear change, they'd probably loose out overall. As a kid, my parents had a Sherpa camper van which we didn't feel went fast enough so we found an old "Essex" V6 to drop in. With the original Sherpa gearbox, it didn't actually go any faster because the gears were far too low and you simply couldn't change them fast enough (the stick must have been 2 feet long for a start)!

Stuff it! I got half way through a witty and erudite reposte and my ^*)^&^*&$£ computer crashed!

I'm off to bed!

The overall 17:1, 160Nm, 1400kg is from an actual SUV.

Yes, I reckon the radius/rolling radius is barely different for our purposes, but at higher speeds start to influence calculations more. That’s why accident investigators always take tyre pressures asap (but cold) where possible, so the actually radius of the day can be used in the future and of course “wrong” tyre pressures in themselves may be evidence. However, if the car was wrecked, hopefully another could be found to then calculate the coefficient of friction on soft/hard tyres – they may in fact increase traction in some scenarios. If another car can’t be found then a similar car can be used and have its weight apportioned appropriately and the tyres in case transferred to new rims if necessary.

Of course the acceleration figures I quote are the maximum for that gear at the peak of torque, but with a torque curve, it can be broken down into a series of finer calculations and distance over time can be calculated with changing gear factored in.

For example if a Police officer is in a pursuit vehicle and claims “We were doing 100mph, put the blue light on and the Ka in front just sped away and we couldn’t catch it”, whilst you and me know something’s not quite right, a calculation settles the matter. Not just maximum acceleration but actual “higher” top speeds when on a down hill gradient, or visa versa.

Know what you mean about Sherpa’s, Dad had several petrols and the last one was diesel. I drove the diesel a few time and I did get the feeling that it needed a good service – I think it’s top speed was 55mph – or it was just too painful to go past that.

Cracked it with the horse box though. The 7.5t trucks are a bit tame, so brought a 12.5t with more grunt and down plated it to max 7.5t to drive on car licences (second hand chassis 12.5t are often as cheap as 7.5t). Where there’s a will there’s away!