Female drivers and snow.
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From: Mortimer on 8 Jan 2010 09:26 "DavidR" <curedham(a)4bidden.org.uk> wrote in message news:7qorv3F5vfU1(a)mid.individual.net... > "Steve Firth" <%steve%@malloc.co.uk> wrote >> DavidR <curedham(a)4bidden.org.uk> wrote: >>> "Keith" <keith(a)mailinator.com> wrote >>> > > > The fact that a lower gear can (*) put more torque to the driveshafts is > not a proper explanation for the advice to use a higher gear when it's > slippery. As such, it is the old wives' explanation. > > I am not an experienced ice driver so cannot claim the advice is > fundamentally wrong but, if it is good advice, I am interested in > something > that's more technically credible. > > (*) The important word being *can*. Not *must* or *will*. Very true: "can" not "will". But when you use a higher gear you are reducing the sensitivity of the car to slight changes in throttle position and so making it less likely that you will apply slightly too much torque, thus crossing the line from the maximum that will still give traction to the minimum that will cause slipping. When you are setting off, you can vary the torque more precisely with the clutch (automatics don't allow this - you can only control the torque with the throttle) but once you are fully engaged (clutch engaged) there is a risk that as you accelerate you might overcook it slightly while accelerating or going from level to uphill gradient. Anything which limits this tendency is a good thing.
From: DavidR on 8 Jan 2010 09:48 "Mike Barnes" <mikebarnes(a)bluebottle.com> wrote > DavidR <curedham(a)4bidden.org.uk>: >>"Keith" <keith(a)mailinator.com> wrote >>> >>> but NOT ONCE have >>> I heard them give any practical tips: use high gear, low revs; >> >>But what is the reasoning? > > My guess: it's less jerky and therefore you stand less chance of > momentarily exceeding the available grip. Given the characteristic of the throttle/accelerator and the contribution of engine friction, I would assume the opposite.. > For an exaggerated > illustration, compare the smoothness of driving at 30 in fifth gear and > in first gear. ....my present car won't do 30mph in either 1st or 5th. That aside, there only seems to be a difference in engine noise.
From: Steve Firth on 8 Jan 2010 10:04 DavidR <curedham(a)4bidden.org.uk> wrote: > > >> But what is the reasoning? Please don't regurgitate the old wives' > >> "there is more torque". > > > > Umm, actually it's the opposite. There's less torque in higher gear and > > less torque at low revs. > > I didn't complete the sentence above Actually you did. You put a full stop at the end, see? > because I didn't expect that some people wouldn't understand the basic > sentiment behind the question. Oh I think I understand that pretty well. What I don't understand is that your statement is the exact opposite of the reason given for using a higher gear. Do you think you could explain that incongruity? Did you perhaps type "more" when you intended to type "less"?
From: Brimstone on 8 Jan 2010 10:24 "DavidR" <curedham(a)4bidden.org.uk> wrote in message news:7qorv3F5vfU1(a)mid.individual.net... > "Steve Firth" <%steve%@malloc.co.uk> wrote >> DavidR <curedham(a)4bidden.org.uk> wrote: >>> "Keith" <keith(a)mailinator.com> wrote >>> > >>> > but NOT ONCE have I heard them give any practical tips: use high >>> > gear, >>> > low revs; >>> >>> But what is the reasoning? Please don't regurgitate the old wives' >>> "there >>> is >>> more torque". >> >> Umm, actually it's the opposite. There's less torque in higher gear and >> less torque at low revs. > > I didn't complete the sentence above because I didn't expect that some > people wouldn't understand the basic sentiment behind the question. > >> This is a matter of fact, not a matter of old >> wives' tales. > > The fact that a lower gear can (*) put more torque to the driveshafts is > not a proper explanation for the advice to use a higher gear when it's > slippery. As such, it is the old wives' explanation. > > I am not an experienced ice driver so cannot claim the advice is > fundamentally wrong but, if it is good advice, I am interested in > something > that's more technically credible. > Why worry about the technicalities? It works, so why not do it?
From: DavidR on 8 Jan 2010 20:05
"Mike" <nospam(a)nospam.com> wrote in message > On Wed, 6 Jan 2010 21:35:49 -0000, "DavidR" <curedham(a)4bidden.org.uk> > wrote: >>"Keith" <keith(a)mailinator.com> wrote >>> >>> but NOT ONCE have >>> I heard them give any practical tips: use high gear, low revs; >> >>But what is the reasoning? Please don't regurgitate the old wives' "there >>is more torque". >> >>> use engine braking to slow; slip the clutch for extra traction. >> >>Eh? There *is* ever only so much traction. > > By selecting a higher gear you effectively reduce throttle sensitivity > at the wheels. Disagree. The accelerator/throttle is essentially (neglecting engine friction) a power controller. Insofar that by holding it at any fixed position, the engine output is same whichever gear is used. The torque adjusts in proportion to speed. When friction is factored in, the delivered power for constant throttle actually rises as revs drop in higher gears (which is why the fuel consumption changes with gear selection). So let's say the minimum engine speed is 1000rpm, then the lowest practical speed with clutch engaged is roughly 10mph. In 1st, we could go down to 5mph but for illustration we stick to 10mph, with the engine doing 2000rpm. The delivered power will be identical but the power lost to engine friction will double. So what's the benefit? For instance, if delivered power is 5bhp and engine friction at 1000rpm is 5bhp, in the limiting case of hitting a a patch of zero traction, then the whole 10bhp will return to the engine which will cause it to rise to 2000rpm. If taken instead in 1st at 2000rpm it would require the same 5bhp delivered but now 10bhp is required by friction. In the same limiting case, all 15bhp returns to the engine which will cause it rise to just 3000rpm. Furthermore, flywheel inertia will give a reduced rate of increase. So overall, a higher base engine speed should give greater damping effect. > Floor the throttle in 2nd or 3rd and you'll still > break traction, drive similar to what you would crawling in 1st in > traffic at the point of bogging down the engine and you make progress. > Many years ago 2nd gear and the base idle control / anti-stall got me I once crawled uphill at 300rpm in a carburettor car. An electronic engine would have a fit. My last one wouldn't maintain stable idle under drive. My present one idles ok but otherwise has the grumps up to 1200. > up a 1 in 6 hill with a near 90 deg bend halfway up it, with freshly > fallen snow as deep Getting up up a 1 in 6 with 2wd needs a mu of at least 0.3 under the drive wheels. That's pretty decent grip. Come back when it's slippery. > as my ground clearance, on what were in absolute > terms the lowest section height tyres on an EU production road > vehicle. > The slightest application of throttle would have broke > traction and left me stranded, just like many tens of others who had > booted the throttle and were trying to use brute force. As it was I, > with a handful of others made it up the hill without problem. So really, the only significant factor is to be gentle with the pedal? |