minumum acceptable acceleration in a general use automobile.

[carnage1]

Are there any guidelines as to the minumums for acceleration? here inthe US I would assume minumum sustainable speed would have to be 75 mph (120kmh).So in the quest for smaller motors for higher fuel mileage is there a limit as to how slow a car you can make?

 

[IRstuff]

Don't know if it's documented anywhere, but some freeway on-ramps in SoCal are awfully short. In particular, on the Pasadena Freeway is an on-ramp with a stop sign, a blind curve for the oncoming traffic and less than about 50 ft to get up to a decent speed, which would be at least 45 mph.

I get about 13 m/s²

TTFN

 

[MacGyverS2000]

Back in '97, Geo's 2-door Metro cranked out a whopping 55 hp. I wouldn't be caught dead behind the wheel of one (which you most likely would be trying to get onto a SoCal freeway in one), but that goes to show you how little power some cars out there have.


Dan - Owner

http://www.Hi-TecDesigns.com

 

[Fabrico]

Or the thousands of on ramps that are nice and long but have a red light at the freeway end. 1/4 mile in 25 seconds?

 

[Comcokid]

I'm not aware of any guidelines for minimum acceleration. There used to be the signs on interstate on-ramps indicating that vehicles under 10Hp were prohibited. Interstates still carry signs indicating (usually) minimum speeds of 40 MPH. These signs are used to keep small motorcycles off interstates. But, these small motorcycles are legal on streets and regular highways.

In Florida, there are lots of slow cars. But, this is a limitation of the driver, not the vehicle!

 

[GregLocock]

IRstuff, that would be fun. Very few cars can accelerate at greater than 1g.

My Mini had 40 hp and was capable of keeping up with London traffic (and if you don't know what that means, fine, but don't be a smart-a).

You'd probably find that gradient climbing at speed is what really sets the power requirement.

I have seen data logged for cars for millions of seconds, it is very interesting to plot rpm vs throttle opening to see what performance real people actually use.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[IRstuff]

I hear you.

Obviously, not many cars actually get up to the minimum safe speed at the onramp. I've drive it once, and it was a harrowing experience, because there's no way to see if there is any oncoming traffic coming up from behind until they get around the bend in the freeway, which is only about 100 ft back of the onramp.

The Pasadena freeway was built back when cars didn't go much more than 40 mph to begin with, so apparently, the designers felt comfortable with the onramp design back then.

The 0-60 mph time for a Porsche 911 can get down to 3.4 seconds, which is only 7.9 m/s²(

http://autos.msn.com/advice/article.aspx?contentid=4024058).

A more pedestrian time would be in the 10-20 second range, so the long side would be about 0.1 g

TTFN

 

[IRstuff]

Here's a link that might be of interest:

http://www.eurotuned.com/0-60.htm

There are several cars, albeit quite old, that take about 20 seconds to do 0-60, so that's about that, other than the faster guys flipping you off for driving so slow as they pass you.

TTFN

 

[GregLocock]

Yes, my guess is that anything faster than 0-60 in 15 seconds is more than capable of keeping up, and in practice most people drive at accelerations equivalent to 0-60 in 20 seconds.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[WGJ]

Shouldn't the issue be more like the power-to-weight ratio rather than outright power?
50hp in a Fiat Panda is about the same as over 200hp in a lot of SUVs.

Bill

 

[NormPeterson]

Torque to fully loaded weight might be a better metric for anything resembling "normal driving". Include gearing if you want to be picky. Consider what happens to the potential acceleration when you add the same several hundred lb passenger load to both the Fiat and the SUV, or when you drive a Honda S2000 without getting within 1500 rpm of the VTEC switch-over point (relative to never dropping below it).

As for the lower limit on acceleration potential, that's a function of whatever minimum value will find sufficiently wide consumer acceptance. Reducing it below some vague point would probably involve psychological stuff and lots of consumer education - I suspect that most people don't care to use wide open throttle very often and would really hate to feel that they had to on a regular basis. And end up hating the car for it.

Greg has previously mentioned the ability to maintain speed on grades, to which I'll add coping with headwinds if a low maximum speed limited by power is selected.

Keep in mind for minimum top speed purposes (presumed reasonably achievable if set barely above normal posted speed limits) that the last few mph is added very slowly, as there is not much surplus torque available to accomplish the acceleration. The last mph could take over half a mile to add.

I'll have to bounce this topic off someone I know who drives an 18-wheel flatbed for a living (OTR) the next time he's in town, just to get thoughts from a different point of view.


Norm

 

[WGJ]

Norm - Quite right on the loading issue, guilty of not thinking. It's Friday and that's my excuse.
Me in a Panda adds about 12% to the weight. Me in a humongous SUV isn't worth working out.

 

[swall]

The times I knew I was underpowered was climbing mountain grades. Had a rental Nissan Stanza once in Utah, and it's top speed going upgrade was about 45mph. Owned a 4 cyl Ford Ranger and its top speed going up the grades of the West Virginia Turnpike was slightly above 50mph.

 

[NickE]

I have to admit I use WOT and the upper rev band (above Torque Peak for defining what the upper band constitutes) every day and love it.....

But then again I'm not your average driver....

(And I dont drive your average econobox... yet.)


Nick
I love materials science!

 

[MacGyverS2000]

I'm with Nick... I'm a driver of the previously mentioned S2000, and a day that goes by without revving it out a little means I'm sick in bed.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[thundair]

I can remember when Honda first came over to the states with a 360cc engine in a little cracker box. and the would get on the freeway with a max speed of 55MPH.( almost hit one in blood alley with a closing speed that made it look like it was parked).
If there is not a minimum there should be. lol


I don't know anything but the people that do.

 

[IRstuff]

It's funny that the newest whiz-bang hybrid cars are trying to get about 40 mpg, when that Honda got 50 mpg when it was at its peak.

TTFN

 

[ewh]

My '78 Civic was rated 42 city/52 highway, but they have since changed how the numbers are figured.

 

[NormPeterson]

Mixed thoughts from the OTR truck driver. Thumbs up if it would serve to cut down on the number of car drivers traveling at 30 mph more than his rig will do. Thumbs down if acceleration to highway speed as actually used is lacking and the ability to maintain speed up long grades is poorer.

To some extent, the above are in conflict with each other. But if it can be assumed that people really won't hold the accelerator pedal flat to the floor for miles on end maybe there's a loosely defined range of top speed that satisfies the other criteria.


Norm

 

[IRstuff]

I can say that when you're trying to pass a semi on a two-lane, there NEVER seems to be enough acceleration.

But the simple answer is that you can divorce the acceleration from the speed by using a governor. An electronic one could even allow you some leeway with a limited duration of higher speeds for passing.

TTFN