你GOOLE一下，谁不会，你懂么？ 解释一下如何买车，车主如何算经济账？
如何买车? 早告诉你了呀。 
I have received several e-mails lately asking which is the best tow vehicle. This is a very difficult question to answer because there are so many factors that go into the make up of a tow vehicle. Often the e-mails I receive are isolating one factor and treating it as though it is the only factor of importance in tow vehicle selection. It got me to thinking that while we have examined most of these factors in detail at one time or another, I have never really written out a complete list. The following is a summarized list of specifications to look at when assessing a tow vehicle, with some of my personal thoughts in each category.
Wheel Base
A longer wheelbase is better for towing, but there is a trade off in manoeuvrability. It also takes a stronger equalizing hitch to transfer weight to the front wheels of a longer wheelbase tow vehicle. When towing a fifth wheel, wheelbase is a very small factor, and a short box truck handles virtually identical to a long box model. The short box has less chassis flex so it will ride slightly better than a long box.
Rear Overhang Ball to Axle
A shorter overhang is better, but which is really better - a short overhang and a short wheelbase, or a long wheelbase with a long overhang?
We look at the overhang as a percentage of the wheelbase. For example, a 116" wheelbase and 40" overhang comes out to a 35% overhang ratio. A 130" wheelbase with a 50" overhang comes out to a 38% overhang ratio - in this case not as good as the shorter wheelbase. Of coarse, an even better combination would be a 130" wheelbase with a 40" overhang, which would come out to 31% overhang.
Rim Diameter
Larger diameter wheels allow you to have a lower profile tire, and still have the same tire load capacity.
For example, a 215/65R x 15" tire has the same load capacity as a 215/60 x 16" tire, but the sidewall is 11mm shorter, and generally, shorter sidewalls have less sway.
Rim Width
The width of the tire is the three-digit number in millimetres. In the case below, 225mm or 8.8" wide. The ideal situation is to have a rim that is almost as wide as the tire, in this case an 8" rim width would be ideal. Today, many SUV's and pickups come with tires 10 or 11 inches wide on 7" wide wheels, so we can dramatically improve the handling by simply installing wider rims, or (more often) narrower tires. (Page Top)
Tire Width & Profile
For example: 225/60R x 16". We want the tire to be as close to the same width as the wheel rim as possible.
The profile is the height of the tire sidewall as a percentage of its width. In this case 225 x.60 = 135 Millimetres. Generally, a tire with a shorter sidewall has less side sway and more precise handling, but also less load capacity. For example, a 225/60 x 16" works very well on a Mini Van, but won't carry a Suburban or full size Pick-up.
Tire Type; Light Truck, Performance Or Passenger
(Performance tires usually carry 44 PSI)
Performance tires are best, but usually have less traction in snow and a shorter tread life. Light truck tires do not have as much traction or as precise handling as performance tires, but offer excellent tread life. We often find that light truck tires have to be used for their load carrying capacity. Passenger tires are usually the least effective.
Rear Suspension Type. Independent, Live Axle OR Mono Beam
Independent suspension is a substantial advantage for a tow vehicle because it projects the suspension stance out to the width of the track. It also has a fraction of the un-sprung weight. A live axle is least desirable.
Suspension Type - Front. Independent, Twin I Beam Or Live Axle
Again, a full independent suspension is superior. Twin I beam is better than a live axle.
Live Axle, Distance Between Springs - Front
Sometimes called suspension stance. In most cases, the wider the better. This is not a factor on vehicles with independent suspension since the suspension stance is the same as the tire track.
Live Axle, Distance Between Springs - Rear
Same as above.
Spring Stiffness
Firm springs are better than soft ones, but not to the point that they are so stiff that they will not work in concert with the equalizing hitch. A slightly stiff but supple suspension is best. If the vehicle sits low at the back when you connect a travel trailer, then the problem is usually in the equalizing hitch. It is OK for a truck towing a fifth wheel to sit a bit low at the back, as long as the suspension still has reasonable travel.
Shock Stiffness
Generally, you cannot have too much shock control. It is far better to have soft springs and stiff shocks than the other way around.
Curb Weight
A heavy tow vehicle has a slight advantage especially if it has a low centre of gravity. You are better off with a light, low profile tow vehicle with independent suspension etc., than a heavy, tall one.
Height
There is a slight economy and handling advantage to a lower tow vehicle. (Page Top)
Inches, ground / bottom of rocker panels. Middle of car
What we really want is a lower centre of gravity. For example, an SUV and a Mini Van are about the same overall height, but the SUV carries its weight much higher and pays a price in handling.
Steering System, Rack & Pinion or Steering Box
Rack and pinion steering has much better steering feel, and gives you more precise handling.
Power Curve Charts for Engine
These are graphs that show the horsepower and torque produced by an engine, ranging from idle to its RPM red-line. If you can find them they will give you a better reference for determining gear ratios.
An engine that produces its torque over a broad RPM range is great for towing. This is why engines that have four valves per cylinder usually perform much better than two valve engines.
Some engines are now available with variable valve timing, which further widens the power band.
Horsepower
Generally, more is better.
Torque
Again, more is better, but look for it over a large RPM range.
Final Drive or Rear Axle Ratio
The best axle ratio depends on many factors. Generally, a numerically higher ratio is better - but not always. Some of the factors are the tire size and the RPM that the engine produces power at. For example, you need a numerically lower ratio in a Cummins diesel that produces its power at 1600 RPM, than you do in a 24 Valve V/6 that needs to run at 2600 RPM. Though axle ratios are often tied to weight ratings, they are really more determined by aerodynamic drag.
First Gear Ratio
There are advantages both ways - a numerically higher first gear ratio will give you more kick of the line, but it will climb steep low gear hills at a lower speed.
With numerically higher first gear ratios, there is likely to be flat spot between first and second gear, unless,, of coarse, you have a 5 speed transmission - then you can likely have your cake and eat it too. (Page Top)
# of Transmission Gears
More is better, especially with a diesel engine that has a narrower power band.
# Valves per Cylinder
Multi-valve engines tend give you the broadest power range. They let you have low-end torque and high rpm horsepower out of the same engine.
Drive Train - Front, Rear, or 4WD
Front or rear drive does not really matter in the selection of a tow vehicle, although front wheel drive has a slight traction advantage, and more efficient power delivery. Four-wheel drive has great traction, but usually some power and economy losses compared to either front or rear wheel drive systems.
Supercharging or Turbo Charging
Can be great for short bursts of power, but you don't want to rely on constant boost for towing.
Tow Vehicle Drag
This is not something you can look up anywhere. The drivetrain has to tow the trailer, and it has to push the tow vehicle as well. For example, a 3.8 Litre V/6 in a mini van gives excellent performance with an aerodynamic trailer, but if we took that same engine and installed it in a 4x4 1 ton dually, it would be doing all it could to move the tow vehicle.
Rear Gross Axle Weight Rating (GAWR)
This is the load capacity of the rear axle of the tow vehicle. Usually, it is not a big concern for towing a travel trailer with an equalizing hitch that is properly set up. It is important with fifth wheels, where all the pin weight is carried on the back axle. To determine the amount of pin weight your truck can carry, weigh the back axle, and subtract the total from the rear GAWR. Remember to allow for loading of the fifth wheel and truck.
The Bottom Line...
When you look for a vehicle that satisfies as many of these criteria as possible, you come up with something like a Corvette. Ah - but you don't want to tow with a CorvetteÉ after all, where would you put the kids and the bikes? Now that you know you are going to compromise, look for the vehicle that suits your lifestyle the best, while giving you the maximum towing prowess. |
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发表于 2008-12-12 18:26:14
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