12 Feb 2010

How to choose a car for regular long hours driving?

If you search in AutoTrader.co.uk for cars less than 10-year old and having 150,000+ mileage, you will mostly get Lexus, Jaguar, Mercedes, Volvo etc.

Few things are similar in all those models, viz. they are all large saloon/estates, diesel and automatic.

Are small cars not capable of doing high mileage? Yes, they definitely are. But the question is, are they comfortable enough?

This is the question I am searching for answer myself at present. I am doing 40+40 miles (= 65+65 km) everyday in a small manual car and feeling the pain in my body!

Then I searched over the internet whether any scientific study has been made on comfort of car seats. I did find few but none were very brief. Most of them include boring PhD experiments data.

In the end, I figured out what are the parameters to look for while choosing a car for daily long commuting. That is why I wrote this blog.

Please note, if you are doing less miles (< 10,000 miles/year) or 3+ hours of driving occasionally then you do not really need all these research. However, if you do around 15,000+ miles a year and spend at least 3 hours on driving seat every day, then understanding how human body behaves in longer journey helps save the body in longer run. To appreciate findings discussed here, you must be spending a lot of time on road driving a car.

Advice #1: Get an automatic car

When you press brake or accelerator, your right foot does not need to travel much as you will rarely press brake and accelerator to the floor (unless it is an emergency braking or you are doing a drag race). On other hand, to change gear, you need to depress clutch fully to the floor (otherwise cogs will scream). So, your left foot needs to travel more than right foot. For long motorway driving, it makes sense to position seat so that your legs are properly stretched and you do not seat too close to steering (if you do, you will feel pain in legs within an hour). However, every time you depress clutch, your left leg travels further than right leg and it twists your body. For this reason, sometimes I need to reposition my posture in seat for best comfort. Also, throwing gear lever by left hand continuously (in stop-start traffic) twists body slightly on left. All these actions for a continuous period day after day put strain on back! An automatic saves this problem! Some automatics have foot operated parking brakes (e.g. most Mercedes, Kia Caren etc.). You engage the parking brake via pressing a pedal (besides fictitious clutch) and release by pulling a lever on right hand side. Now I have not driven such cars for very long distance, but I assume pressing such brake using left foot may bring the same problems faced by operating clutch in manuals! Also, foot operated parking brake in a manual car is complete nuisance (try uphill start).

Advice #2: Get a bigger car First reason is obviously safety.

A small car on motorway is very vulnerable in case of an accident. Besides this, a large car is more stable. While overtaking long articulated lorries at 80 mph (= 130 km/h), I can often feel my small high sided car being affected by side winds! Any high sided car (e.g. SUV) - big or small - has its center of gravity at a high position and likely to topple after a collision. A bigger saloon/estate/MPV is more likely to absorb road irregularities much more than smaller cars. Big cars usually also transmit less noise and vibration to cabin though it depends on particular model.

Advice #3: Get a diesel car

One obvious reason is fuel economy. Diesel tends to achieve a better fuel economy than petrol. Diesel engine produces more torque than equivalent petrol engines. A petrol engine produces 100 Nm/L while a diesel gives 150 Nm/L (L stands of engine displacement here). Gearbox in any car is meant to act like torque multiplier. Since diesel engine produces more torque over a wider range of engine RPM, one does not need to shift gear so much (in case of a diesel manual) compared to a petrol engine. Also, bigger cars require bigger engines and big petrol engines tend to become uneconomical for regular long commuting.

Advice #4: Get a car with all sorts of seat adjustment (and preferably with cruise control)

Only luxury cars will offer steering with reach and rake adjustment and multi-way driver-seat adjustment. Some expensive cars offer electric seat adjustment which is very helpful for getting most comfortable driving position. A good seat must offer at least these - seat forward-backward movement, adjustable lumber and seat height adjustment. There should be a foot rest besides clutch for left foot. Many cars do not have it. This is essential for longer journeys (both in manual & autos). This footrest allows the left lower extremity to exert effective counter-pressure for preventing the forward migration of the pelvis on the seat. This is one of the most important features to look for in a car! I personally find SUV seats are most comfortable as I seat in upright position rather than in a crouching one! Too low seat will make ingress-egress difficult.

Advice #5: Avoid cars with seat/steering/pedal offset

Though surprising it may sound, many cars have offset seat and/or pedals and steering at at angle. Even though these values may be small (to judge in naked eye - without proper measurement), seating in this posture day after day will introduce pain over the body! Steering wheel offset - when center line of seat will not align with center line of steering Pedal offset - pedals are positioned either too left or too right Steering wheel angle - the steering is not exactly perpendicular to cars horizontal cross section line. The old Ambassadors in India had a visible wheel angle problem! This site has good demonstration of offset problems - http://car-seat-data.co.uk/ Even some very expensive cars suffer from offset problem! Often LHD cars are not properly adopted for RHD. Unfortunately, the cars which have all these good features discussed above are often costly!

Few other points worth noting here.

A comfortable seating requires good arm rests. Most cars have one armest on door handles (make sure you can rest your elbow there while holding the steering) but only a few have central armrests. Having both left & right armrests will help to hold the steering with any one hand while cruising. Cruise control of its own not much useful unless it an adaptative type one. In adaptative cruise control, you can adjust cruising speed by controls on steering wheel. Pressing brake anytime (also clutch in manual) cancels cruise control. Cruise Control helps relieving pain in right leg. All the controls and dials should be easily accessible from drivers seat. Visibility all round should be good. You need not twist your body too much during reversing. Since this blog is mainly written for comfort of long journeys, few other points worth noting as you are likely to use your car for some other journeys as well. In general, RWD cars do not perform well on snow (unless you use winter tyres). A FWD car is often easier to use when big freeze happens. How do you figure out which cars are to be short listed for test drive? Simple, look out which models are for sale in AutoTrader those are diesel, automatic and clocked over 100,000 miles yet less than 5 years old. The car you buy must be reliable not and too expensive to run (unfortunately some comfortable cars do tend to cost a lot maintain).

Update:

Last couple of days I was experimenting various seat back (lumber) angles.

So far my conclusion is adjusting lumber at 100 degree angle (from horizontal where 90 degree is just lumber exactly vertical) offers most comfortable position for spine. I also observed that most mini cab drivers keep their seat back at highly obtused angle (ie. even more than 100 degree).

2 Feb 2010

What is Dimension in Oracle data warehousing?

If you are using Oracle (10g upwards) database for data warehousing, chances are you might have come across DIMENSION in Oracle.
What is it and how does it help?

In data warehousing environment, often you find tables do not have referential integreties among them. While, I do not condone it, sometimes it is un-avoidable as source data comes from mainframe where, due to historic design, RDBMS like constraints could not be implemented.

So, when these data are loaded to Oracle tables (without any cleanup), implementing RDBMS constraints becomes difficult.

Without primary/foreign key relationships, Oracle optimizer will be confused as it won't know how to join different tables.

This is where Dimension comes to help!

Dimensions are simply logical constrains defined among tables. A product code may be logically related to transaction table which is defined by CREATE DIMENSION command.

What is the advantage? Well, it then behaves as if it knows foreign key relationships between tables (though not exactly the case).

The main performance gain is that it helps Oracle to rewrite queries so that it can either use a materialized view or avoid complex joins.

If you run a query which does something like this - show all possible aggregations from a very_large_table, it may take a very long time to complete.

Where as, if your DIMENSIONs are defined then Oracle may fetch same result by quering a smaller_table (which may be materialized view).

By defining Dimensions, you tell Oracle where else it may look for to satisfy your complex aggregation requirements.

There are some pre-requisites though, you must enable query rewrite enabled parameter to true beforehand.