weighing vehicles

Technical discussion on converting internal combustion to electric
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Lahzie
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Post by Lahzie » Tue, 19 Aug 2008, 09:19

Grretings all

I'm considering converting a Tarago van and am doing some prelim calcs to spec the motor/controller and battery pack.

First question: Where can weigh my van. Will local mechanics have scales built into their hoists.

Second question: I was going to measure the aerodynamic and rolling drag by getting on a flat piece of road on a still day, getting up to speed and measuring the deceleration when go into neutral. Is this reasonable. Is there easily mountable equipement that can measure speed/wheel rpm more accurately than the speedo.

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woody
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Post by woody » Tue, 19 Aug 2008, 14:51

One way is to go to the tip which normally weigh your car on the way in and the way out, and ask for a weigh, which hopefully will cost you nothing.
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Peter C in Canberra
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Post by Peter C in Canberra » Tue, 19 Aug 2008, 15:11

It should be possible to find the mass of your car mentioned in a listing of specifications or a review on the web. The actual mass shouldn't be much different.
As for your test of wind resistance, I'd say yes and no to your question. Whatever resistance you can deduce from your rate of deceleration will be the sum of wind resistance, rolling resistance of tyre, friction in hubs and so on. However, it is only the total that really matters. Also, an empirical result is better than any theoretical estimate. At higher speed the proportion from air resistance will be higher. What you could do is time your slow down from (say) 100Km/h to 80Km/h. For a simplifying assumption you can ignore that the actually deceleration will not vary in a linear way and just call that the approximate resistance you have at 90Km/h. Similarly, you could time your rolling deceleration from 80 to 60 and call that your resistance at 70Km/h, and so on. This should be close enough to know approximately how much power is required to maintain speed at around 30, 50, 70 and 90 KM/h. You would not be too far out for any speed if you graphed a curve through those results.
All theory on my part. I have not actually done that but can't think of any flaw besides the approximations made along the way.
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Post by Lahzie » Tue, 19 Aug 2008, 21:17

The tip eh I'll give it a go.

For a simplifying assumption you can ignore that the actually deceleration will not vary in a linear way and just call that the approximate resistance you have at 90Km/h. Similarly, you could time your rolling deceleration from 80 to 60 and call that your resistance at 70Km/h, and so on. This should be close enough to know approximately how much power is required to maintain speed at around 30, 50, 70 and 90 KM/h. You would not be too far out for any speed if you graphed a curve through those results.
That was pretty much what I was going to do.

Thanks guys Image

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Post by Tim » Tue, 19 Aug 2008, 22:58

Lahzie wrote: The tip eh I'll give it a go.


Either that or the local sand and metal depot/landscaping supplies.

Most will weigh you and tell you for free, or write out a weighnote for a couple of bucks.

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Post by Lowndsy » Wed, 20 Aug 2008, 00:00

no offence but that method of aerodynamic testing will tell you nothing. the mechanical resistance of your hubs is huge and even the slightest backwind will add up when ur at speed to make quite a difference. saying that, the wind can be overcome by repeating the process in the other direction on the road (how top speed trials are run).

prolly the most effective way of measuring aerodynamics at home is using string and stickers.

cut pieces of string about 100mm long, make sure the colour stands out from your paint. put a sticker (the square ones for name tags are ideal) on one end of each bit of string and cover your car with them (about 20cm apart from eachother). when your car looks sufficiently rediculous, find your flat road. get a mate to drive your van and you drive along side watching how the string blows. in aerodynamic areas, the string will blow back (as you would expect) in low drag areas, but in others watch how it twists/reverses. its quite amazing how even at 70km/h the string will show dramatically how bad some spots are.

areas to keep an eye on in a van will be the tailgate and seperation edge (back edge of roof) as well as wheel arches and above the windscreen.

for an idea as to how much aerodynamics influences performance, a mazda 323 astina was tested by a published book for top speed. it was re-tested with its pop up lights up (not even on, just up) and top speed was reduced by 10km/h.

also, solar cars are able to reach over 100km/h with 2.5hp
Last edited by Lowndsy on Tue, 19 Aug 2008, 14:03, edited 1 time in total.

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Post by Richo » Wed, 20 Aug 2008, 00:33

Lowndsy wrote:no offence but that method of aerodynamic testing will tell you nothing.


No offence but I disagree.

The rate of decelration will give you the total of the forces acting on the car.(not just aerodyncamic)
This will determine the force the motor will need to provide to keep you moving at a given speed.
ie the running efficiency (Wh/km)

Also doing the same test on a dyno should give the running gear forces.
So Total forces - running gear forces = aerodynamic forces.
Which is less useful that the total forces.

The string and sticker method you describe would be ok if you intend to change the aerodynamics of the car.
Not useful for actual measurements that determine the size of engines.
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Post by Richo » Wed, 20 Aug 2008, 00:37

Also Main roads publish a list of public weigh bridges in WA.
Perth Weigh Bridges
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Post by Lahzie » Wed, 20 Aug 2008, 22:13

Also Main roads publish a list of public weigh bridges in WA.
Cheers Richo thats gold

Also like you say I wasn't aiming to optomise the aerodynamics of the van but rather measure the total retarding force at various speeds to predict the performance of various motors.

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Post by Scotty T » Thu, 26 Mar 2015, 17:58

The search on this forum just doesn't work, luckily Google is good or I'd have never found this thread, I'll be using that from now on:

https://www.google.com.au/?gfe_rd=cr&ei ... eva.asn.au

Has anyone got one of these?

http://www.sunbuster.net.au/services/ca ... t-control/

Or these:

http://www.sherline.com/lm.htm

I can't get to a weighbridge, my rego papers don't agree with the official spec of the car, and I really don't want to make a jig and four bathroom scales. The Sherline is apparently on special for $159: http://www.sherline.com.au/

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Post by Johny » Thu, 26 Mar 2015, 18:08

Hi Scott.
The Search does work but the simple search is limited.
When using Search always select Search then Advanced Search.
When in Advanced Search, change the Find Posts drop down to "Any Date.

I also didn't weigh my vehicle (mainly because I forgot before stripping it), so I weighed everything that came out (and went in) and measured the items position relative to the front wheel hub centers and used a spreadsheet to show the weight loss/gain and weight transfer.

I still haven't got around to actually weighing it....

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Scotty T
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Post by Scotty T » Thu, 26 Mar 2015, 19:15

Ah yes I now see that default is only in the last year, thanks for the tip.

I can still weigh mine as I still have the small amount of parts removed so far and could easily place them back where they go to weigh it. Depending where you look the balance runs from 51/49 f/r to 53/47.

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Post by woody » Thu, 26 Mar 2015, 20:20

One of the active Sydney members has a set of scales.

He also drives around the state a bit for work.

if you're in another state you could hire a trailer, take the car to a weighbridge (dump, landscape supplier?) and roll it on 2 wheels at a time?
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Post by Richo » Thu, 26 Mar 2015, 20:42

Scotty T wrote:I really don't want to make a jig and four bathroom scales.


Ha ha that's a good jig.
It's better than a weigh bridge as you can get weight distribution.
I'll only upgrade the 4xbath scales with load cells.
ebay load cell 300KG 131183278928
But you'd need a different jig and electronics for that one to work.


The roll down test mentioned years ago works well too.
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Post by coulomb » Fri, 27 Mar 2015, 03:13

Scotty T wrote: ... I really don't want to make a jig and four bathroom scales.


The jig can be pretty crude...

viewtopic.php?title=weber-and-coulombs- ... 436#p10436

Image

Image

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Post by Scotty T » Fri, 27 Mar 2015, 18:38

coulomb wrote:
Scotty T wrote: ... I really don't want to make a jig and four bathroom scales.


The jig can be pretty crude...


I like that, looks a bit easier to construct than the other one I'd seen. Woolies bathroom scales are $6.50 each at the moment too.

http://www2.woolworthsonline.com.au/?gc ... h%3Dscales

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Scotty T
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Post by Scotty T » Mon, 30 Mar 2015, 01:30

Got the scales and did the job. It was within 5kg of the specified weight.

Image

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Post by Scotty T » Sat, 04 Apr 2015, 01:11

G'day admins, can we make the basic search do all posts ever? This is what I expected, and it is what most people expect. I am a web programmer guy who has done a bunch of stuff with search. People don't like having to choose options like that.

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Post by rhills » Sun, 05 Apr 2015, 05:34

Hi Scotty,

Not sure if this forum will do that, I agree it's what people would expect.

We have a plan to move to a new platform - had a couple of failed attempts with a third-party data converter. I'm now writing a data conversion utility in my (limited) free time. When it happens we'll move to phpBB which seems a much more flexible forum system.

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Post by Scotty T » Sun, 05 Apr 2015, 18:14

phpBB seems to be the standard. I hear the TapaTalk mobile app is great for that too.

Let me know if I can help, I am a .Net, IIS and MSSQL guy with a little php and mySQL experience. I actually recall using the WebWiz forums a very long time ago.

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