Lotus Elan

Speed gear color

PostPost by: TurbineHeli » Wed Dec 19, 2012 3:48 am

Sorry if this subject has been beaten to death before, but the search feature does not seem to be particularly intuitive...
If I recall correctly, 3.9 is blue, 3.7 is black and 3.5 I'm not sure.
My diff gear is 4.44:1 and a blue speedo gear(3.9) is currently installed so obviously there will be speedometer error. So two questions. 1. is there a correct speedo gear available for the 4.44 rear end? Who/where?
2. If not, is it possible to have the speedometer instrument re-callibrated?

Thanks,

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PostPost by: GrUmPyBoDgEr » Wed Dec 19, 2012 6:13 am

I'm not really up to speed on the speedo drive gearing colours.
My Elan with a 3,54 diff' & (please note) 175-60 x 14" tyres has "black" being deemed by a "specialist" as being most suitable.

Yes there are good guys out there who can & will, for a reasonable price, recalibrate your speedo.
They ask you to carry out one or another simple task to help them with the calibration:-

A typical example is for you to push you car in a straight line until the rear wheels have completed 4 rotations.
Whilst doing that you must observe how many times the speedo drive cable rotates.
Obviously this requires the speedo to be removed to obtain access to view that cable; but hey you're going to be sending it away anyway aren't you :wink:

Cheers
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PostPost by: john.p.clegg » Wed Dec 19, 2012 9:21 am

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PostPost by: TurbineHeli » Wed Dec 19, 2012 2:18 pm

Thank you John!
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PostPost by: garyeanderson » Wed Dec 19, 2012 3:04 pm

Tommy Sandham has compiled a data book for the Cortina and about halfway down there is a chart of what driver gear and what driven gears to use with what rear axle ratio and tire size.
click on the link and search for

Speedometer Drive Gears

http://freespace.virgin.net/tommy.sandham/databook.htm

I don't have any idea of where to obtain a 6 tooth drive gear but I have a few of the 7 tooth ones.

hope this helps

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PostPost by: stugilmour » Wed Dec 19, 2012 6:31 pm

TurbineHeli wrote:Sorry if this subject has been beaten to death before, but the search feature does not seem to be particularly intuitive...

2. If not, is it possible to have the speedometer instrument re-calibrated?

Thanks,

AMA


Short answer is yes, and might be preferable...

Here is link to a document I found helpful. Might be more detail than you need...

http://home.comcast.net/~rhodes/speedo.pdf

I had my speedo done by Nisonger due to a diff & transmission change. They are located in Mamaroneck, NY...

http://www.nisonger.com/index.htm

Basically their are two issues to sort out. You sort out the two issues for the odometer reading first, then the speedo reading is checked and calibrated on the bench using a known input rotational speed and the overall TPM.

1. The turns per mile (TPM) of the cable specific to your transmission speedo gear, diff, and tires. You can measure this by pushing the car a known distance or number of tire revolutions & counting the number of speedo cable rotations. Like John mentions above, each repair shop seems to do this a bit differently, but they will all provide you with what they want ahead of time. Some of the different shops I saw want 4 tire rotations, 6 tire rotations, 52.8 feet or 100th of a mile of tire travel, etc., but they all effectively do the same thing which is provide the shop with the actual TPM output at the cable for your present set-up. I opted to just give them the TPM I wanted (see below to determine this).

2. The TPM of the speedo head. For my Plus 2 it is 940 TPM stock; yours may be different. You can see the stock number on the speedo face just below the odometer to the right in small white numbers.

For my MT75 transmission, I opted to leave the gear in the transmission alone (actually I have no idea what it is) and get the speedo head done. I simply calculated the error/difference driving along the Interstate for about 15 or 20 mile posts, comparing to the odometer reading, and then calculating the required TPM from the known 940 TPM on my speedo face. I provided the required TPM number to Nisonger's (1000 TPM in my case) with the speedo head packaging, and they re-calibrated the odometer with an internal gear change and then adjusted the speedometer reading as required for age and wear. Turn around on the job was about six or eight weeks, and cost about $200 IIRC; I think this is their busy time of year.

David has done an excellent spreadsheet for calculating revs per mile etc. for any diff & tire combo. I used this and GPS speed readings to factor in the impact of future tire size changes and to determine that the speedometer and odometer were out of whack with one another, so I knew I needed the speedo head done along with the tachometer. You can use this spreadsheet to see what the impact of the transmission gear change will be ahead of time and see if you will get close to your speedo head TPM with your particular tire, diff & existing/modified transmission speedo gear.

Been a while, but I think the document above provides a pretty complete list of the gear combo's that are readily available for the Smiths speedo's. I made sure I was asking Nisonger for a TPM figure I figured they could easily build up and was within a reasonable percentage error to my custom set-up.

If your tire, diff & transmission speedo gear combo does not work out to a readily available speedo head TPM, they can apparently build up a transmission deal that mounts on the back of the speedo head. I was worried the Lotus would be tight for room for this approach. IIRC the transmission is about 3" long.

Sorry for length; probably more detail than you need. Sounds like you can either try a revised transmission gear or calculate the impact of a new gear and see if you can get close to the TPM requirement on your existing speedo head. Other approach is to leave the gear you have and calibrate the head TPM as required; you can check if this will work for you first with the info above. You may not have to do both, and if you find the speedo & odometer are out of whack you may want to just get it calibrated & serviced (or venture into doing it yourself as per Mr. Rhodes) and leave your existing gear in place.

HTH

PS I just remembered my speedo cable or angle drive broke on my last trip, so I need to fix that up this winter. :)
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PostPost by: msd1107 » Wed Dec 19, 2012 8:24 pm

Stu mentioned there is a spreadsheet to calculate turns per mile and other data.

Read the help text closely. It tells you how to calculate the gears for the speedometer head. That way any possible inaccuracies using the tire rolling methods are obviated. But if you give Nisonger or other speedometer shop the TPM figure, they will know how to fit the right gears in the speedometer head.

The spreadsheet is located here:
elan-f15/updated-spreadsheets-t18445.html

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PostPost by: GrUmPyBoDgEr » Wed Dec 19, 2012 8:39 pm

msd1107 wrote:Stu mentioned there is a spreadsheet to calculate turns per mile and other data.

Read the help text closely. It tells you how to calculate the gears for the speedometer head. That way any possible inaccuracies using the tire rolling methods are obviated. But if you give Nisonger or other speedometer shop the TPM figure, they will know how to fit the right gears in the speedometer head.

The spreadsheet is located here:
elan-f15/updated-spreadsheets-t18445.html

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Hi David,

that's interesting but for me a bit confusing.
In asking this I'm contradicting my philosophy on "nit-picking"
Anyway, here goes. You wrote:-

"That way any possible inaccuracies using the tire rolling methods are obviated".

How is a calculated (theoretical?) solution more accurate than an actual measured "rolling tyre method".

My question is really rhetorical & not my intention to demand a scientific explanation.
It would go over my head anyway :oops:

Cheers
John
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PostPost by: stugilmour » Wed Dec 19, 2012 8:41 pm

Thanks for the link to the latest and greatest version David. I found the sheet very useful and accurate.

John, I used the sheet a bunch to calculate speed to rpm in all five gears for two tire combo's I have run and it is quite accurate. Keep in mind the empirical method only uses about 1/100th of a mile, so probably has some built in error as well. Thats the reason (besides ease) I calibrated to mile posts over a significant distance. One speedo company wanted three readings using the tire rotation method, so they are well aware of the inherent inaccuracy of the method.

In addition, my tach precision varies considerably with temperature, even after a re-build to work with my Pertronix. Prior to rebuild I found it varied by about 300 to 500 rpm, so went with the calculated values and GPS speed readings as the benchmark numbers. Since a recent tire change my speedo is out by about 3% which I can live with. Probably should have accounted for this by requesting 1024 or 1025 TPM instead of 1000 TPM as required, as I was pretty sure I was changing to stock tire size with a slight change in rolling circumference.

The whole deal is kind of a horseshoes affair. Will be interesting to hear from David if he has put in a tire inflation bugger factor though; on page 23 Rhodes talks about an "industry standard" bugger factor of about 3%.

AMA, another quick point. If you sent the speedo head off for service, tell them if you want the odometer adjusted to zero or left with the present reading. They phoned me to check this point.

Cheers!
Last edited by stugilmour on Wed Dec 19, 2012 9:16 pm, edited 1 time in total.
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PostPost by: msd1107 » Wed Dec 19, 2012 9:09 pm

John,

The tire rolling method counts turns for a certain number of tire rotations. The fewer the turns, the greater the statistical error, plus the possible error in counting the speedometer cable turns.

Using tire turns per mile give you a good fixed figure.

Of course, both of these methods have errors.

Even if the rolling method was error free, it captures a correct figure for the tyres at one point in their service life. Tyres can change by as much as 3% during their life.

Published turns per mile (even manufacturer specified values) can vary by 1% or more and, again, capture the tyre at one point in its service life.

Accurate distance measurement might be accomplished using a GPS based odometer, as long as you stay in sight of enough satellites for good accuracy. I use an iPhone app to measure distance I row or set a count down distance measurement for a fixed distance race. http://performancephones.com/speedcoach-mobile/

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PostPost by: GrUmPyBoDgEr » Wed Dec 19, 2012 10:22 pm

OK I think I understand your theory now but the mind boggles at who would go to the effort of counting how many revolutions the tyre makes per mile.

Well in any case the main function of the speedometer is to ensure that whilst driving on public roads that you can be sure that you know when you are most likely exceeding the legal limit.
I think that most manufacturers calibrate their speedo's fast to ensure that they don't get tangled up in legal arguments about speeding convictions etc.
Apart from that proud owners are lead to believe that their cars are faster than they actually are :lol:

Cheers
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