Pagoda SL Group
W113 Pagoda SL Group => General Discussion => Topic started by: al_lieffring on November 20, 2006, 11:33:20
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When I have been driving around in my pagoda. The speedo seems to be reading much lower than I think I am going.
I remember from when I used to drive the car that the speedo was about 2% of the tach reading, Is that correct? Now when I am running at 3000rpm in high gear the speedo reads about 50mph.
I tigntened up the spanner nut on the drive flange, that made the speedo read steady, but is still seems too low.
The tach checks out ok by an electronic tach and the odometer readings match up with the roadside mile markers, so I am sure that the drive gear in the trans is no longer sliping.
Does anyone have a rpm to speed graph like the ones in old R&T articles?
Who does good speedo work in the Atlanta area?
Thanx in advance
Al
66 230sl
113-042-10-014715
904/396 blue, Ivory Tex
condition: not-as-rusty-as-before-bucket
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Al, this happens all the time.
Seems that age has an effect on the internals of the speedo.
They can be recalibrated by sending to a speedo shop.
I had this happen to me as I drove a friends 1959 300d adenauer on the freeway. The speedo showed 65mph, but the car following me (another friend) told me I was going nearly 90mph!
That was a major need for a recalibration. Whew, nothing happened to the car.
Walter
1967 220SL-diesel
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Al
There have been quite a few previous discussions regarding speedo accuracy. If you do a search you'll find a discussion of how the readings vary and what the causes could be. Here is a link to a site that Arthur Dalton posted (whatever happened to Arthur :( )that allows you to punch in tire size, differential ratio and tach readings to give you an accurate speed, but I think most find that depending on the rear ratio, 3000 rpm is around 60 mph ( about 63 or 64 mph for me with a 3.69 and 205/70's....could it be you've swapped rear ends?
Hope that helps
http://wahiduddin.net/calc/calc_speed_rpm.htm
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I was thinking the same thing the other night as I was putting down the freeway....seemed like I was going much faster than indicated on the speedo. I chalked it off to the lower ride height from the daily SUV driver.
1969 280SL
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Just starting to look at speedo calibration. Several BMW/Porshe sites talk about VDO odometer problems and repairs. One has a picture of the magnet and gearing. I have a 280SEL speedo apart and the assembly is the same with the exception of the lower worm gear. Here is a pic of what I'm looking at. How is calibration or adjustment accomplished. Gear changes?
Seems to me that the white gear at the bottom is important to accuracy because it appears to drive the speedo needle and odometer. These should be in sync to get accurate readings. Comments?
Download Attachment: (http://images/icon_paperclip.gif) Speedo.jpg (http://www.sl113.org/forums/uploaded/jeffc280sl/20061128194734_Speedo.jpg)
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Jeff C.
1970 280SL 4-speed
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quote:
Originally posted by al_lieffring
Does anyone have a rpm to speed graph like the ones in old R&T articles?
It's not a graph, but there's a table for my 230SL with 3.75 rear axle ratio, 814 tire revolutions per mile, and transmission ratios of 1, 1.53, 2.28, and 4.42.
I couldn't figure a way to keep the format in this posting, but if you tilt your head just right, I think the table is readable. Columns from left to right are rpm, mph(4th), mph(3rd), mph(2nd), mph(1st), km/h(4th), km/h(3rd), km/h(2nd), and km/h(1st)
750 15 10 6 3 24 16 10 5
1000 20 13 9 4 32 21 14 7
1250 25 16 11 6 40 26 17 9
1500 29 19 13 7 47 31 21 11
1750 34 22 15 8 55 36 24 13
2000 39 26 17 9 63 41 28 14
2250 44 29 19 10 71 47 31 16
2500 49 32 22 11 79 52 35 18
2750 54 35 24 12 87 57 38 20
3000 59 39 26 13 95 62 42 21
3250 64 42 28 14 103 67 45 23
3500 69 45 30 16 111 72 49 25
3750 74 48 32 17 119 78 52 27
4000 79 51 34 18 127 83 55 29
4250 84 55 37 19 134 88 59 30
4500 88 58 39 20 142 93 62 32
4750 93 61 41 21 150 98 66 34
5000 98 64 43 22 158 103 69 36
5250 103 67 45 23 166 109 73 38
5500 108 71 47 24 174 114 76 39
5750 113 74 50 26 182 119 80 41
6000 118 77 52 27 190 124 83 43
6250 123 80 54 28 198 129 87 45
6500 128 84 56 29 206 134 90 47
Chris Earnest
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What about slippage 'When the rubber meets the Road?'.
quote:
Originally posted by Ricardo
Al
There have been quite a few previous discussions regarding speedo accuracy. If you do a search you'll find a discussion of how the readings vary and what the causes could be. Here is a link to a site that Arthur Dalton posted (whatever happened to Arthur :( )that allows you to punch in tire size, differential ratio and tach readings to give you an accurate speed, but I think most find that depending on the rear ratio, 3000 rpm is around 60 mph ( about 63 or 64 mph for me with a 3.69 and 205/70's....could it be you've swapped rear ends?
Hope that helps
http://wahiduddin.net/calc/calc_speed_rpm.htm
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Chris
Thanks For posting the table.
I have just been just assuming that my tach readings are accurate, The digital meter that I have hunts all over the scale for readings in such a wide range that the readings arent of any use. It is possible that both my speedo and tach are incorrect. I just have been following traffic, in Georgia that would be about 70 in a 55 zone.
Jeff the gears in the speedo head only effect the odometer reading.
The magnet spins inside a steel disc that is attached to a hairspring and the indicator needle, as the magnet spins the disk tries to follow the rotation of the magnet but is held back by the spring, so the only internal components that are adjustable would be the spring tension and the relative position between the indicator and the spring. More tension, lower readings--Less tension, higher readings.
Al
66 230sl
113-042-10-014715
904/396 blue, Ivory Tex
condition: not-as-rusty-as-before-bucket
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NM
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Al,
Thanks for the input. I've taken the worm gear out of the SE speedo and agree with you (assuming similarity of design) that there are no gears in the path between the speedo cable and indicator arm. I suppose this leaves one with making changes to the spring or magnet strength to recalibrate/adjust the speedo. The speedo and odometer need to operate together in order to receive accurate information. If the speedo was changed by magnet or spring the odometer would also need to change.
I have run a test where I used the 280SE 4.5 speedo in place of my SL speedo. The actual speedo readings and the rpm calculated results were very close (less than 1% variance) This means that the W108 4.5 axle and speedo were matched. I this case MB did not make different transmission speedo drive gears (manual trans) to adjust for rear axle ratio variations. They used different speedos and odometers.
Can the speedo guts from the 280SE 4.5 be transplanted into the 280SL speedo?
Jeff C.
1970 280SL 4-speed
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I've taken a look at the two speedos. First major difference is the top speed. The 4.5 goes up to 160 mph. And scale may be the main difference betwwen the two. It looks like the indicator arms sweep over the same size area. The SL goes up to 140 and the 4.5 to 160MPH.
I suppose its possible that one day I will need the 160 mph scale. Anyone brave enough to try a run at that speed. One would need to hit 7000 rpms to make that speed in my car. I don't think so.
Jeff C.
1970 280SL 4-speed
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Jeff in reply to your off forum question, I am not sure how difficult it would be to move the indicator to a different position on the shaft, I have never tried it.
What would happen if one were to do this? I will have to use a graph to explain.
(http://www.sl113.org/forums/uploaded/al_lieffring/20061129205121_graph.jpg)
the RPM scale represents the propeller shaft speed.
MPH scale, self explanitory
The red line represents the actual vehicle speed with the new axle ratio.
The green line, the speed indicated on the unrecalibrated speedo head.
In your message you asked what would happen if at say 60 mph you were to move the needle on the shaft to a corrected speed you calculated (lets say 67mph) to match your new axle ratio.
This results would be represented by the blue line with that calculated (67mph) being the point where it crosses the Red.
This graph is for illustration purposes only, and is exagerated to show the results more clearly.
It would take a combination of adusting the spring tension and needle position to make the speedo respond accuratly through the entire range.
and of course the odometer gears would need to be changed to make the mileage read accuratly.
I hope this expalains things more clearly.
The internal components of my 230SL speedo are different than 108 sedans of the same era, I dont know if your 280SE 4.5 internals will interchange with a 280SL speedo.
Al
66 230sl
113-042-10-014715
904/396 blue, Ivory Tex
condition: not-as-rusty-as-before-bucket
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Thanks Al,
I understand your explanation. I know you have constructed the blue line for illustative purposes. I'm hoping the blue line will more closely follow the red. The only way for me to know for sure is to run some tests. I'll use the SE speedo as the rat and see what happens. Interesting observation: When I separate the magnet from the speedo the indicator arm does not return to the stop. It is spring loaded but the loading is not sufficient to return the indicator to the stop. If I return the magnet the indicator returns to the stop at once. I'm pretty confident that if I adjust the indicator arm as proposed the magnet will assist and return it to the stop. If this happens maybe the blue line will start closer to zero and follow the red one more closely.
I've read where speedos have been recalibrated at a speedo repair shop. Assuming the magnet is good and spring is functioning I suspect they do exactly what I'm proposing when "calibrating". The alternative is to add/remove pieces of the magnet or change the spring tension somehow. I doubt this is how most if not all are calibrated.
Thanks for your input.
Jeff C.
1970 280SL 4-speed
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For those interested here are some pics of the inside of a 1972 VDO 280SE 4.5 speedometer. Similar in many ways to our SL speedos.
Download Attachment: (http://images/icon_paperclip.gif) hair spring.jpg (http://www.sl113.org/forums/uploaded/jeffc280sl/20061130164116_hair%20spring.jpg)
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Download Attachment: (http://images/icon_paperclip.gif) magnetic cup.jpg (http://www.sl113.org/forums/uploaded/jeffc280sl/20061130164149_magnetic%20cup.jpg)
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Jeff C.
1970 280SL 4-speed
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Ran a couple of tests after adjusting the indicator needle. I have two hand drills. One runs at 1250 rpm and the other at 4700 rpm. Here are the results. In test #2 without much fine tuning I tried to return to the factory setup.
0rpm 1250 rpm 4700 rpm
Factory setup 0mph 34mph 125mph
test#1------- 0 25 110
test#2------- 0 32 123
test#3------- 0 46 138
Al, do you have any comments? The test were run on the 280SE speedo.
Jeff C.
1970 280SL 4-speed
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quote:
Originally posted by jeffc280sl
Ran a couple of tests after adjusting the indicator needle. I have two hand drills. One runs at 1250 rpm and the other at 4700 rpm. Here are the results. In test #2 without much fine tuning I tried to return to the factory setup.
0rpm 1250 rpm 4700 rpm
Factory setup 0mph 34mph 125mph
test#1------- 0 25 110
test#2------- 0 32 123
test#3------- 0 46 138
Al, do you have any comments? The test were run on the 280SE speedo.
Jeff C.
1970 280SL 4-speed
Jeff
I put a calculator to your test results, here is what I came up with.
The test rpm of 1250 was then increased to 4700 or by a factor of 3.76 so the second speed reading of each test should be 3.76 x the first one.
test 1- 25 x 3.76 = 94, result 110
test 2- 32 x 3.76 = 120.3, result 123
test 3- 46 x 3.76 = 172.9, result 138
Your results more closely follow the blue line of the graph than the red line.
in test 3, the difference of the lower speed was 12mph faster than the original setting, and the difference of the higher speed was 13 mph so the +12 change in speed is almost the same through the entire range. Or as shown on the graph the blue line runs parallel to the green but 12mph higher.
66 230sl
113-042-10-014715
904/396 blue, Ivory Tex
condition: not-as-rusty-as-before-bucket
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Thanks Al,
The calculated adjusted speed assumes the low rpm test was a good starting point. I suspect it is "off" by some factor which is not taken into consideration. From what I have read others who have used this method of adjustment start with a mid point speed say 60mph. If the speedo should read 72 when it actually reads 60 one would hold the cup at 60mph and gently reposition the indicator needle to 72mph. Since I don't have an rpm input in this range I may try to set the indicator at 60 mph and then observe results on the low and high side using my drill input. I'll post results later.
Jeff C.
1970 280SL 4-speed
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Decided to try and calibrate the SL speedo. With the new rear axle my speed was off about 20%. I moved the needle and cup to 60mph and then held the cup and turned the needle to 72mph. Put everything back and here is the data.
rpm actual calculated
1500 40 34.5
2000 50 46
2500 59 57.5
3000 69 69.1
I don't know if the tach is accurate. Plan to buy and electric tach to check on its accuracy.
Jeff C.
1970 280SL 4-speed
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Jeff and all
Have you had a chance to test your speedo after repositioning the needle? Is it giving you results that you are satisfied with?
Basicly you have pushed the 20% error off to points on the scale where it makes less difference (0 and 144 mph). And if that's OK with you it is fine by me.
Because the error in my speedo is not as far off (-5mph @ 3000 rpm) as the one your axle ratio conversion created, I will be doing the same thing.
About 1/2 of my speed error was my own perception, I didn't know that I had the looooong 4.08 axle ratio (you'd think that sometime in the last 30 years that I would have checked) and the speed to RPM is actualy slower than I expected.
Al
66 230sl
113-042-10-014715
904/396 blue, Ivory Tex
condition: not-as-rusty-as-before-bucket
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Hi Al,
The results I posted were from a test using the adjusted speedo. I'm going to buy an electric tach and see if that changes the calculated results. I think I'm going to leave the speedo where it is for a while. I suppose I'm most concerned about the highway speed so I'm glad that area appears accurate. The new rear axle is taking some getting used to. I find the car more quiet and smoother with the lower rpm which makes me think I should be going faster. I do really like the axle change?
Jeff C.
1970 280SL 4-speed
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Caveat: I've never seen the details of the speedo's functional design nor the inside of one. The following explanations of how the calibration can get out of whack and whether the problem will present as a constant offset or a change in slope of the relation between magnet rpm and indicated speed is based on simple physics and the descriptions and pic's shown in some of the notes below. Since I'm an engineer, I always look for root causes... what changes and what effect that change will have. I offer the following for general information on what causes what kinds of changes in the speedo's indicated vs actual speeed when a calibration changes.
The variables in the speedo's method of indicating the analogue speed is thru 2 primary forces.... an electromagnetic force and a spring force.... both are invariates.... i.e. neither should change over time nor amount of use... since both are material properties, not settings or adjustments that are made or which can be made outside of cutting new metal. Material properties are constants unless the material has undergone excessive environmental exposures i.e. become badly rusted (oxidized) in the case of non-stainless varieties of steels (iron containing metals).
The electromagnetic force is created by a solenoid effect... magnet spins, generating a non contact electromagnetic force on the steel (iron) disk.
The disk is attached to a helical spring or other shaped spring, since otherwise it would spin continuously limited only by it's frictional components.
The spring force acts against the magnetic force applied by the magnet to the disk. As the rpm of the magnet increases so the magnetic force increases proportionately.... therefore the force to spin the disk increases proportional to the rpm. The helical spring force acts against the magnetic spinning force, but at a lower force at it's neutral (0) position. The difference in the helical spring constant (force vs displacement), and the magnetic force of the magnet acting on the disk let's the disk (therefore pointer on the dial) move thru an arc which is proportional to the magnet's rpm.
There are several variables that can modify the forces.... mostly simply friction.... the disk's friction on it's bearings, the spindle shaft's (attached to pointer from disk) friction on it's cylindrical bearing surface, both of which increase the forces acting against the rotation of the disk... effectively adding to the spring constant of the helical spring. When this has occurred, to whatever extent, it changes the slope of the speed vs rpm relationship.... since frictional force will generally increase with increasing relative velocity of the contacting surfaces, and if for no other reason than the fact that any of the normal lubricant's used (oils, greases) increase viscosity with increasing shear force... i.e. relative speed of moving parts being lubricated, at near constant temperature.
The spring itself is a metal which will not relax it's modulus with time and the displacement of the helical spring when wound to it's fullest won't cause it to reach anywhere's near it's elastic limit, therefore the spring contsant over it's entire displacement range will not change with time.
Another source of a change in friction within speedo are at the bearings for the gears that drive the odometer... since those are also driven by the shaft's rotation, but other than create a constant drag on the shaft, this source of friction won't cause the speedo to have an offset or slope change with shaft rpm..... other than a momentary and temporary (less than a fraction of a second) lag .... & then only when initially accelerating from a full or near full stop.
The other variable that can effect calibration is the location or attachment of the helical spring to it's fixed end points... and if either of them move it changes the offset of the speed indicated.. i.e. the speed is always indicated to be Actual +/- x... x is constant at all speeds, since the spring constant hasn't changed... just the position of the position of the spring's zero force point.
The magnetic force on the disk can only change if the location (distance) of the magnet to the disk changes, and I don't know whether there are wearing members that will increase or decrease the distance over time and wear. However, if this occurs then the relation of the magnetic forces applied to the disk changes, so that the helical spring's countering force relation to the magnetic force changes, and this will cause a change in both the slope AND offset of the speed vs shaft rpm. Assumedly, if this occurs, the distance between the magnet and disk will increase, decreasing the magnetic force, thus making the spring constant of the helical spring relatively too large... reducing the indicated speed vs that actual speed at an increasing rate... i.e. the higher the speed the lower the indicated speed relative to the actual. If the distance decreases the converse ocurrs.
You can't change the spring constant of the spring without changing the spring's length, width, or thickness between the fixed ends of the spring.... shortening the length increases the spring constant, which adds to the countering force on the disk, and drops the slope of the indicated vs actual speed... lower indicated speed than actual with an increasing error as speed increases. Increasing the length of the helical spring between it's fixed ends does the reverse.
Generally speaking, I'd expect the frictional and shaft/bearing wear points to be the cause of most error in W113's speedo's. ...i.e. friction, therefore pure offsets, but not a change in slope. Unless the spring's fixed end points have changed so that it's changed the length of the spring, the spring constant won't change. ... so other than that I can't think of any means by which the slope of the relation would change unless the distance between magnet and disk have changed.
Could the magnet material itself lose some of it's magnetism over time? Possibly, if it's not a natural magnetetic material. If so, it would change the magnetic force acting on the disk, so that the spring's countering force would no longer have the same relationship as the magnitic force increased with increasing rpm....i.e. it would change the slope of the relationship (magnet rpm to indicated speed), as well as the offset between them.
There is another cause for change in indicated vs actual speed that has nothing to do with the speedo itself, rather the shaft connected to it. The shaft runs in a tube...and thus has frictional forces while turning in the tube. Those frictional forces restrict the ability of the shaft to rotate in the tube and retard the shaft for a period of time... small sub-second periods... as the torsion in the shaft winds up under frictional forces in the tube, the speed shaft is retarded in it's rotation momentarily... and when it winds up enough, the force to unwind overcomes the friction in the tube, and it' unwinds ... speeding up it's rotation relative to what it should be, and indicating a faster speed than actual. What you really see on the speedo dial is a wobbly speed indicator... bouncing back and forth around the actual speed. Lubricating the shaft in the tube with graphite will solve that symptom.... though it indicates a wear problem has occurred within the tube, so the graphite solution may not do much for the symptom if wear is extensive (and / or wear debris is extensive)... it will reduce the wobble, but not eliminate it, or if it eliminates it, it may not do so for long... a year's worth of intermittant driving, perhaps.
A speedo calibration shop knows the design relationship beteen spin speed of the magnet and indicated speed on the speedo dial.... either because they have the spec's or because they've evaluated a lot of "in calibration" speedo's over time. They spin the magnet with known rpm, and mesure the relationship. If the relationship shows an offset exists (after cleaning and / or replacing bearing surfaces), there has to be a feature in the speedo that let's them change the location of one of the fixed end-points on the spring attached to the disk... which either adds or subtracts the force beign applied by the spring to the disk... depending on how much offset was indicated.
If the slope's off, frankly, I'm not sure what a speedo shop can do to effect calibration unless there's an adjustment feature in the design that let's them increase or decrease the fixed length of the spring... effectively shorten or lengthen it.... maybe up to 5% - 10%?.
Normally, cleaning and/or replacing the worn or dirty bearing surfaces, with and adding a few drops of the correct weight lubricant should result in an almost immediate "recalibration" without changing anything else. Since I'm not familiar with design details, I can't describe what other adjustments there may be designed into the device for initial setting and calibration.
Longtooth
67 250SL US #113-043-10-002163
'02 SL500 Sport
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Slippage in the cable-to-speedometer linkage? I have a speedometer where the speed indication is constant (i.e. it works well, even if the speed indication is not accurate) but where the odometer sometimes does not move at all, or at a very slow rate.
It must be internal to the speedo since a separate (calibratable -- is that a correct word) mileage counter attached via a T-gear to the same speedo cable does indicate miles (odo) correctly.
Peter
Check out http://bali.esweb.nl for photographs of classic car events and my 1970 280 SL
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quote:
Originally posted by vanesp
Slippage in the cable-to-speedometer linkage? I have a speedometer where the speed indication is constant (i.e. it works well, even if the speed indication is not accurate) but where the odometer sometimes does not move at all, or at a very slow rate.
It must be internal to the speedo since a separate (calibratable -- is that a correct word) mileage counter attached via a T-gear to the same speedo cable does indicate miles (odo) correctly.
Peter
Check out http://bali.esweb.nl for photographs of classic car events and my 1970 280 SL
Peter
This is a very common problem on the mechanical VDO speedometers.
The alloy metal disc that drives the rotating digits (next to the 1's wheel)it is press fit onto the shaft that the numbers are mounted on. It will work loose from the shaft and the numbers will not advance any more.
Al
66 230sl
113-042-10-014715
904/396 blue, Ivory Tex
condition: not-as-rusty-as-before-bucket