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Can Gearing Calculation be Wrong?


Adrian Williams

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I have a gearing calculation which works out mph/1000revs in all six gears and therefore theoretical max speed on the limiter in all gears. Recently I've compared the figures with those from both my DL1 data logger and speed traps at sprint venues. To date the speed given by the logger has always been exactly the same as those given by a speed trap as indeed it was at last Saturdays club event at Curborough.

 

The fact that the speed trap figure always corresponds to the data logger figure has lead me to believe that these figures are correct. However if that's the case then my gearing calculation is about 5% too high which seems odd.

 

Can anyone shed any light on this 🤔

 

For the record details are as follows:

Rover K revving to 8600

Std 6 speed box

3.92 diff

Rear tyre circumference is 1694mm according to the manufacturer (kumho) and 1692mm when I measured it.

 

I make that a theoretical top speed of 139mph in 6th and 105mph in 4th.

 

I have data logging info that says it was 135mph and 99mph?

 

 

Adrian

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You get any tyre blooning - like dragsters (very dramatically) get?

 

Actually, that's testicles isn't it . .. . blooning would give you a lower indicated reading than an actual reading 😳

 

Edited by - AdamHay on 21 May 2008 16:00:41

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Adrian, I'm Peter C will be along in a moment to explain at length how this can be but I would hazard a guess that the error may lay in the tyre measurements that your gearing calculation uses.

 

*arrowright* *arrowright*Harry Flatters *arrowright* *arrowright* *thumbup*

AKA Steve Mell - Surrey AR (Until 30th May) and Su77on Se7ener

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Is your engine really spinning at 8600?? only a few rpm short would give you that variation.(sorry can't be bothered to work out exactly how many *tongue*, brain ache at this time of the day 😬)

 

RED 2.0 HPC 230BHP *thumbup* *smile* here

 

Edited by - Gambo on 21 May 2008 16:22:39

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Did you measure the tyre circumference off the car? This will be slightly greater than the rolling circumference, with it taking the cars weight. This may explain the variation.
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Concur with your calcs.

8600rpm = 104.8mph (4th), 138.4mph (6th).

Given the gear ratios are fixed, the only variables are tyre circumference at speed and rpm measurement (and speed measurement!).

How does the DL1 calculate speed? If it is from a wheel sensor then prone to tyre variation; if from GPS then you need constant speed for a couple of seconds for it to be accurate. The response time it too slow for transient values, but if you let it settle it should be +/- 1mph I believe.

How are you logging rpm? My ACES has some averaging to remove spikes so that may not be keeping up either...

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DL1 uses GPS, speed is never constant while sprinting (which is when I use it) other than flat in top which has only happened at one venue and for no more than a couple of seconds. The fact that whenever there is a speed trap it reports trhe same figure lead me to beleive the logging figure was pretty accurate.

 

RPM is via the Stack and ACES which use the same feed, limiter is set on the MBE at my request so not assuming factory settings.

 

Tyre circumference was indeed measured off the car.

 

Adrian

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is the rev counter calibrated ? or is the hrad cut coming in too early ?

 

not unusual for them to be 2-500 rpm out. My stack rev counter is 100/200rpm different to the emerald software readout on the laptop at 5000rpm

 

 

Taffia rear gunner

 

 

Edited by - Dave J on 21 May 2008 17:28:08

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Some figures taken from the Polley website (don't think they're on there now)

 

AO21R 185/60x13, Dia=558mm X pi = 1753mm, (unloaded circumference)

 

Rolling circumference given as 1702mm, about a 3% difference.

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Dave, the rev counter hasn't been calibrated but does tally with the hard cut point on the MBE. Is it possible for the MBE to under read and send the same wrong signal to the rev counter or does the rev counter take it's signal from the coil in which case they both read the same figure which you'd assume is then correct (ish) 🤔

 

Different rolling circumference when loaded would do it but I can't get my head around why the tyre shrinks. I can see how the load would reduce the radius measurement but the tread is still there it just isn't round anymore and will still have to pass across the road.

 

Adrian

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Just taken a look at some Tyre web sites (Avon, Kumho, Toyo & Yoko) and they all talk about revs per mile (apart from Yoko who don't give any information on the subject) being a definitive number not some sort of range depending on load, pressure or whatever.

 

So if the rolling circumference can change the figures on the web sites are nonsense 🤔 🤔

 

Adrian

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Just taken a look at theoretical wear of say 4mm on Kumho's stated circumference figure. This reduces the circumference from 1696mm (it measures 1694mm at the present wear rate) to 1670mm. Put that into the calculation and you get 137mph in 6th and 104mph in 4th, only slightly closer to the data logger and speed trap readings.

 

I doubt it's the wear that's causing the difference. *confused*

 

Adrian

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Been thinking about this since we chatted. I now think that the calculations ie the mechanicals can not be wrong. They are fixed! They are metal to metal. Gears spin at etc. etc.

 

Each of the other "Measuring" items rely on data. Revs signals. Breaking a timing beam - twice! and SatNav info!

 

Therefore all the measuring devices are wrong!

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Adrian,

 

Sorry, didn't spot that you'd already measured the circumference.

 

How does the datalog compare against the gear calculator at 7000rpm? Is it spot on? Could it be that you're not actually reaching 8600rpm due to rev limiter bounce? This would explain why the 5th gear "true" speed reading is further away than the 6th gear reading, as she would bounce more in 5th.

 

Willie

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Simon, the tyres aren't metal to metal. Also I'm happy to accept that the data logger is wrong but why would all the speed traps we've been through be wrong by the same amount?

 

Willie, interesting point but the rpm log doesn't work properly so I have no way of checking speed at a given rpm point from the data to check against the gear calculation. I'm sure it's not bouncing, I know I hit the limiter in 4th as I crossed the line and speed trap at Curborough last Saturday and can see it on the video to back it up.

 

Adrian

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I don't know how soft cut limiters work, but on a tintop I have, if you hit the soft cut limiter, the revs do drop noticeably. Its not really a bouncey thing, but they definetly do drop.

 

A limiter must limit the revs and if it cuts spark/something at 8600rpm, it seems logical that there would be a lower value that it drops to before coming back to life. So if you are "on the limiter" it would seem reasonable that you are not maintaining a constant 8600rpm, but something a little lower.

 

 

 

 

 

 

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Willie, the data trace when the limiter is hit becomes a horizontal line with a very slight wave formation where the speed fluctuates by much less than 1mph up and down so any 'bouncing' is tiny and nowhere near 5mph.

 

Dave, I'll give it a go this will tell us if the GPS speed is more accurate when maintaining a speed rather than accelerating. If the logger does become more accurate at a constant speed I'm still bemused why the speed traps at Curborough and Mira have always confirmed the data logger as accurate.

 

Adrian

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Adrian...

 

GPS signal will very rarely be absolutely spot on....mainly as the signal has to be triangulated at every point as you move to give 100% accurate results...

 

I'm not sure that GPS software takes sample date infinite numbers of times a second. As I accelerate in the car with GPS on, it never quite keeps up with the speedo....At constant speed for they tally however...

 

We use GPS surveying equipment on sites, and depending on times of day, cloud cover, wobble on the satellite, geodesy, there can be reasonable variations in the quality of the data obtained.

 

Dannyboy *tongue*

 

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Actually Dannyboy . . .. with the DL1 and the way that it uses it's accellerometers AND the GPS signals, the speed reading is very very very accurate *tongue* or so they say. And the fact that Adrian's DL1 speed is always the same as trap speeds, I guess they're right.
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