Warning:- Before carrying out any modifications to the standard design seek written approval from the Kit Designer. Use only high quality materials and fasteners and lock all threads. This applies particularly to Safety Critical items such as Brakes, Steering & Suspension Components.
Wheels and Tyres
A full set of 5 x 61/2 J Alloys were purchased with the kit, and these were put aside in the boxes to avoid damage during the build. The car being built on the Sierra donor wheels. Also the cost of 5 new tyres was to say the least a bit daunting. (£305.50 - April 2004) There does however come a time when you need a proper set of wheels and tyres on the car to facilitate fitting the Front and Rear mudguards in the correct position. (Obviously, 61/2 J's are much wider than standard Sierra tyres) Also the exact position of the spare wheel carrier needs the correct tyre/wheel combination.
The recommendation of RHSC's was used to specify the five tyres which were the new AVON ZV3's 15" 61/2 J 205/50/R15 A quick measure with the tape confirmed a circumference or rolling diameter of 1841mm (72.48") which is similar to the Sierra tyres. ( I am a little confused on this point as the Haynes Manual gives no less than 6 Tyre sizes, non of which are identical to the above Avons, The Ford Gearbox Manual gives the same number of tyres.) The correct circumference is essential if your speedometer is to operate with any degree of accuracy. ( Tested during the SVA) You can of course use whatever tyres or wheels you see fit, but you must alter the speedometer gearing if you alter the circumference of the rear tyres. The tyres were bought from my local supplier Jim Jeffrey of Warrington ( he is also the cheapest for miles ) who takes extreme car with fitting tyres to Alloys and can fit the balance weights on the inside if requested. Give him a ring before you go to ensure he has the stock required.( Warrington 01925 247 560 )
The same does of course apply to a change of Differential (Back Axle) ratio. Unless the Speedometer, Diff Ratio and Tyre Rolling diameters are all as the original donor vehicle your speedometer will have errors. The actual gearbox ratio's are not be a problem as the speedometer take off point is on the propshaft output. See (3) below. See the Chris Longhurst site to calculate possible errors.
There is a section on Caster, Camber, & Toe setting in Section X Front Hubs and Disc Brakes
Speedometer Calibration
According to the Avon tyre manual, the 205/50/R15 has an outside rolling diameter of 586mm or 1841mm circumference. This equates to 72.48" ( I have used Imperial measure as we are working in Miles per Hour, not Kilometer's per Hour ) The SVA test checks the speedometer calibration which must be accurate within designated limits, it is therefore a good idea to carry out your own checks prior to setting off for the SVA. Since you cannot run the car without Insurance we are in a Catch 22 situation. A rolling road at your local MOT station is the best bet.
The acceptable tolerances can be found in the SVA manual and are also shown below. The speedometer must not indicate a speed LESS than the true speed. See Section 20, Page 2 of the SVA manual. No minus error but a very generous plus.
|
Indicated Speed (mph)
|
Min. True Speed (mph)
|
|
35
|
26
|
|
40
|
30
|
|
45
|
35
|
|
50
|
39
|
|
55
|
44
|
|
60
|
48
|
|
65
|
53
|
|
70
|
57
|
Table correct as at March 2004
The table below should be used to determine the accuracy or otherwise of the speedometer. This is correct only for Avon 205/50/R15's or tyres of the same rolling diameter. ie:- 72.48" If you measure wheel rpm you can ignore diff or gear ratio's. See (1) below.
If you have used all the transmission parts from a single donor vehicle you should have no problems. If you are able, it is a good idea to calibrate your rev counter at this stage. I used a "Compact" hand held laser unit, but some "Gunson" Ignition timing strobe testers have a similar facility.
|
Diff Ratio
|
Speed in MPH
|
Actual Wheel RPM
|
Rev Counter (4th Gear 1:1)
|
|
3.92:1
|
20
|
291
|
1,142
|
|
3.92:1
|
30
|
437
|
1,713
|
|
3.92:1
|
40
|
583
|
2,285
|
|
3.92:1
|
50
|
728
|
2,856
|
|
3.92:1
|
60
|
874
|
3,427
|
Formulae for other ratio's etc:-
(1) Miles per Hour to Wheel Revs = mph x 1056 / Rolling Circumference in inches. (gear ratios do not come into the equation)
(2) Miles per Hour per 1,000 rpm in 4th gear 1:1 ratio and with a 3.92:1 Diff = 17.53 mph.
(3) Revs per Minute to Miles per Hour in any gear = RPM / Selected Gear Box Ratio / Back Axle Ratio x Rolling Circumference (in inches) / 1056
Note:- On all 5 speed Sierra gear boxes Top is 0.82:1 and 4th gear is 1:1 ( 5th is in effect an overdrive top ) On all 4 speed boxes Top is 1:1
Fitting a Non-Standard Speedometer
Fitting a new Speedometer is not as easy as it seems. The easy option, but the most expensive, is to buy a modern electronic unit. This can be calibrated to suit any Tyre and Diff ratio combination and full instructions are supplied with the unit. All modern cable driven Speedometers are more or less the same, manufactured to give 1,000 turns of the spindle per mile. ( 620 for Speedometers calbrated in km/hr ) Whatever mechanical unit you are using the Rear Tyres, Diff and Gearbox must match. It is possible to alter the Speedometer output spindle ratio at the gearbox, but this needs a partial gearbox strip. If you are coming from the single donor source this is not a problem, any 1,000 turn Speedo will work. There are other solutions such as a magnetic pulse unit on the Speedo spindle and custom ratio gearboxes. All expensive. Re-calibration is not possible for the Mileometer bit as is 100% gear driven, the Speedometer can be calibrated within certain limits. To check out your car see below, but 21 turns of ONE rear wheel should turn the Speedometer cable EXACTLY 24 turns whatever Diff or Gearbox you fit. Or put another way 874 turns of the rear wheels ( Car covers 1 mile ) = 1,000 turns of the Speedo spindle ( Mileometer indexes 1 mile ) See also the method suggested by Speedy Cables Speedy use the formulae:- Tyre revs per mile x Diff Ratio / Speedometer Output Ratio. eg:- 874 x 3.92 / 3.426 = 1,000 turns of the Speedo per mile.
Actual Worked Example:-
With one side of the car jacked up, 42 turns of the rear road wheel turned the Speedo cable exactly 24 revolutions. Since only one wheel was going round ( the other was on the floor ) we can assume 21 turns of both rear wheels will still turn the Speedo cable 24 turns. ( this is precisely an overall ratio of 7 : 8 ) 21 turns of the road wheel move the car ( 21 x 72.48" / 36 ) = 42.28 yards. 1,760 yards / 42.28 = 41.63 x 24 = turns of the Speedo cable per mile = 999.12. This is as close to the 1,000 figure in the above paragraph as makes no difference. This would suggest that the Speedo output spindle is geared at 3.43 :1 for a 3.92 Diff and is used by Ford for all Tyre combinations. All the above does of course relate ONLY to Ford Sierra's.
Table showing Speedometer Output Spindle Gearing for various Tyre and Diff Combinations. ( Ford Sierra Only )
|
Tyres
|
165/80/R13
|
175/70/R13
|
185/70/R13
|
185/65/R14
|
195/65/R14
|
195/60/R14
|
205/50/R15
|
|
Circumference
|
73.49"
|
71.14"
|
72.87"
|
73.70"
|
75.31"
|
72.90"
|
72.48
|
|
Diff Ratio's
|
-
|
-
|
-
|
-
|
-
|
-
|
-
|
|
3.14
|
8 : 22 (White) 2.75:1**
|
||||||
|
3.36
|
8 : 24 (Grey)
|
8 : 23 (Pale Blue)
|
8 : 24 (Grey )
|
8 : 23 (Pale Blue)
|
8 : 24 (Grey)
|
-
|
|
|
3.38
|
8 : 24 (Grey)
|
8 : 23 (Pale Blue)
|
8 : 24 (Grey
|
8 : 23 (Pale Blue)
|
8 : 24 (Grey)
|
-
|
|
|
3.62
|
7 : 23 (Blue)
|
7 : 22 (Yellow)
|
7 : 23 (Blue)
|
7 : 22 (Yellow)
|
7 : 23 (Blue)
|
-
|
|
|
3.92
|
7 : 24 (Black) 3.43:1**
|
||||||
Table courtesy of the Ford Motor Company Limited Technical Information Centre UK
** It is not possible to calculate exact worm drive ratio's using the number of teeth alone, other factors such as worm pitch, starts etc.need to be considered.
Note: As you can see from the above table there is no EXACT match for the Avon 205/50/R15 the 185/70/R13 & the 195/60/R14 are very close. The Chris Longhurst site will calculate any errors. Go to the Car Maintenance Bible, 4th option down.
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