Ties
 
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Hi,

I had my van checked up by my local garage in preparation for it's upcoming MOT, and all 4 tyres needed replacing. (already saw it coming but was hoping they still have one more MOT in them...)

Anyway, the current tyres are all 6.70x13 from different brands, unfortunately tyres in that size are not really available here in the netherlands.
Looking around online I found an eBay add selling an old 185/70R13 tyre that came of a bedford CF with the same engine configuration and from the same era as mine, so I assume that a 185/70 R13 tyre (witch is quite readily available and affordable) should work on my van in place of the old 6.70x13 tyres, right?

I just wanted to make sure before I order 4 brand new tyres that wont fit...

Postby Ties » Sat Sep 23, 2017 7:41 pm


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Phil Bradshaw
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185/70R13 has a smaller rolling radius than 6.70x13 and never was specified for CFs. The only other tyre specified for 13-inch rims was 6.40x13 for the early 14 cwt model with 1598cc engine and three-speed transmission.

If any of the wheels have 10 holes then an inner tube is advisable because the rims have no bead rebates for tubeless tyres.
5-hole wheels should have bead rebates both sides of the rim.

Another option is to find a set of 14-inch wheels (5-hole preferably) and fit 195R14 van tyres, or 185R14 van tyres for fuel economy and slightly less heavy steering but less grip when the van is trying to go sideways.
    What is real is not the external form but the idea, the essence of things. Constantin Brâncuși

Postby Phil Bradshaw » Sat Sep 23, 2017 8:42 pm


Ties
 
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Phil Bradshaw wrote:185/70R13 has a smaller rolling radius than 6.70x13


Ok, so after a lot of googeling and calculating, I think I follow. If my math is correct:

The original 6.70x13 tyres have a rolling radius of 23,72" (80% of 6.70" is the height of the sidewall = 5.63". 5.63" x 2 + 13 = 23,72")
185/70R13 tyres would have a rolling radius of 23,197" (about 2.2% smaller than the current tyres)
175/80R13 would give me a rolling radius of 24,024" (around 1.3% larger than current tyres)

So to me this last option now seems the most sensible option, since the rolling radius is closer to the current tyres, and the tyre width is closer to the current tyre width aswel.

14" wheels would be nice but unfortunately aren't really an option right now. They are very hard to find used, and a set of refurbished or new ones is way out of my budget.

Assuming all my wheels are 5-hole wheels, would the non-standard 175/80R13 tyres cause any issues apart from a difference in engine speed relative to road speed and a slight inaccuracy in the speedometer?

Postby Ties » Sun Sep 24, 2017 12:25 am


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Phil Bradshaw
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Nearest equivalent to 6.70x13 is 185R13 (i.e. 80% aspect ratio) or 205/70R13.

(Similarly for 700x14 tyres used for earlier CF350 models - 185R14 (specified option) or 205/70R14.)

The only radial tyre specified for 13-inch wheels is 185SR13 for the CF220 Code 880 which replaced the earlier 14 cwt model with 6.40x13 tyres.

Rolling radius is a function of loading and inflation pressure. Even for van tyres (8 ply rating, greater load capacity) the difference between cross ply and radial tyres can be significant in terms of speedometer error.

Speedometer driven gear is the same for 6.70x13 and 195R14 for a particular final drive ratio, e.g. 18 teeth for 8/37 final drive.

For 185R13 tyres the gear for 6.40x13 is close, 19 teeth for 8/37 final drive.

Anything else usually will require calibration using a correction adaptor on the drive: more expense ...

Compared to the taller 6.70x13 tyres 185R13 look silly on a CF anyway, but that's a personal view. :) Roadholding is far superior on radial tyres nonetheless.
    What is real is not the external form but the idea, the essence of things. Constantin Brâncuși

Postby Phil Bradshaw » Sun Sep 24, 2017 3:25 am


Ties
 
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Phil Bradshaw wrote:Nearest equivalent to 6.70x13 is 185R13 (i.e. 80% aspect ratio) or 205/70R13.


Thanks! I looked for some 205/70R13 tyres and they are still quite hard to find and very expensive, but I did find an inexpensive 185/80R13 tyre.

Phil Bradshaw wrote:Even for van tyres (8 ply rating, greater load capacity) the difference between cross ply and radial tyres can be significant in terms of speedometer error.


To be honest the difference in the speedometer doesn't really bother me, its quite inaccurate as it is.
Im more worried about the difference in road speed. Is there any way to calculate how much faster the engine would have to spin to achieve the same speed on 185/80R13 tyres vs the current 6.70R13's?

Postby Ties » Sun Sep 24, 2017 9:03 am


Ties
 
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Also, I just went out to check if I have 5 or 10 hole wheels, but couldn't get the chrome hubcaps of... :oops:

Is there a proper way to do this or should I just pull harder until they come off?

Postby Ties » Sun Sep 24, 2017 9:51 am


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You need to measure the height of the rear hub centre above ground for 6.70x13 and 185R13 with the van at its normal weight and tyres inflated to the correct pressure (3.1 bar for 6.70x13, 2.8 bar for 185R13 at 1300 kg axle weight).

Ratio of hub centre heights will give a multiplying factor, smaller height divided by the larger for reduction factor when going smaller.

For a quick estimate of speed reduction factor divide the speedometer driven gear teeth number for 6.70x13 (18) by the number for 185R13 (19): approximately 0.95 or 95%.

This could be an over-estimate: my first CF managed to lose about 8% when going from 6.70x13 to 185R13 and calculating road speed using a stop watch and counting Motorway markers when they were 16 to the mile.

Hub caps are sprung over 3 lugs formed in the wheel disc. Use a pry bar or long screwdriver against wheel rim and close to one the the lugs to lever cap from wheel.

Image
    What is real is not the external form but the idea, the essence of things. Constantin Brâncuși

Postby Phil Bradshaw » Sun Sep 24, 2017 12:41 pm


Ties
 
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Phil Bradshaw wrote:If any of the wheels have 10 holes then an inner tube is advisable because the rims have no bead rebates for tubeless tyres.
5-hole wheels should have bead rebates both sides of the rim.


I seem to have 10 hole wheels, so I'm also going to need inner tubes. Could the old ones be reused? Or should thees be replaced at the same time as the outer tyres?

Postby Ties » Mon Sep 25, 2017 6:50 am


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Fitting new tubes may be best but old tubes in good condition can be re-used.

An experienced tyre fitter will be able to tell you once old tubes are out and can be examined.
    What is real is not the external form but the idea, the essence of things. Constantin Brâncuși

Postby Phil Bradshaw » Mon Sep 25, 2017 9:17 am