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Sahara Travel Forum Topics specific to North Africa and the Sahara down to the 17th parallel (excludes Morocco)
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Desert Travels - Motorcycle Journeys in the Sahara and West Africa!

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  #1  
Old 4 Jul 2002
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7.50 x 16 or 235/85's on a Range Rover

Can anyone tell me what needs to be done to run larger tyres on a Range Rover. And how do you work out the difference in gearing?
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Old 5 Jul 2002
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Let’s be clear about this. You are talking about bigger diameter tyres, not bigger width.

There are three things you have to sort out.[list=1][*] Is there enough room within the wheel arches so that the bigger tyres don’t hit the bodywork when the suspension is fully compressed?[*] Similarly can you turn the front wheels from full lock left to full lock right when the suspension is fully compressed?[*] You will have to recalibrate your speedometer.[/list:5d7eb60958]

Range Rovers are fitted with many rim and tyre profile combinations, but they all end up with roughly the same static radius, eg a 235/70 R16 has a static radius of 367.7mm, a 255/65 R16 has 368.95mm, and a 255/55 R18 has 368.85mm. So the speedometer reading is hardly affected (the difference between a new tyre and a worn tyre is much greater).

So, you are going to increase the static radius of your wheels by approximately 34mm (that’s just under one and a half inches in old money). If you fit new springs that are two inches longer, you will probably be OK. BUT borrow a set of wheels and try them for size before you spend any money. Try talking to Scorpion Racing about springs on 020-7485-5581 - they may know someone that has already done it.

That takes care of items (1) and (2). Now comes the complicated bit which is all about rolling circumference, ie the distance that a tyre travels along the ground in one complete revolution.

You have to be careful here because a tyre’s rolling circumference is not the same as its static circumference. This is because the steel within the tyre makes it “deform” in use. It is designed to do this.

So you need to know the manufacturer’s figures for the tyres you are coming from and the tyres you are going to.

The Michelin 235/70 R 16 4X4 - A/T has a rolling circumference of 2,245mm, whereas its static circumference is 2,311mm. The Michelin 7.50 R16 4x4 O/R has a rolling circumference of 2,450mm, and both the Michelin 235/85 R 16 4X4 O/R and the BFGoodrich LT235/85 R 16 All-Terrain T/A are 2,442mm.

The difference will be at least 9%, and you have two solutions to enable you to display your correct speed. You find out the precise difference and get a gearing box made up and attached to your speedometer cable. I have found outfits in the US that can do this for some US vehicles, but I don’t know of anybody who can do it for a Range Rover. If anybody out there knows of someone please shout. The simple solution is to buy, fit and calibrate an electronic speedometer. Europa Specialist Spares sell a nice one, see http://www.europaspares.com/acatalog...Isspro_15.html . You will need access to a rolling road to calibrate it.

The difference between the rolling circumferences will also be the amount of overgearing you have introduced. Talk to Ashcroft Transmissions on 01582-750400 about changing the gearing of your Transfer Box to get back down again. They have a web site at http://www.autoconv.com/ .

(Sorry - missed the last bit out in the first post)


[This message has been edited by Terry Davies (edited 05 July 2002).]

[This message has been edited by Terry Davies (edited 14 September 2002).]
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  #3  
Old 5 Jul 2002
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The other way to guage your speed is just to use a GPS all the time, with the inaccuracy of 101 speedos and the addition of 20" unimog tyres, the GPS is invaluble to me - you could just use that to mark up your existing speedo

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  #4  
Old 11 Jul 2002
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Hello,

I drive a Range Rover Clasic from 85 in the Sahara and have used 7.50R16 from Bridgestone (VSJ) and Michelin XS-F

To compensate for the load the RR has Bearmach +40mm HD Springs all arround.

On the front I removed this spoiler thing.

Only the XS-F requires slight trimming on the rear wheeh arch (never tried them front).

I feel that I should lower the front axle bumpers about 1 cm.

The gearing with the 7.50 is 13% longer - according to my GPS!
Low range is still low enough, but the gearing in high range is less then optimal, the first is often too long, frequent changing between HR/LR is not really comfortable.

So I will change the hight range ratio in the transfer box to a 20% shorter one (from 110).

The speedo could be corrected: take the gear from a 110.

A good compromise would be the dimension 215/85R16.

Bye, Yves



[This message has been edited by Yves (edited 11 July 2002).]
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  #5  
Old 13 Oct 2002
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I run 235/85-16s on a Discovery, but I had to cut away the trailing edge of the rear wheel arches. The piece I took out was about 35mm wide by 120mm long. It is hardly noticeable if you get the cut to follow the original lines. I would guess that similar body mod might be needed on a Range Rover.

A 7.50-16 is fractionally bigger, though considerably narrower.

Terry's info was spot-on, but you generally needn't worry too much about the difference between rolling and static radius. It is usually only a couple of percentage points difference, and will apply across the board, to all intent and purposes, proportionally.

So if you work out the nominal (static) diameter of your orginal tyre, and the percentage by which your proposed tyres are bigger (again, nominal diameter), you will have all the info you need, for both gearing and speedometer calibration purposes.

Regards,

Michael...
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  #6  
Old 13 Oct 2002
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There’s another thing I missed out of my original post - Shock Absorbers.

If you put longer springs on your vehicle, eg two inches (approx 50cm), then you should change your shock absorbers for ones with two inches additional travel. This is because if one of your wheels drops into a hole, its movement downwards will be halted by the shock absorber, not the spring, ie the shock absorber will “bottom out”. Shock absorbers cannot take that kind of punishment.

I know that Bilstein do not make longer travel shock absorbers with standard mountings. I haven’t been able to get any information out of Koni.

The following companies do make them:

De Carbon - contact Scorpion Racing tel: 020-7485-5581 http://www.scorpion-racing.co.uk/.

Old Man Emu - contact Frogs Island 4x4 tel: 01235-832100 http://www.frogsisland4x4.com.

ProComp - contact Simmonites tel: 01274-833351 http://www.simmonites.co.uk/.
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