Does anyone know what the gearing effect would be if I changed the standard 
235 - 60 -17" wheels and tyres for 235 - 55 -18" - would the reduction in 
profile offset the change from 17" to 18" wheels?

thanks

"jscott"  gurgled happily, sounding much like they
were saying:

> Does anyone know what the gearing effect would be if I changed the
> standard 235 - 60 -17" wheels and tyres for 235 - 55 -18" - would the
> reduction in profile offset the change from 17" to 18" wheels?

<passes calculator>

The profile is a percentage of the tread width (in mm).
There's 25.4mm in an inch.
3.14 is good enough accuracy for Pi for these purposes.
The circumference is Pi x diameter.

Go fer it.

Adrian wrote:

> "jscott"  gurgled happily, sounding much like they
> were saying:
> 
>> Does anyone know what the gearing effect would be if I changed the
>> standard 235 - 60 -17" wheels and tyres for 235 - 55 -18" - would the
>> reduction in profile offset the change from 17" to 18" wheels?
> 
> <passes calculator>
> 
> The profile is a percentage of the tread width (in mm).
> There's 25.4mm in an inch.
> 3.14 is good enough accuracy for Pi for these purposes.
> The circumference is Pi x diameter.
> 
> Go fer it.

Or alternatively, Google for tyre rolling radius, which will lead you here:

http://homepage.ntlworld.com/cattermole/tyres.htm

Chris

-- 
Remove prejudice to reply.

"Adrian"  wrote in message 
news:6k3liqF5oq6nU4@mid.individual.net...
> "jscott"  gurgled happily, sounding much like they
> were saying:
>
>> Does anyone know what the gearing effect would be if I changed the
>> standard 235 - 60 -17" wheels and tyres for 235 - 55 -18" - would the
>> reduction in profile offset the change from 17" to 18" wheels?
>
> <passes calculator>
>
> The profile is a percentage of the tread width (in mm).
> There's 25.4mm in an inch.
> 3.14 is good enough accuracy for Pi for these purposes.
> The circumference is Pi x diameter.
>
> Go fer it.

I make it 2.241 metres per rev of tyre for the 235/60 17" 
(17x25.4)+(235/100x60x2)
and 2.247 metre per rev of tyre for the 235/55 18"

So, (if correct) an extra 6mm travelled per tyre rev for the later. Its very 
slight, but if it was larger it would be interesting to see how this can be 
related to speedo reading or gearing ratio,  but unfortunately at this early 
hour of the day, my brain can not troubled to fathom it just yet.

"Mark"  gurgled happily, sounding much like they were
saying:

>>> Does anyone know what the gearing effect would be if I changed the
>>> standard 235 - 60 -17" wheels and tyres for 235 - 55 -18" - would the
>>> reduction in profile offset the change from 17" to 18" wheels?

>> <passes calculator>
>>
>> The profile is a percentage of the tread width (in mm). There's 25.4mm
>> in an inch.
>> 3.14 is good enough accuracy for Pi for these purposes. The
>> circumference is Pi x diameter.
>>
>> Go fer it.

> I make it 2.241 metres per rev of tyre for the 235/60 17"
> (17x25.4)+(235/100x60x2)
> and 2.247 metre per rev of tyre for the 235/55 18"
> 
> So, (if correct) an extra 6mm travelled per tyre rev for the later. Its
> very slight, but if it was larger it would be interesting to see how
> this can be related to speedo reading or gearing ratio,  but
> unfortunately at this early hour of the day, my brain can not troubled
> to fathom it just yet.

Clue: "Negligable" is a perfectly acceptable answer...

It's also worth thinking about the effect of the difference in tread 
depth between a brand spankin' and a knackered tyre.

Chris, Adrian, Mark - thanks for your help.

"Adrian"  wrote in message 
news:6k3o8nF5oq6nU6@mid.individual.net...
> "Mark"  gurgled happily, sounding much like they were
> saying:
>
>>>> Does anyone know what the gearing effect would be if I changed the
>>>> standard 235 - 60 -17" wheels and tyres for 235 - 55 -18" - would the
>>>> reduction in profile offset the change from 17" to 18" wheels?
>
>>> <passes calculator>
>>>
>>> The profile is a percentage of the tread width (in mm). There's 25.4mm
>>> in an inch.
>>> 3.14 is good enough accuracy for Pi for these purposes. The
>>> circumference is Pi x diameter.
>>>
>>> Go fer it.
>
>> I make it 2.241 metres per rev of tyre for the 235/60 17"
>> (17x25.4)+(235/100x60x2)
>> and 2.247 metre per rev of tyre for the 235/55 18"
>>
>> So, (if correct) an extra 6mm travelled per tyre rev for the later. Its
>> very slight, but if it was larger it would be interesting to see how
>> this can be related to speedo reading or gearing ratio,  but
>> unfortunately at this early hour of the day, my brain can not troubled
>> to fathom it just yet.
>
> Clue: "Negligable" is a perfectly acceptable answer...

Yes, obviously negligable. As in "very slight". Just was curious to take 
this further to see what a bigger difference would make on speedo reading, 
but the website that does it for us automatically has now taken all the fun 
out of going any further.

Mark wrote:

[...]

> Yes, obviously negligable. As in "very slight". Just was curious to take
> this further to see what a bigger difference would make on speedo reading,
> but the website that does it for us automatically has now taken all the
> fun out of going any further.

Sorry to have spoilt the fun. FSVO fun, obviously...

Perhaps I got it out of my system in the sixties and seventies, when I raced
motorcycles for four seasons, then went on to rally a Mini for a couple of
years.

I spent hours producing charts of speed versus rpm for all combinations of
tyre sizes, final drive ratio, gearbox internals etc. All by pencil and
paper, for affordable pocket calculators were still a twinkle in Sir
Clive's eye then!

Chris
 
-- 
Remove prejudice to reply.

Nedavno jscott napisa:

> Does anyone know what the gearing effect would be if I changed the
> standard 235 - 60 -17" wheels and tyres for 235 - 55 -18" - would the
> reduction in profile offset the change from 17" to 18" wheels?


Check here:


http://www.miata.net/garage/tirecalc.html




-- 
      ___   ____
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    /  / \/  /\  \     ** Registered Linux user #291606 **
   /__/\____/--\__\    ** http://counter.li.org/ **

jscott wrote:
> 
> Does anyone know what the gearing effect would be if I changed the standard
> 235 - 60 -17" wheels and tyres for 235 - 55 -18" - would the reduction in
> profile offset the change from 17" to 18" wheels?
> 
> thanks

Increase the wheel diameter by 0.3%, this should be ok.

More info from a classical tyre calculator tool at 
http://www.miata.net/cgi-bin/tirescgi

"jscott"  wrote in message 
news:6k3lbnF5u94mU1@mid.individual.net...
> Does anyone know what the gearing effect would be if I changed the 
> standard 235 - 60 -17" wheels and tyres for 235 - 55 -18" - would the 
> reduction in profile offset the change from 17" to 18" wheels?
>
> thanks
>
I do not know why the rest of the guys are attempting some long 
calculations, here is my 1p [from what I remember from school maths about 25 
years ago].
You increase your rim (diameter by one inch = 25.4mm) and reduce the height 
of your rubber by 5mm x2 = 10mm.
Overall increase of wheel diameter is therefore 15.4mm, divide that by 
original wheel diameter (17 x 25.4mm + 2 x 55mm = 541.8mm)
the result is 0.028 - just under 3%.
So your speedometer will show your speed to be 3% less than actual (that is 
just 2mph at 70mph) and the same goes for distance covered... 


** Posted from http://www.teranews.com **

Nedavno GArlington napisa:

> "jscott"  wrote in message 
> news:6k3lbnF5u94mU1@mid.individual.net...
> > Does anyone know what the gearing effect would be if I changed the 
> > standard 235 - 60 -17" wheels and tyres for 235 - 55 -18" - would
> > the reduction in profile offset the change from 17" to 18" wheels?
> >
> > thanks
> >
> I do not know why the rest of the guys are attempting some long 
> calculations, here is my 1p [from what I remember from school maths
> about 25 years ago].
> You increase your rim (diameter by one inch = 25.4mm) and reduce the
> height of your rubber by 5mm x2 = 10mm.


235/55 R18

235 is tire width
 55 is tire highth as percentage of its width
R18 is rim diameter

so in this case tire highth is 235x0.55=~129mm (not 55mm)




-- 
      ___   ____
     /__/  /    \      ** Registrovani korisnik Linuksa #291606 **
    /  / \/  /\  \     ** Registered Linux user #291606 **
   /__/\____/--\__\    ** http://counter.li.org/ **

Nedavno GArlington napisa:

> I do not know why the rest of the guys are attempting some long 
> calculations, here is my 1p [from what I remember from school maths
> about 25 years ago].
> You increase your rim (diameter by one inch = 25.4mm) and reduce the
> height of your rubber by 5mm x2 = 10mm.


235/55 R18

235 is tire width
 55 is tire height as percentage of its width
R18 is rim diameter

so, in this case, tire height is 235x0.55=~129mm (not 55mm)


-- 
      ___   ____
     /__/  /    \      ** Registrovani korisnik Linuksa #291606 **
    /  / \/  /\  \     ** Registered Linux user #291606 **
   /__/\____/--\__\    ** http://counter.li.org/ **

"GArlington"  gurgled happily, sounding much
like they were saying:

> I do not know why the rest of the guys are attempting some long
> calculations, here is my 1p [from what I remember from school maths
> about 25 years ago].

One whole penny?

It's overpriced.

> You increase your rim (diameter by one inch = 25.4mm) and reduce the
> height of your rubber by 5mm x2 = 10mm.
> Overall increase of wheel diameter is therefore 15.4mm, divide that by
> original wheel diameter (17 x 25.4mm + 2 x 55mm = 541.8mm) the result is
> 0.028 - just under 3%.
> So your speedometer will show your speed to be 3% less than actual (that
> is just 2mph at 70mph) and the same goes for distance covered...

Nice theory. Shame it's based on a misunderstanding.

On Fri, 26 Sep 2008 09:27:51 +0000, Adrian wrote:
> 
> Clue: "Negligable" is a perfectly acceptable answer...
> 
> It's also worth thinking about the effect of the difference in tread
> depth between a brand spankin' and a knackered tyre.

...unless you drive one of those ultra-sensitive 4WD cars where worn 
rears with new fronts would burn out the viscous centre diff - Volvo was 
it?