TimC
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| posted on 15/2/06 at 05:01 PM |
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Wheel offset sussed?
Have spent a few mins playing with wheel offsets and calculations. Can someone tell me if I've got this right.
Offset is basically the distance between the mounting face of the hub and the centre of the wheel measured in mm.....
This is usually negative, i.e. the wheel centre line is closer to the cockpit/diff/whatever rather than outside the hub.
Based on this:
A lot of the kits e.g MK Indy are designed to use 7x15" alloys with a 38mm offset.
7" = 177.8mm. If you half this and subtract the offset, you get the distance that the edge of the rim sticks out from the hub mounting face.
In this case 50.9mm.
Similarly, a 6" rim with an offset of 23mm is 152.4mm wide resulting in a distance from mounting face of 53.2mm i.e. only 2.3mm further out.
Does this make sense?
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flak monkey
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| posted on 15/2/06 at 05:07 PM |
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Yes, but the rim width is measured across the tyre seats, not the overall width of the rim. IIRC.
David
Sera
http://www.motosera.com
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cerbera
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| posted on 15/2/06 at 09:44 PM |
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quote:
Originally posted by flak monkey
Yes, but the rim width is measured across the tyre seats, not the overall width of the rim. IIRC.
David
If this is the case, how wide is a 6" rim when measured across the tyre seats?
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flak monkey
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| posted on 15/2/06 at 09:46 PM |
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6" 
The overall width of the rim (from edge to edge) will be a little more. But not a huge amount.
The method Tim describes gives a good indication of how much the wheel sticks out.
David
Sera
http://www.motosera.com
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Wadders
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| posted on 15/2/06 at 10:17 PM |
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split rims loose 1/2" per side between rim outer edge and tyre seat i.e a 7" overall
measurement would be a 6" rim, not sure if cast rims work the same.
If this is the case, how wide is a 6" rim when measured across the tyre seats?
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tadltd
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| posted on 15/2/06 at 10:46 PM |
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Wheel offset data here (it's FREE info' - I'm not trying to sell you
anything!)
Best Regards,
Steve.
www.turnerautosport.com
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stevebubs
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| posted on 16/2/06 at 12:23 AM |
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quote:
Originally posted by tadltd
Wheel offset data here (it's FREE info' - I'm not trying to sell you
anything!)
Like your avatar, Steve!!
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TimC
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| posted on 16/2/06 at 08:13 AM |
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So in summary - and ignoring scrub angles or whatever (not got my head around that lot yet) if a kit is designed for 7x15" ET 38, using a
13x6" with ET23 wheels will place the outside of the rim 'close enough' to where the designer intended the 15" wheel to sit?!
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02GF74
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| posted on 16/2/06 at 09:08 AM |
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quote:
Originally posted by TimC
So in summary - and ignoring scrub angles or whatever (not got my head around that lot yet) if a kit is designed for 7x15" ET 38, using a
13x6" with ET23 wheels will place the outside of the rim 'close enough' to where the designer intended the 15" wheel to sit?!
you have one 15 inch wheel and one 6 inch or one 7 inch wheel and one 13 inch???
Assuming you mean 15x7 and 13x6 (diameter x width) then without thinking too much about it, to get the outside of the wheel to sit in the right
place, you need to move it by half the difference in the widths, i.e. 12.7 mm hence your ET is 26 (so yes, 23 will be close enough).
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TimC
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| posted on 16/2/06 at 10:06 AM |
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Erm - nothing like a bit of clarity - and that was nothing like clarity. Sorry.
I was theoretically talking about
15x7" and 13x6" (Dia x width) wheels.
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DarrenW
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| posted on 16/2/06 at 10:28 AM |
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Steve's link confirms exactly what you thought. Yes - you are right.
You just need to consider brake clearance as well. i know Jos has fitted 13" wheels to his Mac#1 with no problems. Might be worth a quote mail
to ask him what spec wheels he has used. i also know of a Westfield Megabusa with 13" wheels (flashy image split rims!!).
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tadltd
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| posted on 16/2/06 at 10:42 AM |
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In short yes, your wheel will be in the same position (the difference is only 2.25mm). So you don't need to worry about how they'll look
under the arches.
I'll post a pic of how I worked it out in a moment...
As promised:
 
wheel offset
[Edited on 16/2/06 by tadltd]
Best Regards,
Steve.
www.turnerautosport.com
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Lippoman
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| posted on 16/2/06 at 11:22 AM |
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Other things changing
Yes, the change in offset will get the outside of the rim at the approx. same position as with 15" rims, but other things change:
- track is increased by about 25 mm
- the roll center at static ride height is moved up slightly
- scrub is increased
- the forces on the wheel bearings increase
(all assuming same total height of the wheel)
Basically, you end up with the right look but with a different suspension set up. If the total height of the wheel is changed the suspension setup is
further changed.
If changing the offset, these considerations should be made. As the locost is lighter than the donor car, the wheel bearing loads will not be a factor
for moderate changes of offset.
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Marcus
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| posted on 16/2/06 at 05:06 PM |
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Lippoman is right here, it's not just the looks, if a hub or upright is designed for a certain offset, it's wise to stick with that offset
(or very close). If you were to use a wide rim with large offset, the wheel would 'look' to be the same as a narrower wheel with small
offset, BUT, would screw the handling up!
I wouldn't use a Sierra offset wheel on my Cortina uprights, 38mm vs 15mm
Marcus
Marcus
Because kits are for girls!!
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t.j.
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| posted on 16/2/06 at 08:12 PM |
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quote:
Originally posted by Marcus
Lippoman is right here, it's not just the looks, if a hub or upright is designed for a certain offset, it's wise to stick with that offset
(or very close). If you were to use a wide rim with large offset, the wheel would 'look' to be the same as a narrower wheel with small
offset, BUT, would screw the handling up!
I wouldn't use a Sierra offset wheel on my Cortina uprights, 38mm vs 15mm
Marcus
So? Tell my?
Cortina front uprights:
13" ET ?
14" ET ?
15" ET ?
Sierra front uprights:
13 "ET ?
14" ET ?
15" ET ?
Sierra rear
13"ET?
14"ET?
15"ET?
Sierra uses tires 165x13 up to 205/50-15
all with Rad. of prox. 285 mm.
Now we need only the ET's and we're happy
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Marcus
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| posted on 16/2/06 at 09:13 PM |
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The size of the rim makes no difference, the offset for Sierra rims is 38, for Cortina ones 15-19 (depending on who you ask!)
For offset read ET
Marcus
[Edited on 16/2/06 by Marcus]
Marcus
Because kits are for girls!!
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