Wheel size

CitizenErazed

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Ok I'm a little miffed here, I'm putting some 17" alloys on to allow bigger brakes and therefore improve handeling but I've seen people on here say that bigger wheels slow the car down. I don't really understand this as the way I look at it a bigger wheel is like a bigger cog, turn the smaller cog and for every rotation you cover a larger area with the bigger cog, if that makes sense. So a rotation of a 17" wheel covers a greater distance then a rotation of a 16" wheel. So by turning that wheel at the same speed you're covering a greater distance in the same time, thereby going faster. the only restriction to that being weight perhaps but light weight alloys should overcome this?

But from what people are saying thats not how it works, so can someone please explain to me how it works?

Also how much speed is actually lost as you go up the wheel sizes? I may be putting an inch bigger wheels on but I'm also putting on ultra low profile tyres so the overall size will stay the same (hopefully that means I wont have to have my spedo etc.. re-configured) so I'm liable to loose any speed?
 

HDi fun

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The idea is to keep the overall rim and tyre the same size as the original items.

If you stay within 5% then you shouldn't have problems.

I can knock up a little spreadsheet calculator if that would help.
 

CitizenErazed

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Thanks, by putting on ultra low profile tyres I should be going up an inch in alloys and down an inch in tyres to keep the overall size the same

But i don't understand how it works when people say that bigger wheels will make the car slower, to me it should make it faster provided the weight doesn't surpass the benefits, was wondering if anyone could explain it to me?

a spreadsheet calculator of gains/ losses from different wheel sizes would be usefull actually. If it's not any trouble that is :)
 

pgarner

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what are they on about when it slows the car down, top speed or accelaration ?

the bigger wheel also affects the final drive ratio making accelaration slower.
regarding weight - remember theres 4 wheels
 

HDi fun

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Download this and save to you computer. It's a scrappy little speadsheet I originally knocked together in about 1998 to work out overall wheel diameters and suitable wheel/tyre combinations.

This is simple geometry and arithmetic. You need to seek specialist advice with rim widths etc. There is also no allowance for tread flex. However, if you keep within 5% of standard overall diameter you should not have too much difficulty finding a suitable fitment.

http://www.mycomputer.ltd.uk/tyres.xls

note that you can use other reference speeds but the text in B15 remains locked at 60mph (Sorry!). I'll amend this for clarity at some point.
 
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CitizenErazed

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Cheers! I'll have to download it on wednesday as I'm on a mac at the mo. You make these sort of things as part of your job then or was this just made as a reference?

Pgarner I'm not sure but based on that I'd probably say they meant acceleration. surely 4 wheels would just be 4 times the weight difference of one wheel? so if it's 1KG lighter then 4KG overall, and visa-versa
 

pgarner

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yeah but i dont think theres only a kilo between wheels, should really have wieghed the 17" wheel and the 16"s to see what the difference was but would say theres a big differnece.

if the wheel size is altered by 5%, lets say it alters the final drive by 5% as well - need to have this confirmed as not sure on ratios. so if your car does 0-62 in 10.5 seconds ( based on standard 1.4 tigra ) then 5% more makes it 11.02 seconds 0.5 second more. this is before the extra weight is taken into account.

like i said it need to be confirmed by someone who understands the gearing
 

CitizenErazed

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fair enough, the wheel size should be ok as I'm switching to ultra low profile tyres at the same time but I think I better see a specialist anyway, to, as you say, confirm about the gearing. I wana do it right after all and I haven't even looked into widths yet. I think I need to see a specialist anyway as I need to sort out my cold air feed but am not sure if the gains from cutting a vent in my bonnet would outweigh the loss in aerodynamics, so need to see someone who can do a lot of maths with cars Lol.

As a side note, does that mean then that if I put 13" wheels on, my car would fly off like a rocket? (just wouldn't be able to brake as well Lol)
 

HDi fun

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fair enough, the wheel size should be ok as I'm switching to ultra low profile tyres at the same time but I think I better see a specialist anyway, to, as you say, confirm about the gearing. I wana do it right after all and I haven't even looked into widths yet. I think I need to see a specialist anyway as I need to sort out my cold air feed but am not sure if the gains from cutting a vent in my bonnet would outweigh the loss in aerodynamics, so need to see someone who can do a lot of maths with cars Lol.

As a side note, does that mean then that if I put 13" wheels on, my car would fly off like a rocket? (just wouldn't be able to brake as well Lol)

You do not need to see anyone about the overall gearing. My spreadsheet gives you all of that information. As I said, it's simply based upon static geometry.

You can certainly fit smaller wheels of smaller overall diameter. And you'll get the speedo higher round the dial in the same time.

But who wants a 0-62mph (indicated) in 8 seconds when in fact the car has only made 49mph in those 8 seconds.
 

obi_waynne

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It does take a bit of getting your head round but it all makes sense in the end.

It does cover a greater distance as you say but this is acually the problem. It now takes (for arguements sake) 2.5 engine drive shaft rotations for each wheel revolution rather than just 2.

Borrow an old 12 inch record player and put 3 blobs of bluetack on the record. The blob nearest the center and the one on the outside will be spinning at very different speeds although they both rotate the same amount!

:eek: Ok so I'm just confusing everyone even more now! Lets get HDi's calculator up.
 

CitizenErazed

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It does take a bit of getting your head round but it all makes sense in the end.

It does cover a greater distance as you say but this is acually the problem. It now takes (for arguements sake) 2.5 engine drive shaft rotations for each wheel revolution rather than just 2.

Borrow an old 12 inch record player and put 3 blobs of bluetack on the record. The blob nearest the center and the one on the outside will be spinning at very different speeds although they both rotate the same amount!

:eek: Ok so I'm just confusing everyone even more now! Lets get HDi's calculator up.

No that's not confuseing more, thats the exact way I've been looking at it, take your blue tack on vinyl analogy, the bit on the outside is traveling faster, they both have to compleate a single rotation at the same time, because they are attached, the middle can't rotate twice whilst the outside rotates once because it is one solid attached object, but obviously the center has a much smaller circle to rotate in, so whereas maybee the center has a 10cm circle perimiter to rotate the outside could have 100cm but because they both have to rotate at the same time, that means the outside will be covering 10* the distance in the same time frame, so is going 10* faster. and if it was another 10* the distance away it would be going 100* faster then the center. Say a rotation took a second, in the center it's spinning at 10cm a second but this translates to 100cm (1M) a second at the end of the wheel. Put a 10* bigger wheel on and whilst the center (the driveshaft) is still only spinning at 10cm a second, the SAME as it was, the wheel edge and therefore the car is traveling at 10M a second.

That's what I ment about bigger wheels make sense being faster to me, ok, maybe it takes more effort from the engine to make a rotation of a bigger wheel and it could be slower in that respect but the only way I can see it needing more effort is for the added weight, however I was looking to buy lighter alloys then factory standerds.

I've clearly got it compleatly wrong as everyone else is posative otherwise so I will look at HDIs thing tomorrow and see fi I can work out the flaw in my logic.
 

pgarner

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top speed will be up as long as the engine can cope if the overall size is bigger however acceleration will be down as the size of the wheel will affect the final drive ratio of the gearbox.
if you stick to very similer sizes overall theres not much difference
 

HDi fun

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You're also likely to mess up the suspension geometry by changing the overall diameter. If you fit a smaller wheel then the ride height will be lower as well. The converse also applies.
 

Sid447

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Ok I'm a little miffed here, I'm putting some 17" alloys on to allow bigger brakes and therefore improve handling but I've seen people on here say that bigger wheels slow the car down..............
..........Also how much speed is actually lost as you go up the wheel sizes? I may be putting an inch bigger wheels on but I'm also putting on ultra low profile tyres so the overall size will stay the same (hopefully that means I wont have to have my spedo etc.. re-configured) so I'm liable to loose any speed?

What is your wheel and tyre combination at the moment?
What size tyres are you going to for the 17"-ers?



p.s. Why is the spell-checker in American English?? It's tyre not tire!
 

HDi fun

TC ModFather
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If the overall diameter is the same then performance will be unaffected. The goal is to keep the overall rolling diameter as close to standard as possible.
 

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