Borednow's Guide to Wheels, Tyres, Stretch, Poke etc

So I've been looking around and have noticed that there are many questions about how do I get these wheels to fit or can I put 20" wheels from a Mercedes on my mk1 etc etc.
So rather than blowing up and making too many sarcastic comments I thought I'd do a little guide to help out a bit.
Disclaimer!!!!!!!

I'm not an expert on this stuff. I don't work in the wheel or tyre industry. This is stuff I have picked up over the years. I have used some products and many different sets of wheels on numerous cars. PLEASE use this info with caution. If you're unsure or concerned please ask questions. Your local tyre fitter will probably be able to help you out.

PCD - Pitch Circle Diameter. This is the spacings and number of holes in the wheels. Golfs have 4no  holes and a PCD of 4x100. this means that there is 100mm between the holes when measured diagonally. Anything else will not fit. ie 4x108, 5x100, 5x120 etc ect. PCD varies between vehicle manufacturers and even models from the same manufacturer. There is the possibility of using PCD changing spacers to fit different wheels from different models.

Tuck - The wheels and tyres are inside the arches and the top of the tyre (and possibly the wheel) is hidden by the bodywork. This is most often seen on the front of air cooled vehicles.
 Can be achieved by using narrow wheels (4-5" wide) with a high offset (ET) generally around ET50-ET45 on the wheels. When used with a narrow tyre 145-165 width the wheels will tuck under the arches.
Issues may be with steering lock if too low and rubbing on the inside of the tyre against the suspension arms.

Poke - The edges of the wheels stick out past the side of the arch. Tyres are then normally 'stretched' onto the rim to ensure that the tread of the tyre is inside the arch of the bodywork. Seen on many Jap cars and many Watercooled VW's. Usually taken to extremes with 13" wheels on Golfs.
Can be achieved by using wide wheels 9-10 inches wide and wheels that have a very low offset ET5 to ET-10 etc.
Issues will be with the tyres hitting the lip of the arch. Tyres can also come off the rim. Grip is also compromised. The police are not big fans of poke and stretched tyres when taken to extremes. Camber is also needed usually to get the tyre tread near the arch.

Stretch Tyres - Putting a narrow tyre on a rim to wide for it.
Enables low off-set or wider wheels to be used and still keeps the tyre tread under the arch of the bodywork thus keeping things 'within the law'. A small amount of stretch is safe and can solve some minor issues. Extreme stretch 145 tyre on a 8" rim is (in my opinion) dangerous. The tyre doesn't want to stay on the rim when cornering. I have seen many posts where younger drivers wonder why their tyres keep coming off the rims. Obviously this is not good for you or the car or anyone who happens to be in the general vicinity when the tyre comes off. (BTW my Golf is on 'normal tyres' but my Derby runs stretched tyres so I'm not against them per se).

Wheel Off set - this is what determines how far the back of the bolt face is from the centre line of the wheel. It is general stated as an ET38 is about standard for mk1 alloys and ET45 is generally the standard off set for mk1 steels (I think!!!!!!!) it is also measured in millimetres.
By changing a 4.5" wide wheel with an ET of 45 for a 4.5" wide wheel with an ET of 38 for example, the outside of the rim of the wheel will be moved 7mm closer to the arch. The inside rim of the wheel will b moved 7mm away from the suspension strut.
this needs to be taken into account when getting wider wheels as if the ET changes then suddenly a wheel that is relatively narrow won't fit.

Camber - This is the angle of the wheel to the floor. On front wheel drive cars with no power steering they used to put negative camber on the front wheels to make it easier to steer at low speeds whilst parking. Negative camber (small amount) gives better handling in corners as there is more tyre in contact with the tarmac when the steering wheel is turned. Normally when the wheel is turned the amount of tyre contact is reduced thus decreasing grip in a corner. Camber can also be used to bring the top of a tyre inside the arch of a car, thus making it legal again. Mostly seen in epic proportions on the back of bugs etc.
Issues - Causes uneven tyre wear, reduces grip in a straight ahead line thus reducing braking efficiency.

Spacers - used behind a wheel to push the entire wheel further out towards the arch.
You MUST use longer wheel bolts if using spacers!!!!! Also there are 2 types available Hub-centric (best) and non hub centric (bad). Hub centric spacers are vehicle specific and fit over the hub where the wheel mounts. The wheel mounting hub is then re-produced on the spacer so the wheel fits snugly onto the spacer.  this means that the wheels will be central to the hub and will spin correctly.
Non hub centric spacers are just flat pieces of metal with holes in for the wheel bolts. They can cause major wobble on the wheels if not fitted correctly. If using these I would suggest changing from wheel bolts to studs.

More to come. Please feel free to add anything you may know. Please stick to facts though!

Hi there, thought I'd just add a little about spacers. I'll include the same disclaimer, this isn't my job, it's just stuff I have picked up.

As well as the hubcentric vs non-hubcentric spacers, there are two different types of hubcentric spacer to think about.

Where the spacers required are small (generally <20mm, though not exclusively) the spacers may have 4 or 8 bolt holes, and the raised bevel to locate on the hub & inner wheel. They will require you to use longer wheel bolts. These type are cheaper, and usually come with 2 different pcd's (hence the 8 holes) in the manufacturers bid to make them somewhat multi-purpose. on our little cars you're not likely to run into many problems.



Where the spacers are larger (generally >20mm but, again, not exclusively) the spacers will have 8 bolt holes, and the raised bevel to locate on the hub and inner wheel. They will require you to use 4 bolts to attach the spacer to the hub, and then another 4 to attach the wheel to the spacer. Well, usually they will come with pre-installed studs to mount your wheels on, rather than threading the spacer. But there's no industry standard for these things. This is also how they construct PCD adapters, if that's what you're into  .  If you want to run spacers >20mm, I would highly recommend this style over the previous. I personally have trust issues when it comes to extra-long wheel bolts, but that's just opinion 

As for non-hubcentric spacers, well there's some debate. My favourite quote is from a Ralliart forum I trawled through a few years ago:

IMO I'd rather go with extended wheel studs and some kind of steel or billet type spacer; holds up better. Stay away from the cheap aluminum type spacers that you can source through your local auto chain store; they will break on our high speed cars. Hub centric bolt on type spacers tend to be weak and break over time, thus my suggesting the other route. Just my experience with spacers, do what you like.

A few years ago there was a spate of cheap hubcentric spacers failing when under high load. Similar failures to "those" rotas   and so they suffer similar criticism, and for a while there was a reaction back to using simple plate-type spacers. However, they're a pig to balance properly, and can give one hell of a wheel wobble.

I personally can see no issue with hubcentric spacers anymore, so long as you use a little bit of your brain when you choose how much you want to spend. If you want to throw yourself around a racetrack, or rallyx, or grasstrack, or whatever… it's probably not a good idea to penny pinch on the bits that join the car to the ground.

Another (hopefully obvious) note is that installing spacers will affect the offset of your wheels. Using bored's example, if your wheels are 4.5" alloys with ET38, and you install 10mm spacers, you're now running an effective ET 28. This also means the outer edge of your wheels is 10mm closer to the arch, and the inside of the wheel is 10mm further away from the strut. Similarly if you're running serious negative camber, installing large spacers will put the inner edge of your wheels further out, lowering the vehicle slightly.

That's all for my brain dump (for know). Hope it helps a little. B

Wow, that and what borednow has written is very impressive and very informative!  

I just wished I understood 50%!  
Really nice work Guys.  

But benr I've just got to ask, what the heck do you do for a living then?
You can PM me if you want to keep it off the 'Books'.  

Great bit of info

For people wanting to go the stretches route this is a great site to help you get an idea of how they will look giving specific tyre sizes and rim dia etc

Tyrestretch.com

Good guide fella!

www.willtheyfit.com is also a good site to use to look at your poke/ stretch

Thought I'd add a bit about tyres.

Remember the ONLY bit of your fantastic ride that actually touches the tarmac. Forget those big brakes and massive BHP's. If your tyres are rubbish, wrong or worn out non of what else you do matters!

First a bit on safety. Boring? - maybe, essential knowledge? - Deffo!

40% wear on a tyre equates to a LOSS of grip of over 60%. This increases in the wet! Bear this is mind when contemplating how long your tyres will last.

At 60 mph on a wet road your grip is half that at 40mph. If your tyres are also worn this can equate to ONLY 25% of the grip on a wet road at 70mph with worn tyres as you have at 30mph on a dry road with new tyres. That's why there are so many crashes on wet roads!

Part Worns - Basically please don't buy these. Most are imported from other European Countries where the legal limit for tyres is less than the legal limit here. There are NO restriction on imports at all. No safety checks whatsoever. It is down to the fitting person to check that they are suitable. Comments about reputable tyre fitters/garages could be made at this point, however that would be an opinion. I'd suggest part worns can be used for checking sizes, and fitments but I'd strongly suggest that they aren't used for real road use of any sort of mileages. This is different to buying a car with tyres on it or indeed buying wheels with tyres already in them. When buying second wheels if they come with tyres then use your judgement. If they look ok, are in date and hold air then just get them checked over by your local tyre fitter. Most will do a free check of tyres (the idea is that they'll find something wrong in some cases and sell more tyres). I recently bought some second hand tyres and was assured they were ok. when they were spun up on the rims on the balancing machine a 'wobble' in the tyre could be seen. I ended up buying 4 new tyres in the end. Lesson Learnt.

Rolling Radius

All cars come from the factory with a specific 'Rolling Radius' of wheel and tyre combination. This relates to the size of the wheel (ie 13") PLUS the addition of the tyre. If this 'rolling radius' is changed the speedo will read incorrectly and MPG will be effected, it also effect handling and gearing.

By changing the rolling radius the car may also ride higher or lower than factory original. If the rolling radius is reduced too much a car may ground out and if it is increased too much tyres may strike the bodywork. This is assuming the suspension stays the same.

We'll come back to this later.  

Tyre Specs

Ever wondered what the markings on the side of the tyre are all about?  

Example my Passat has tyres whcih have the following on the side 245/35 R17 94 XL.

I'll break it down for you "245/35". 245 refers to the width of the tread in millimetres. ie 245mm of tyre width in contact with the road. The 35 refers to the height of the sidewall as a percentage of width of the tyre. Therefore the height of the sidewall is 35% of 245mm. Therefore a little over 85mm.

The R17 refers to the diameter of the wheel measure in inches. In this case 17 inches.

94 refers to the 'speed rating' of the tyre. This is effectively how fast the tyre can go before it MAY be damaged or in worse cases explode. The 94 DOES not mean that at 94mph the tyre will explode. It is a CODE not an empirical measurement. Your tyres should never be 'Down rated' as your insurance will become invalid. You can go up though

XL refers, in this case, to the load rating of the tyres. Again this is an code not a measurement. Load ratings are specified for the vehicle and should not be down graded. Too much load on a tyre will cause it to fail.

Changing Tyre Sizes.

People want to change the size of their tyres for many reasons, to add grip for cornering for example or to be able to increase the wheel size or just for looks. To be able to do this and maintain everything working as it came from the factory a bit of maths is involved!

In the example above my passat has a rolling radius based on its wheel size in INCHES (R17) and it's sidewall profile (35% of 245mm) in Milimetres!

So calculating the Rolling Radius is a bit of a faff. After asking Google I've found that 1 inch = 25.4mm. Therefore my wheel diameter in mm is 431mm. We divide this 2 to get 215mm We then add the size of the tyre 85mm to get 300mm for the rolling radius of my wheels. If I want to change the width of my tyres I need to know the original rolling radius so I can make sure that I don't change the speedo.

So lets say I want to go to 18's instead.
Rolling radius originally is 300mm

18" equates to 228mm which leaves 72mm for the tyre. Lets assume I want the same width tyre (245mm) to get the same rolling radius I will need a tyre that is only 72mm high in the sidewall. If I go to a tyre that is 30% of 245 then the sidewall is 73.5mm high, which is fine.

So by running 245/30 on R18 wheels I have the same rolling radius as 245/35 R17.

Good innit  


So if you want to go up a size in wheel in inches come down a size in the profile of the tyre!

It also seems to work with width. If you from a 185/60 R14 tyre you can go to 195/50 R14 tyre with no probs. I won't go through the maths as I'm falling asleep writing it!!

However NOT all tyre sizes exist! So you may follow my maths and find the tyre size you need isn't actually made. In that case go for the nearest that is made and bear in mind that your speedo will not be as accurate as it was. Use the calculator at www.willtheyfit.com as mentioned my Mr Nosmonkey to find out the error. Your sat nav may become your best friend if the speedo is miles out.

hope I didn't bore you too much!

Comments and Corrections welcomed

Awesome thread about the tire/wheel calculations mate. Will keep that one in mind. Actually will never forget that EVER again. I got a speed fine back in South Africa with my white CTI (on my engagement day of all days). 180km/h in a 120km zone, because I forgot the engagement ring in my Navy cabin…………Long story short….. I argued with the Police that I was "ONLY" going at 165…….. Now I know why….    

Great read thank you. I've taken a lot in but I'm a bit thick . I'm after a set of split rims for my mk1 golf. I've got a set of 15"s on now and want to stick with that. I would like however wider wheels with a rear offset. I still don't understand what width wheels I'm looking for and the 7J and 8J talk I haven't got a clue  
Is it just a wider lip you have on the rears or complete wider wheel? How wide can you go with standard arches with out having issues? 

Bit of info here as well


Jellybelly's Guide to the world of wheels & tyres !!! - The Mk1 Golf Owners Club

That's great thank you. I'm getting there now slowly. What is the average offset everyone has with 15" ?

I'm looking to replace tyres on my 83 golf mk1 any ideas where to look ?