Wheel Bolt Lengths With Spacers

[TripleBob]

Hi folks, need some reassurance/confirmation I'm doing the right thing!

I've picked up some Bimecc spacers for my Leon - and gone for the tried and tested (and often recommended) combo of 12mm's front and 15mm's rear.
I ordered a kit that said it came with the 12+15mms, 16 appropriate longer length bolts and 4 longer locking nuts... The kit has arrived and all the supplied longer bolts are 40mm long.
My understanding was that the longer bolt should be at least the length of the original bolt + spacer... I've measured one of my existing ones and make it as 27mm long? (thread length) - so for the 12mm spacers that's 12 + 27 = 39mm, so the 40mm bolts should be fine as long as they don't rub where they protrude behind the hub?
My issue is with for the 15mm spacers - as 15 + 27 = 42mm. So the new longer bolts for the rears will be 2mm too short.

I'm thinking this is a deal breaker, and I should go back and request longer bolts for the back? Or is 2mm short fine? I can't tell if there is excess thread length on the OEM bolts... Will 40mm give me safe enough depth to secure the wheels, or do I need 42mm minimum, maybe even 43mm to have a small excess?

 

[SuperV8]

Hi folks, need some reassurance/confirmation I'm doing the right thing!

I've picked up some Bimecc spacers for my Leon - and gone for the tried and tested (and often recommended) combo of 12mm's front and 15mm's rear.
I ordered a kit that said it came with the 12+15mms, 16 appropriate longer length bolts and 4 longer locking nuts... The kit has arrived and all the supplied longer bolts are 40mm long.
My understanding was that the longer bolt should be at least the length of the original bolt + spacer... I've measured one of my existing ones and make it as 27mm long? (thread length) - so for the 12mm spacers that's 12 + 27 = 39mm, so the 40mm bolts should be fine as long as they don't rub where they protrude behind the hub?
My issue is with for the 15mm spacers - as 15 + 27 = 42mm. So the new longer bolts for the rears will be 2mm too short.

I'm thinking this is a deal breaker, and I should go back and request longer bolts for the back? Or is 2mm short fine? I can't tell if there is excess thread length on the OEM bolts... Will 40mm give me safe enough depth to secure the wheels, or do I need 42mm minimum, maybe even 43mm to have a small excess?

I would be looking to use bolts with at least an equivalent thread engagement as the OEM bolts.
If the OEM bolts protrude out the back of the hub 2mm then should be fine.

 

[mjj4]

It will be fine. People often run up to 5mm spacers with stock bolts. Unless they were expensive (I've never used Bimec) then I wouldn't worry. If they were top dollar then I'd want them to be mm perfect.

If you think about it from a physics perspective, the bolt reaches it's maximum strength when it has been threaded into the hub by the same diameter as the bolt (M14 in this case). So once it goes past 14mm into the hub you're wasting your time as its weakness is now the diameter.

The Leon hub is m14x1.5mm pitch. So you need 9.3333 turns to have 14mm of thread contact.

Bear in mind that different alloy wheels have different thickness bolt holes which can affect this either way too.

 

[SuperV8]

I've picked up some Bimecc spacers for my Leon - and gone for the tried and tested (and often recommended) combo of 12mm's front and 15mm's rear.
I ordered a kit that said it came with the 12+15mms, 16 appropriate longer length bolts and 4 longer locking nuts... The kit has arrived and all the supplied longer bolts are 40mm long.
My understanding was that the longer bolt should be at least the length of the original bolt + spacer... I've measured one of my existing ones and make it as 27mm long? (thread length) - so for the 12mm spacers that's 12 + 27 = 39mm, so the 40mm bolts should be fine as long as they don't rub where they protrude behind the hub?
My issue is with for the 15mm spacers - as 15 + 27 = 42mm. So the new longer bolts for the rears will be 2mm too short.

I'm thinking this is a deal breaker, and I should go back and request longer bolts for the back? Or is 2mm short fine? I can't tell if there is excess thread length on the OEM bolts... Will 40mm give me safe enough depth to secure the wheels, or do I need 42mm minimum, maybe even 43mm to have a small excess?

I have an old front hub - this measures 13mm thick where the bolt threads are.
I have measured my winter alloy thickness at 'roughly' 6mm where to where the taper starts.
Our discs are 7.2mm thick.
Plus your 15mm spacer.
I make the total thread approx. 6+13+15+7.2=41.2
You would probably be missing out on about 1x thread engagement vs original.
The variable is the alloy thickness - I don't know how much this varies between sizes/styles?

 

[TripleBob]

I have an old front hub - this measures 13mm thick where the bolt threads are.
I have measured my winter alloy thickness at 'roughly' 6mm where to where the taper starts.
Our discs are 7.2mm thick.
Plus your 15mm spacer.
I make the total thread approx. 6+13+15+7.2=41.2
You would probably be missing out on about 1x thread engagement vs original.
The variable is the alloy thickness - I don't know how much this varies between sizes/styles?

Amazing thank you! I've contacted the supplier but haven't heard back yet - I'm running the OEM 18" sport alloys (part number 5F0601025F 8Z8), which I think are a little thicker at the centre where it mates to the hub
I offered them up as a test, and couldn't get the OEM spec 9x full rotations at the correct torque - and given the tolerances involved I'm not happy that 'missing' 2mm's of grip to the hub is safe

 

[andylong]

Are you fitting the spacers so you can fit new wheels or to fill out the arches more?

 

[TripleBob]

For the time being just to fill the arches better, although long term I'd like to pick up some 19" OEM wheels for the summer

 

[TripleBob]

So the store are sticking to their guns and saying being too short is fine, and that 5-6 turns is adequate on the bolt... But by my maths, that's maybe 9mm into a 14mm hub... That's only 65% of the OEM grip!

 

[RUM4MO]

I've been there done that when my older daughter bought her Leon Cupra then chose to buy a set of winter wheels/tyres, I came to the conclusion that alloy wheel suppliers try to stick to using/selling a couple or so lengths of bolts, obviously to make like easy for them, just like aftermarket providers of road springs and dampers, and even brake pads.
So, to satisfy myself, I removed one wheel pressed a single SEAT wheel bolt firmly into its fitting area, and measured the projection of the threaded bolt out the inner side of the wheel. Then grabbed a new winter alloy wheel and did the same for it with its supplied bolts and found that they were maybe over 6mm longer - I've forgotten the exact length, so bought in a new set of wheel bolts that were shorter, in fact their length was the same as the SEAT or VW Group supplied wheel bolts - 27mm rings a bell, although these aftermarket ones had taper seats instead of conical seats, that worked for me in as much as it meant that the projecting bolt length was the same for each wheel fitment.

I'd suggest, if it is not too late, that you do that, and if that means buying in some new bolts different lengths front and rear, then do that and identify the different lengths by painting the heads different colours - which will get covered by the normal bolt caps.

 

[TripleBob]

I've been there done that when my older daughter bought her Leon Cupra then chose to buy a set of winter wheels/tyres, I came to the conclusion that alloy wheel suppliers try to stick to using/selling a couple or so lengths of bolts, obviously to make like easy for them, just like aftermarket providers of road springs and dampers, and even brake pads.
So, to satisfy myself, I removed one wheel pressed a single SEAT wheel bolt firmly into its fitting area, and measured the projection of the threaded bolt out the inner side of the wheel. Then grabbed a new winter alloy wheel and did the same for it with its supplied bolts and found that they were maybe over 6mm longer - I've forgotten the exact length, so bought in a new set of wheel bolts that were shorter, in fact their length was the same as the SEAT or VW Group supplied wheel bolts - 27mm rings a bell, although these aftermarket ones had taper seats instead of conical seats, that worked for me in as much as it meant that the projecting bolt length was the same for each wheel fitment.

I'd suggest, if it is not too late, that you do that, and if that means buying in some new bolts different lengths front and rear, then do that and identify the different lengths by painting the heads different colours - which will get covered by the normal bolt caps.

Cheers, yeah I've not installed yet as I've been weighing up my options. I'm actually now debating getting a stud conversion kit instead of bolts -
It's a stronger system than longer bolts
Zero chance of anyone mixing up front and rear bolts of different lengths
Allows me to change spacer sizes without having to rebuy bolts
Less risk of wearing the hub threads out when I change wheels (I have summer and winter wheels)
Can swap between OEM and aftermarket wheels with different seating types

Cost wise there's basically nothing in it, only thing I can't find ATM is any R13 (radius/ball seat) locking nuts - only 60° taper, and I don't really want to use ball to taper adapters...

 

[SuperV8]

Cheers, yeah I've not installed yet as I've been weighing up my options. I'm actually now debating getting a stud conversion kit instead of bolts -
It's a stronger system than longer bolts
Zero chance of anyone mixing up front and rear bolts of different lengths
Allows me to change spacer sizes without having to rebuy bolts
Less risk of wearing the hub threads out when I change wheels (I have summer and winter wheels)
Can swap between OEM and aftermarket wheels with different seating types

Cost wise there's basically nothing in it, only thing I can't find ATM is any R13 (radius/ball seat) locking nuts - only 60° taper, and I don't really want to use ball to taper adapters...

One negative of studs is it's more tricky cleaning your hubs when you come to change your discs.