Help me choose my MB wheels

[Mcalvert]

Right - I have bored enough people with silly questions to help me select the right dimension of MB wheels. Shaun was very helpful earlier and based on this, I am getting nearer to understanding what I need. I just don't want to make an expensive mistake!

 

The objective is to fit 215/50R13 Kumhos front and back. Ideally, it would be nice to be able to swap around front and backs, but this is not a must. I have a set of RIF front wings on order.

 

I understand that the perfect rim for a is 8" but most people go 7.5" - Is there a practical reason for this?

 

We measured the backdepth of my current wheels at 4 1/2", and there is less than 1/2" netween the inner of my tyres currently and the wingstay, so the additional width compared to my current 6" rims must go on the outside.

 

So - what are the measurements that I need to ask for before I have a chat with Mr Barnby please? The website here offers a wealth of choices....

 

Thanks - Michael.

[Adrian Williams]

Michael

 

IMO the best size at the front is a 5" inner with a 2.5" outer to give you a 7.5" rim. This clears the wing stay as the 5" inner is used with a 1.5" outer on the 'standard' 6.5" rim and gives you an equal track front and rear if a 'standard' MB rear is used. The 215's sit nicely on these so I can't see the point in going to an 8" rim.

 

At the rear you could use the same size which is what I would do (I think David was using 7.5" rears this year) or go up to 8" or even 8.5", I think the size you choose could have more to do with future plans than anything else. Standard rears are 6" inners with 2.5" outers so if you stick with a 2.5" outer the track stays the same front to rear and all you'd have to change is the inners if you want to go wider in the future. Alternatively a 7.5" rear can become another front rim if you want two sets in the future. The other thing to bear in mind is clearance for the rear arb drop links with an over slung bar, 5" inners will be fine but 6" inners are likely to mean the tyre will rub against them when cornering so you might have to re-think the arb layout and move to under slung or a Freestyle jobbie.

 

I have two each of the following (all with Kumho's) sitting in the garage, why not bring the car over and try the different combinations on it:

5" inner with 1.5" outer and 185's

5" inner with 2.5" outer and 215's

6" inner with 2.5" outer and 215's

6" inner with 2.5" outer and 235's

 

 

 

 

 

 

 

Adrian

[david nelson]

Michael

 

I have been running 215 on 7.5" that 5" and 2.5". I plan to put these on the front for next year.

 

If your staying in class 3 you could get 4 of this size and move the tyres around to increase there life.

 

Are you sure your current ones are 4.5" inners. were these a special order just seam a non standard size?

 

On the rear what ARB do you have ? under or over slung? this might make a diffrence to the size for the rears

 

David

[Mcalvert]

Thanks Guys.

 

I do not have a rear ARB at all, although I did have the pick-up points added by Arch last year so that I could add one if I wanted to. Based on this, I take it that I could select an ARB that would fit with the wheels if I ever decided to go that route rather than worry now?

 

Adrian - really appreciate your offer that I would ordinarily take you up on like a shot. However, I tried to give the car a final run of the year yesterday but It's decided "season over" unfortunately. It's thereore now off the road and the engine/gearbox out stripdown has started...

 

I can't see myself replacing the Crossflow with anything else, so not too worried about the ability to go with wider rears. So, it would look like 5" rims and 2.5" outers should be the way to go. Does this wider track do much to the suspension geometry that needs to be thought about?

 

Regards,

 

Michael.

[Jason Plato]

I ran a 3.5" outer front with a 5" inner back in 2005 with the 8.5 ACB all round and then you reap the benifit of another 2" track at the front.

 

dont forget to increase the spring rates to gain the use of the extra grip

 

the front geo remains the same other wise and ackerman isnt too badly affected.

 

here is C7 TOP

Taffia rear gunner

 

Edited by - Dave Jackson on 26 Oct 2008 16:54:54

[Adrian Williams]

Geometry remains the same Michael as DJ has confirmed, go with the 5" + 2.5" all round.

 

Deal with the arb after the rims but with 5" inners you can use any of the three options.

 

A 5" inner gives around 4.5" backdepth as the hub has a half inch step between the hub flange and rim flange. The inner rim dimension is NOT the backdepth.

 

 

 

Adrian

[Englishmaninwales]

Michael

As discussed on Sunday, what are the ET and backdepth diamensions on your new 2.5/5" MBs?

 

thanks

 

Malcolm

[Mcalvert]

Malcolm - My measured back-depth on my existing 8 spokes which I provided was 113mm but I hope to receive the new MBs tomorrow and will take dimensions then.

 

Mike is very knowledgeable and worth chatting with too...

 

Michael.

[Englishmaninwales]