Wheel Stagger on a GT-40

[Tom]

Wheel diameter stagger, that is. Anyone out there running 17" rears and 15" fronts on their GT-40? Pro's or cons?

I've found a little more freedom in selecting tire combos this way when matching the same tire brand. However, I'm not sure what a 2" difference in wheel diameter would look like (i.e. Ford GT runs 1" between front/rear). Anyone have pictures?

Thanks,

Tom

 

[Ian Anderson]

Surely you want bigger rims on the front too.
They accommodate bigger brakes etc

That said I know what you mean I run different pattern Kumho front and rear
Ian

 

[Tom]

Ian, thanks for replying. I'm not totally sold on the 17" look on a GT-40 but as we all know, the choices in 15" street tires are dwindling fast so I may not have a choice a year from now. I am widening a set of 17x10" rears for now, and of course, have the option to go to 17" fronts. Just wondering what that stagger would look like.

Although I love the look of the big brakes, keeping the car as light as possible is more of a goal for me. If larger brakes are eventually needed, then it's not a big deal to make the necessary changes.

 

[Malcolm]

Ultima's run stagger like you describe. No idea of benefits to a 40 though.

 

[Dave Bilyk]

I am surmising here based on front to rear balance,
the 17" rears would be stiffer than 15", so they will change the front/rear balance toward oversteer.
How much, I would guess a small to medium effect, small if you are running relatively soft springs, more if you are running relatively stiff springs.
It would also depend on the tyres used, as the change could also involve different front/rear tyre choice and so different front/rear grip.
The actual effect on balance will depend on how your car is setup in the first place.
If setup/balance is neutral in the first place, you will tend to a little more oversteer, if you already have that, the change could increase oversteer, if your initial car balance tends to understeer, you might find an improvement.

Dave

 

[Keith]

I thought "stagger" was more to do with side to side rather than front to back?

 

[Dave Bilyk]

That's right Keith, that's my true understanding too, important for oval racing and used in reference to different suspension setup camber, tyre pressure from side to side as well. I think CobraJet has gently hijacked the term and turned it 90degrees

Dave

 

[Keith]

OK I understand, but stagger is introduced by increasing/decreasing the tyre pressures thus increasing/decreasing rolling radius and together with corner jacking or wedging makes an oval car appear "caddywumpus" when viewed from the rear if you get my meaning. Referring to different wheel sizes front to rear as 'stagger is really more than just a "gentle" hijack!

 

[Mike Pass]

Height difference front to rear is "rake".
Cheers
Mike

 

[Tom]

If I worded the title incorrectly, my apologies as I see the thread has drifted. What is your suggestion for a proper title based on my question? I'll get it changed or start a new thread.

I have seen the term staggered tire sizes, and staggered rim sizes in both diameter and width, in past discussions. I know of body rake but have not heard the term 'rake' when used to describe using wheels of different diameters.

 

[Dave Bilyk]

Cobrajet,
I suppose that what you are referring to is rim diameters, so could be 'Differing front/rear wheel rim diameters on a GT40'.

Dave

 

[Dalton Veitch]

That heading doesn't have the same ring to it. I was intrigued by the thought of wheel stagger, wondering if Tom meant the wheels on the RHS being 100mm ahead of the wheels on the LHS, or vice versa. Perhaps, finding that most roundabouts went anti-clockwise (not sure about the Iraqui desert), or most racing circuits he was on went clockwise, or whatever, he would stagger the wheels accordingly. But no, it wasn't this crazy.

 

[Darrell]

Hi Tom
I fitted 17'' fronts on my DRB and 18'' rears for a few reasons.One was to try to get the stance of the early cars as they had huge diameter rear rubber and the second was to up the gearing for good highway cruising. I dont think there is any real issue with handling, as with most modern built GT40s as the shocks and sway bars are all adjustable and with a little trial and error you can get the result you are happy with.
Link to my car
http://www.gt40s.com/forum/drb-gt40s/27217-drb-46-now-road.html

Darrell
DRB LS1 G50

 

[Keith]

Yes but are you keeping the rolling radius the same?

I believe Cobrajet's original question centred around different rim diameters, as opposed to different rolling radii front to back.

I am not sure if I would be happy with different rim diameters and certainly not different rolling radii front to back.

Might tend to have a bit of 'dragster' look and do nothing at all for the handling? :worried:

 

[Dave Bilyk]

Keith, but they are already different rolling radii. e.g my tyres rear 255/60/15 and front 215/60/15 gives about a 15mm diff in rolling radius.
Try this Visual Tyre Size Calculator | Kouki Tech

Dave

 

[Mike Pass]

Different rolling radii front and rear is not a problem unless you have four wheel drive with a locked centre diff which will wind up. Old F1 cars and F5000 had tyres which were very different in rolling radius as well as width.

The main problem with different tyre radii front and rear is that to get the rake correct it may mean that the suspension height may not be optimal. In changing the height the wishbones will move. The angle of the wishbones at rest should be in the correct position to give the ideal rate of camber change in bump and droop. The most common rake for a GT40 used on the road is with 4.00" front and 4.5" rear ride height. As a very rough guide when set at this level the lower wishbone should be level/angled slightly down and the top wishbone angled up at a greater angle than the lower. If the suspension is being run "out of range" then it will not act as intended. The idea is that the camber increases in bump and decreases in droop to keep the tyre flat to the road as the car rolls in the corner. If the tyre diameters are extreme then the suspension rest position may be too far from the ideal rest position. Most GT40 suspension setups are designed around original style tyre diameters and if using something very different and the normal rake then check what your suspension is doing in moving from that rest level. Weird handling due to the roll centre moving oddly because of incorrect suspension height setting is unpleasant and hard to cure. The front and rear rollcentres and relative positions have a big bearing on whether a car handles nicely or is a pig.
Just something to bear in mind when working things out and doing the setting up.

Cheers
Mike

 

[Keith]

Yes, of course, it's quite obvious really... :blank:

 

[Tom]

Yes but are you keeping the rolling radius the same?

I believe Cobrajet's original question centred around different rim diameters, as opposed to different rolling radii front to back.

I am not sure if I would be happy with different rim diameters and certainly not different rolling radii front to back.

Might tend to have a bit of 'dragster' look and do nothing at all for the handling? :worried:

Keith,
I was referring to using different wheel diameters, but keeping the overall tire diameter the same as it would be if I were to use 15" wheels all around.

For example, a Hoosier R6 245/50/15 radial front tire would be approximately 24.8" in diameter which is about as tall as I can go with an RCR (<25.5"). For the rear, a 17" wheel with say, a 315/35/17 or 335/35/17 tire, would come out to ~25.6". That's roughly a 3/4 inch difference in overall diameter between front and rear tires, and retains the ride height difference front to back of just under 1/2 inch. I believe this option would allow for a few more performance tire choices. Just not sure what a 2" difference in wheel diameter would look like vs a more accepted practice of using 1" difference in front to rear wheel diameter like what the Ford GT uses, even perhaps the Z06.

I'm willing to bet your 'rolling radii' is different front to back.

 

[Mike Pass]

Low profile tyres tend to lose the original look which of course had tyres with deep sidewalls. They don't look "right" IMHO but as you say you can get a much larger range of tyres especially in 17" dia wheel size. On the original 15" wheel size the Avon cr6zz gives fairly good grip and has a theoretical friction coefficient of 1.33 which is pretty good and up there with modern low profile tyres. The european classic GT40 racers use them in the wet . One of our replicas over here showed 1.1+g on the data logger at the Donington track day.
They are quite soft and will wear quite quickly but mine have done 3 years duty and have a great ride on a long journey. With my mouth full of fillings and quite hard springs and shocks this is good for me.
Whether the modern low profiles will outperform them I don't know.
The modern low profile tyre will react faster and have a lot less "squidge" on turn in etc. but a much harder ride. The larger wheels are also popular for performance as they will allow bigger brakes and have maybe a bit less unsprung weight.
The looks are pretty much in the eye of the beholder. The original look folks go for the 15" high profile tyres and the track folks go for low profile handling, braking and grip. My view is that I prefer the look of the high profile tyres on 15" wheels and do the best I can with the brakes etc. The original race cars still running today are not exactly slow on them either!
Maybe our old friend photo shop can help you decide by doing some cutting and pasting of different profiles onto a pic of your car.
Cheers
Mike

 

[Tom]

Mike,
Yes, I believe the Avons are the best way to go for a 15" wheel on a GT-40. As it turns out I will end up with complete sets of 15 and 17 inch BRM wheels (from Vintage) before the car is completed, so I can play with the combination and set the car up accordingly. I am having the 17" rears widened at the moment and perhaps the 15" rears later.

Like you, I prefer the 15" look and will most likely go in that direction for the majority of the time. For occasional track use, I can switch over to the 17's but will have to setup the car's suspension accordingly which will require track time.

Thanks all!

Tom