Gt6 Mk3 15" Wheels

[DaveKent]

yes they'll physically fit your car, but...... read the rest of the thread! :)

[DaveKent]

not only the wet but also the dry!   remember that the design of the rear suspension changes the angle of the tyre contact area in relation to the road over the course of its travel.  in basic terms a higher profile tyre will flex enough to mantain a similar contact pattern over the whole suspension movement, whereas a lower profile tyre is square and more rigid, therefore as the suspension angle changes you only end up with the edge of the tyre in contact with the road.      consequently although you think you've got fatter tyres and more grip, in reality you could have less tyre surface in contact with the road.     as most people say, 185 is good, using max 14" rims . most of us run 185/60 x13  or 175/70x13 for good all round driveabilty.

[Tim Bancroft]

Nicely explained.

[starman]

xyv wrote:

not only the wet but also the dry!   remember that the design of the rear suspension changes the angle of the tyre contact area in relation to the road over the course of its travel.  in basic terms a higher profile tyre will flex enough to mantain a similar contact pattern over the whole suspension movement, whereas a lower profile tyre is square and more rigid, therefore as the suspension angle changes you only end up with the edge of the tyre in contact with the road.      consequently although you think you've got fatter tyres and more grip, in reality you could have less tyre surface in contact with the road.     as most people say, 185 is good, using max 14" rims . most of us run 185/60 x13  or 175/70x13 for good all round driveabilty.

Bingo Dave--you nailed it.  

That's why I'm not a fan of the 15 and 16 inch wheels and really low profile tires on the Spitfire, especially with a swing axle setup and the inherent large amount of camber change with suspension movement.  A 50 or 45 aspect ratio tire (or lower), especially one with stiff sidewalls, will not flex enough to maintain a good contact patch with the road at the large camber angles that occur with the swing axle rear (and don't forget, contact patch is not simply a function of tire pressures because the tire itself has non-zero stiffness and compliance; real tires are not idealized thin-walled balloons).  With a rotoflex or similar non-swing axle/fully independent rear suspension, especially one with long virtual swing arms, then things like 15 inch wheels and lower profile tires might be OK.  

Another thing to consider here is rotational inertia.  Smaller wheels and tires have their mass concentrated on a small radius, thereby reducing rotational inertia and thus making the wheels easier to accelerate and deccelerate.  Rotational inertia (moment of inertia) goes with the square of radius, so having more mass concentrated farther from the hub hurts performance in a hurry.  Moreover, most readily available larger diameter wheels and tires will be heavier than their smaller counterparts.  And one note on wider tires--yes they can generate more grip than narrower ones, all else being equal, due to the fact that more grip is achieved at any given slip angle of the rubber (although ultimately they cannot generate as large a slip angle as a taller/narrower tire but that's beside the point that they can achieve higher levels of total adhesion) provided they have the right relationship to the road afforded by the suspension.  Think about this--if bigger and bigger wheels with lower and lower aspect tires were the end-all be-all of performance, don't you think you'd see such things on every racing swing-axle Spitfire, or every race car for that matter?

The entire suspension--wheels, tires, springs, linkage geometry, dampers, etc.--are a system wherein all the component pieces need to be considered in their relation to the other pieces such that the system work optimally as a whole.  But then again, if you are merely going for looks and don't care as much about handling or safety in an emergency manuever, that's another matter.

One last thing about wheels.  Remember to consider offset.  It's really important--not just for avoiding interference but also for distributing bearing loads and affecting steering behavior.

[jango]

pics of 14" wheel on the move

[Jimmmy0_0]

Ok so if you could have any wheel tyre option, what would it be?

James

[neilnaz]

13" 6J Revolution alloys with decent 175/70/13 tyres which is exactly what I have got. I could be persuaded that 14" also look good with 185/65/14 or similar but as people have discussed, any more is ruining ride and handling.

Cheers, Neil

[GT6 Mike]

892 wrote:

Even though the width of the tyre has increased, the patch area is still 100% exactly the same

How does that work ?

Ignoring the camber changes as Dave has already explained the issues.

Wider tyres give a larger footprint which spread the weight of the vehicle over a larger contact area, overall ground pressure is reduced, this affects the tyres ability to move water.  

[neilnaz]

Mike sorry to disagree but theory goes Pressure = Force/Area so if the weight of the car remains the same (which it will do more or less when changing between tyre and wheel set-ups) and the tyre pressure is kept the same then the area will remain the same. This is provided the tyres have similar flexibility (etc)

If you double the width of the tyre then with the same pressure then the tyre will squish less keeping the same contact patch.

Regards, Neil

[GT6 Mike]

Neil, no problem, I agree with the theory but when changing tyre width, wheel diameter and tyre aspect ratio, tyre pressures will need to be adjusted (reduced in my case).

[Deleted User]

GT6_Mike wrote:

Wider tyres give a larger footprint which spread the weight of the vehicle over a larger contact area, overall ground pressure is reduced, this affects the tyres ability to move water.  

No no no !

Wet grip, and the point of aquaplaning is all about how quickly the tyre can expell the water.

If the tyre pressure is the same, the contact patch area is the same, so the contact patch MUST be wider, therefore the water has further to travel to be expelled.  

When the wheel is rotating so fast that the water being displaced from the centreline doesnt have time to escape either into the tread pattern or out the SIDE of the contact patch, the tyre will aquaplane.  With very few exceptions, this occurs at a lower rotational speed as the tyre width increases.

The tread pattern has an influence, and also provides somewhere for the water to go, but when there is so much water that the tread in any given area is full, then the tyre is on the point of aquaplaning.

[starman]

Jimmmy0_0 wrote:

Ok so if you could have any wheel tyre option, what would it be?

James

Hi James-

My opinion for swing-axle Spitfires and GT6s with no flaring of fenders or major suspension changes besides spring rates, ride height, dampers and bushings: 13x6 alloy wheels w/ about +25mm offset and 185/60-13 tires, and replace the stock 3/8"-24 wheel studs w/ M12x1.5 studs from a Land Rover Freelander.  Hard to go wrong with this combo under any circumstances.  The speedo will read about 5% faster than your actual speed, but so what.  175/70-13 on these wheels are good too, especially if you have not modified the suspension from the factory configuration (and your speedo will read accurately).  A low-budget option is to use 13x5.5 wheels from a TR7 (which are close to +25mm offset) and modify them for lugcentric mounting on the Spit.

Alternatively, on a properly prepared swing axle Spit or on a Spit or GT6 with a fully independent rear suspension (a la rotoflex or rotoflex type with CV jointed halfshafts or Jango's new rear end), then 14x6 alloys w/ about +25mm offset and 185/55-14 tires can work.  

[Deleted User]

666 wrote:

pics of 14" wheel on the move

Rear discs? do tell! :)

[Deleted User]

380 wrote:

Rear discs? do tell! :)

Read his thread on his rear end conversion, really nice stuff, I am hoping he does interest free credit so I can make an order ;)

[Deleted User]

892 wrote:

Iain. :P

Is that your car in your Avitar?  

[drofgum]

Iain,
Yes cost was a major factor in the choice of tires and it still is, but another issue was the fact that 70% aspect ratio tires were only introduced to the market in the later years of  Spitfire and GT6 production. So it really shouldn't be a surprise that Triumph didn't use them. The change to radial tires probably brought about a larger improvement in handling than the change to 70 series tires would have made. I certainly wouldn't want to try my car on bias ply tires.                                      
                                                                          Cheers,
                                                                          Paul

[cook1e]

When you buy a qulity modern car you often get several options on wheels diameter and width. My wifes Mini had wheel /Tyre Options ranging from 15" 185 width up to 17 inch 225 with various different alloy styles. My Jag had Wheel and Tyre Options from 16 up to 18 inch. Therefore the suspension geometry must be designed to cope with different widths and diameters. I guess in our Triumphs time manufacturers just stuck to a standard size though....

[Deleted User]

892 wrote:

Yes.
Iain

have you a blog? tell us more about it, it looks nice.

[tiggrr1]

Sorry for the thread drift


Rob checkout page 31 of thread "Who's who on the forum" reply 463 (in the chit chat section)

Nice Car Indeedy :)

[AllanE]

The issue of width and tyre patch seems a little misunderstood:

Generally (ignoring tyre pressure, tread design and compound), increasing the width of the tyre will INCREASE the tyre contact patch.
The pressure (not force/load) between the tyre and the road will decrease, as the load (weight of vehicle) is spread over a larger area.
Generally the friction coefficient will increase with an increase in contact patch.

Wet grip is something else; tread design, water depth, speed of vehicle all come into play - conversely a merely damp road can actually have more grip than a dry one.

As 'optional' wheels increase in diameter, the aspect ratio will decrease to retain the overall diameter, so a wider tyre is usually required to get an acceptable load rating - the load rating is roughly proportional to the volume of air contained by the tyre.

[josok]

Simply, if the tyre pressure remains the same, the contact area tween tyre and road will be the same, whatever width the tyre has.  The only thing that changes is the pattern from a longer oval to a rounder oval. It's simple a matter of pounds per square inch. Wear will be the same, because the weight per square inch is the same. Friction will be the same. Rolling resistance will be less, because there is less rubber to deform in the longitudinal direction. And because the oval becomes wider, it becomes more difficult to get rid of water. But wider tyres look a lot better!

[AllanE]

Josok,
Why don't F1 cars use 155R13 tyres, then?

[Anthony]

I wrote all about this on another forum

I'll try and dig it out and post it on here

[Anthony]

Here ya go......

http://www.club.triumph.org.uk/cgi-bin/forum10/Blah.pl?m-1313249082/

[Hugh]

[quote=KevinR]
No no no !

Wet grip, and the point of aquaplaning is all about how quickly the tyre can expell the water.

If the tyre pressure is the same, the contact patch area is the same, so the contact patch MUST be wider, therefore the water has further to travel to be expelled.  

When the wheel is rotating so fast that the water being displaced from the centreline doesnt have time to escape either into the tread pattern or out the SIDE of the contact patch, the tyre will aquaplane.  With very few exceptions, this occurs at a lower rotational speed as the tyre width increases.

The tread pattern has an influence, and also provides somewhere for the water to go, but when there is so much water that the tread in any given area is full, then the tyre is on the point of aquaplaning.

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