lighter rims

[Petrus]

an interesting scientific study:

https://iopscience.iop.org/article/10.1088/1757-899X/355/1/012023/pdf

The most important observations are:
- lighter wheels greatly improve roadholding
- in combination with abs the responsiveness under critical braking is markedly better
- the limits of control under crital conditions are higher
The latter too logically follow from the first.

As the Mr2 is a light car the above effects are relatively large as the ratio sprung vs unspung is improved relatively more.

[Petrus]

Part two.

Why will I stay 15"?

First look at the construction and weight distribution of tire and rim:

The tire has a rigid tread surface, a strong bead and a flexible sidewall connecting the two. The thread is the heaviest, then the bead and the sidewall is very light.
That means that a larger rim tire gets héavier because the bead circumference grows.

The rim is simpler still. The heaviest part is the outer rim. With a larger diameter the circumferance grows exponentially thus the weight of it too, making the wheel disproportionally heavier.

Thus a larger rim will make for a heavier wheel.

Going for a lighter material rim can compensate that but why?

So let´s look at the properties of a larger diameter rim.
The only funtional differences are a lower sidewall and less air chamber.

The lower sidewall will make the connection between rim and tread shorter thus directer/stiffer, making the steering directer.

Less air chamber makes the air suspension stiffer; the tire following the surface worse and transmitting more to the car.

All in all I can even for track application not see any advantage in larger diameter rims unless this is needed for a specific tyre giving a notable advantage.


Back to the air chamber, the air suspension, for a moment. Ever seen the Formula One slow motions with the ultra high speed camera of the tires flexing? In F1 this air chamber allowas the travel of the car´s suspension to be short and hard. With less of it, the car´s suspension needs really be redesigned, at léast be .... sóftened up.
Larger diameter rims and lowering = stiffer springs will only work on the smoothests of surfaces. On real world road surfaces it will seriously reduce road contact thus traction/grip.

[Petrus]

Part three:

A good article starting with the summit of wheels; formula one:

http://www.formula1-dictionary.net/wheels.html

Please note that F1 still runs on 13" rims.
Pirelli has for yéars been pushing for way larger rims, 18" even but sofar constructors have resisted. For 2021 they may need to rethink suspension though.
Why is Pirelli pushing this? Well, marketing obviously.

[Petrus]

and now the for the time being last one:

A confusing aspect is braking distance and mass. High school physics teaches us that as the friction of the tyre is linear with the vertical force pressing it on the surface ánd equally linear in kinetic energy, mass can be stricken from the equasion. Thus mass theoretically has no influance on braking distance with all other variables the same; ceterus paribus.
Now, apart from heavier cars crashing harder into things when still on the move, we all knów that they do need more road to stop.

The answer is tyres.
Tyres do NOT have a linear increase in friction as the vertical load increases but are load sensitive:

https://en.wikipedia.org/wiki/Tire_load_sensitivity



Simply put; the more the vehicle weighs, the lower the max fiction is )between the max of 1 and min. of 0..
This staggering little know phenomena tells us that a heavier vehicle doés have a longer brake distance because modern cars all have brake systems well capable of exceeding the grip available.

In the real world there is yet anóther factor upsetting the theoreatical model; road surfaces are not smooth; bumps will deflect the wheel vertically, increasing/lowering contact load.
Nów we get back to lighter wheels. Lighter wheels have less deflection = better road contact = more optimal friction = shorter braking distance.

Same goes for acceleration and cornering limits for a given mass/output but the braking distance is simplest to explain.

The tyre is a variable ofcourse.
Tyre weight is at least as important because the weight is further out to the rim. This is however a very tricky thing to use as a decision criterium because the friction coefficient is more important still.
Thus it is best to go for the stickest rubber for the conditions* and only opt for the lightest if there is a choice between comparable ones.

* operating temperature, wet grip etc.

[tom256]

an interesting scientific study:

https://iopscience.iop.org/article/10.1088/1757-899X/355/1/012023/pdf

The most important observations are:
- lighter wheels greatly improve roadholding
- in combination with abs the responsiveness under critical braking is markedly better
- the limits of control under crital conditions are higher
The latter too logically follow from the first.

As the Mr2 is a light car the above effects are relatively large as the ratio sprung vs unspung is improved relatively more.

Here is great guide for handling (series has 10 parts):
https://motoiq.com/The-Ultimate-guide-to-Suspension-and-Handling-Part-1-Wheels-and-Tires/


Optimal rims size for MR2 Roadster (according to interview with Techno Spirit Pro):
-road use 15" front 16" rear
-track use 16" front, 17" rear

[Petrus]

Optimal rims size for MR2 Roadster (according to interview with Techno Spirit Pro):
-road use 15" front 16" rear
-track use 16" front, 17" rear

The difference between track and street is easily understood.
Because of the smooth surface of the track the advantage of a bit lighter rim is less than the advantages of a lower sidewall and stickier race tires.

The stock 16" rear needs a bit of explanation.
This is principally for the FL which is set up for that.
Also it obliges a wider 215 tread as the overall diameter cannot grow too much and a higher sidewall would increase creep/oversteer.
Imo Toyota only went for 16" at the rear to counter the optical effect of the rear 15" appearing smaller in the wheel arch. -and to soften the oversteer with a wider tread.
Weight wize it in not advantageous as the 16" with wider tyre is a kilo heavier that the 15" one.
I would not fit a 16" rear to the pre FL unless you want it because it looks better which imo it does.

Speaking of higher/lower section rear rubber and the overall diameter we are touching on aerodynamics btw. Marginal gains but it is there.
A slíghtly lower front than rear will create a minimum of ´ground effect´ downforce. The stock diaper is not half bad assisting the air under the car to flow upwards at the rear, creating a lower pressure area.
Although I think the diaper looks .... not nice, it does have a function.