I know this has been posted before by Brandon but quite frankly that thread was a F-up

I've realised that nobody is going to read past the first page of the tyre tech thread that brandon J's started.
why bigger isn't necessarily better when it comes to Tyres and rims

wider tyre.

Cornering
Pro's
- Tyres react faster to steering inputs
- Tyres do have ultimately a higher capacity for "lateral force"
Cons
- tyres are heavier which means higher unsprung mass which means increased disturbance by bumps (harder to keep contact on bumpy surfaces)
- tyres are more sensitive to changes in camber
- tyres will be breakaway in a less progressive way at their limit
- tyres will steer themselves more (increased self-aligning torque)

Ride
Cons
- tyres are heavier which means higher unsprung mass which means increased disturbance by bumps (degrades ride on bumpy surfaces)

Lower Profile tyres and increased wheel diameter

Cornering

Pros
- Improved steering feel
- Improved steering response
- Lower rolling resistance
- Improved sidewall stability under braking
- Room for bigger brakes
- Less camber distortion when cornering. (not sure 100% how this effects)
- Greater steering accuracy
Cons
- tyres will breakaway in a less progressive way at their limit
- More susceptable to sidewall damage
- Heavier (harder to keep contact on bumpy surfaces)
- More expensive
- More susceptable to tram tracking
- More critical to tyre pressure

Ride

Cons
- tyres are heavier which means higher unsprung mass which means increased disturbance by bumps (degrades ride on bumpy surfaces)

In general bigger wheels
Further to that larger sizes of wheels will usually have an increased rolling radius.


Now my theories about the e34 in general and these tyres-

- the M5 was designed to utilise the 17 in wheels with 235s and 255s (which have roughly the same width of sidewall)
- "lesser" e34s uses narrower tyres with larger sidewalls

M5

m5 rolls less and the wheels are keep flatter on the road during corner (stiffer springs and sway bars)
- this was not done by accident, in fact they probably did this in order to use the wider tyres better.
- Ride has been comprimised slightly in this car for sportiness this is a result of the stiffer suspension setup.

m5 bushes have been beefed up
- increased "reaction forces" in lower profile and wider tyres. places added stress on suspension pivots.
- increases preciseness of setup and thus sportiness

m5 has alloy control arm in place of steel arm
- counteracts the increase of unsprung mass with the bigger wheels/tyres thus helps ride/tyre contact.

M5 shocks are stiffer
- since we have more unsprung mass we must stiffen the shocks in bounce in order to maintain wheel contact.
- this increase the force transmitted to the rest of the car reducing ride
- need to increase because of stiffer springs to help ride quality (otherwise will be underdamped)

M5 brakes are bigger (and lighter?)
- well you can fit more under there so why not?
- car is faster so needs more brakes
- (and lighter?) unsprung mass again

M5 has staggered setup rear tyres are larger
- i'd expect that is to help high speed stability which is reduced by putting more responsive tyres on
- however i agree it is also because this is where the torque is transmitted to the road is also another reason.

Now lesser e34s

cheaper is better in components we don't necessarily need as much handling but ride is still important also.

body rolls more and thus wheels experience more camber, but less stiffness helps ride somewhat.
- narrower tyres with larger sidewalls can handle more camber but are not are precise and do not give as high of a limit in handling.
- setup is more forgiving due to predicability at the limit of the tyres.
- ride is good also due to less unsprung weight in the wheels and less stiffness in the setup overall.

bushes are cheaper.
- don't need them to be as strong. tyre doesn't stress bushes so much due to less tramlining besides only m5 drivers goto the track

steel arms
- less unsprung mass due to smaller wheel and narrower tyre
- steel's cheaper

Shocks are softer
- reduced unsprung mass
- softer springs need softer shocks to maintain ride.

Brakes are smaller
- cheaper
- might not fit m5 brakes under 15"s



If you want to use wider tyres and thus smaller side walls:

they will give you more cornering
- KEEP in mind the setup with regards to reducing body roll and thus keeping the tyres flatter on the road during cornering.
- the wider the tyre the less it tolerates camber, and will fall out of its optimal area for cornering and braking/acceleration.

they will be less predictable at the limit,
- too bad thats just the way it is. see next point.
- might consider tuning a little understeer at the limit (anyone for 255 at the rear?)

they will be more precise and respond quicker.
- thats actually related to the previous point so thats your consolation.

Your ride will suffer (due to more unsprung weight) (doing anything in this area is good for grip also)
- look for a lighter rim if you can
- get lighter brakes
- get alloy m5 arms

your ride will suffer (stiffer springs and sways)
- make sure you have adequate shocks to counteract the springs (helps handling also)
- other than that like everything its a trade off.

increased tramlining/wandering/high speed instability
- would be trying increase in toe in. due to a increased rolling resistance on each tyre the dynamic toe in (that is the toe when the vehicle in motion) is less than with smaller tyres. but this will decrease turn in responsiveness a little
- get stiffer bushes should help reduce play in the steering.


"So What you're saying is if i get 300 wide tyres and stiffen up my suspension enough as well as do shocks Its worth it?"
- No because there comes a point where the increase grip due to the width of a tyre isn't enough to make up for the increase in unsprung mass (and its effects on both ride and grip)
- Perhaps for a road car: we want good ride as well, a kind of trade off. This is what the M5 has set out to do and has achieved.
- Unsprung mass' effect on grip (keeping wheel in contact with road) can be reduced by increasing spring rates and shock rates however this will make the ride even worse in the process of aiding the grip.

Thus we reach our "limit state" for the usefulness of bigger rims and tyres on the e34 road car.

it is determined by:
- by the roughness of the road you drive on regulary (on smoother roads larger wheel and tyre combinations because the disadvantage of higher unsprung mass is proportional to this.)
- unsprung mass of desired wheel/rim setup.
- unsprung mass of e34's suspension links springs shocks and struts. (alloy arms and lighter brakes help)
- Weight of the e34 in total (and thus sprung mass, more sprung mass = better ride)
- how harsh you can tolerate the ride.
- how responsive/twitchy/unstable at speed you like your car (less progressive behaviour at the limit, high speed instability/tramlining is a downside with more responsive tyres)
- how expensive the setup is.


M division has done a good job at balancing these. I think there are many lessons to be learnt from how they've modified the base e34 to achieve a good balanced car.
Some of these may not be immediately obvious and by no means do I understand the setup in its entirity.
I intend eventually to map out some of the suspension geometry of the e34 and gain some more insight.

So. in case you haven't noticed its not an easy subject. but you weren't seriously expecting to be smarter than companies that spend millions of $s in R&D?
There is still room for experimentation of course but at least theres some rationale to start people off.

I cannot recommend highly enough the book by Fred Puhn How to make your car handle

Now most of this stuff is untried but its sound in theory.
One thing that someone pointed out was the difference in geometry of the M5 if someone could enlighten me on the differences between normal e34s and m5's suspension geometry I'd be all ears.

This thread is to stimulate disscussion so people please feel free to share experiences of your wheel sizes and how they went. and your personal theories.