Rob101's explanations for e34 suspension components

[Brandon J]

I am posting this in his own thread as requested to share his insight to suspension for the e34:

"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 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)



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
Did you?"


The above is Rob101's explanation of suspension in the e34. He had it posted under my tire tech thread so I moved it to its own since it is talking about suspension.

[rob101]

thanks for that i actually meant for yang to post that in a new thread.
otherwise that first statement doesn't really make sense lol
I really confused as to why it times out... if i write more than a couple of paragraphs.
anyway have a go at me if you want pick apart my arguments these are my impressions from what i've learnt and my musings they aren't by any means complete and aren't a substitute for experimentation perhaps some sort of rationale at interpreting the things people have experienced.

[rob101]

And its not really talking only about suspension.....

[krzysiom5]

My current M5 a(nd my previous M5) came with stock 235/45 17s all around. The staggered set was an option later in the years. The 18in wheels on the latter M5s used the same width front and rear tires, 245. Wasn't the M5 designed for same size tires front and rear just like the same width tires of non M5 e34s? I think the M5 calipers and carriers are almost the same weight of the non M5 e34s, but the rotors are definitely heavier because they are larger. I know the aluminum arms in the M5 were also found on some 525s, 530, and 540 e34s that came here to the states. They all came with stock 15in wheel and tires, I was wondering why did they add them to those e34s too when the wheels and tires were the same size?

[rob101]

I didn't know about the alu arms on other e34s that came into the US. regardless its a good idea to have the lightest and suspension arms you can have really the only downside is cost. less unsprung mass is universally good. you don't NEED to have 17inch wheels to benefit from that

[Turbo Ready]

Should be made a sticky

[rob101]

perhaps we should make a knowledge repository thread where all the big write ups are kept. and thus stickied

[krzysiom5]

Alright. Yeah, I already knew that about not needing 17in wheels to benefit from aluminum lower control arms. Thats a given.

I am in the process of fine tuning my M5 suspension. Many M5 owners still face the front tire outside wear and understeer. BMW even made strut mounts for the M5 to give more camber. While the M5 suspension is good I noticed how Dinan's M5 suspension set-up actually rode better and handled better? Can you tell me your experiences with different e34 suspension set-ups that you have worked on. thanks

[rob101]

Alright. Yeah, I already knew that about not needing 17in wheels to benefit from aluminum lower control arms. Thats a given.

I am in the process of fine tuning my M5 suspension. Many M5 owners still face the front tire outside wear and understeer. BMW even made strut mounts for the M5 to give more camber. While the M5 suspension is good I noticed how Dinan's M5 suspension set-up actually rode better and handled better? Can you tell me your experiences with different e34 suspension set-ups that you have worked on. thanks

I'll be honest I haven't worked on e34s, most of the stuff i did was on open wheelers. but in the future i really want to map out the e34s suspension geometry. perhaps when i am on hols in Xmas. I can't really "see" whats going on exactly until i know exactly what the geometry is doing. you need to get under the car and measure with plumb bobs really before you can know whats going on

[rob101]

Alright. Yeah, I already knew that about not needing 17in wheels to benefit from aluminum lower control arms. Thats a given.

I am in the process of fine tuning my M5 suspension. Many M5 owners still face the front tire outside wear and understeer. BMW even made strut mounts for the M5 to give more camber. While the M5 suspension is good I noticed how Dinan's M5 suspension set-up actually rode better and handled better? Can you tell me your experiences with different e34 suspension set-ups that you have worked on. thanks

actually i'd be interested to know what kind of mods they did to the M5 strut to give it more (negative?) camber. that is something I don't really know about (hence didn't mention)