[rallybull]

I had been doing some research on trying to find an answer to the question:

Does a light weight wheel increase the amount of bumps that the passengers feel or does it actually decrease the bumps?

There seems to be some amount of misinformation on the internet on this subject and perhaps I'm guilty too of adding to the misinformation in a separate thread on this forum on the M-performance suspension.

I consider tirerack a credible source for information and they feel that heavier wheels improve the comfort. Here is a link to a tirerack article: http://www.tirerack.com/wheels/tech/...jsp?techid=108

Here is a quote from that article:

On the road, this combination was only praised for its ride quality. Our experience showed that the heavyweight wheel's reluctance to be moved as the vehicle rode over expansion joints, patches and potholes actually damped the impact harshness transmitted to the suspension, and forced the tires to absorb more of the jolt.

On the other hand I found at least a couple of articles that have stated that by increasing the sprung to unsprung weight ratio (like by reducing wheel weight?) the bumps actually reduce.

Here is the first article which seems pretty well written:

http://www.rqriley.com/suspensn.htm

Here is a snippet from that

Early pioneers believed that the primary job of the suspension system was to absorb bumps and smooth out the ride. Today we understand that an equally important function of the suspension is to keep the tires in contact with the road. This is not as easy as it might appear to be. When a tire encounters an irregularity the resulting forces tend to reduce contact pressure and therefore degrade adhesion. Obstacles impart a vertical acceleration to tires that increases in proportion to the forward speed of the vehicle and the size of the obstacle. The greater the accelerated mass (unsprung weight) the greater the kinetic energy. In a sense, a raised obstacle throws tires away from the roadway. A depression causes the surface to rapidly drop away leaving the tire to follow along when inertia can be overcome by the downward pressure of the springs. Both occurrences reduce the tire's contact-pressure and tires can actually become airborne if the forces are great enough.
The forces generated by roadway irregularities (bumps) must be overcome by the springs in order to keep tires in contact with the road. The force of the springs comes from the compressive load imposed by the weight of the vehicle. The lighter the vehicle, the less compressive force is available, and the easier it is for the vertical motion of the wheels to overcome the inertia of the sprung mass and transfer motion to it as well. The ideal combination occurs when the ground pressure is maximized and inertial forces are minimized by a high sprung-to-unsprung weight ratio. A high ratio keeps the tires more firmly in contact with the road, and it also produces the best ride.

Also wikipedia article on unsprung mass actually makes it a bit clearer:

http://en.wikipedia.org/wiki/Unsprung_mass

It appears that lower unsprung mass (like lighter wheels) results in a less bumpy ride over poor roads. However, the downside is that it could increase the "fine vibrations" that you feel over some "relatively smooth surfaces" such as concrete roads or an asphalt road with gravels.

Based on the above, I would like to conclude that tirerack was actually wrong in making that statement. Light weight wheels do make for a less bumpy ride.

Anyone care to agree/disagree?

[dinonz]

Originally Posted by rallybull

I had been doing some research on trying to find an answer to the question:



There seems to be some amount of misinformation on the internet on this subject and perhaps I'm guilty too of adding to the misinformation in a separate thread on this forum on the M-performance suspension.

I consider tirerack a credible source for information and they feel that heavier wheels improve the comfort. Here is a link to a tirerack article: http://www.tirerack.com/wheels/tech/...jsp?techid=108

Here is a quote from that article:



On the other hand I found at least a couple of articles that have stated that by increasing the sprung to unsprung weight ratio (like by reducing wheel weight?) the bumps actually reduce.

Here is the first article which seems pretty well written:

http://www.rqriley.com/suspensn.htm

Here is a snippet from that


Also wikipedia article on unsprung mass actually makes it a bit clearer:

http://en.wikipedia.org/wiki/Unsprung_mass

It appears that lower unsprung mass (like lighter wheels) results in a less bumpy ride over poor roads. However, the downside is that it could increase the "fine vibrations" that you feel over some "relatively smooth surfaces" such as concrete roads or an asphalt road with gravels.

Based on the above, I would like to conclude that tirerack was actually wrong in making that statement. Light weight wheels do make for a less bumpy ride.

Anyone care to agree/disagree?

Agree.

[ronin951]

Originally Posted by rallybull

I had been doing some research on trying to find an answer to the question:



There seems to be some amount of misinformation on the internet on this subject and perhaps I'm guilty too of adding to the misinformation in a separate thread on this forum on the M-performance suspension.

I consider tirerack a credible source for information and they feel that heavier wheels improve the comfort. Here is a link to a tirerack article: http://www.tirerack.com/wheels/tech/...jsp?techid=108

Here is a quote from that article:



On the other hand I found at least a couple of articles that have stated that by increasing the sprung to unsprung weight ratio (like by reducing wheel weight?) the bumps actually reduce.

Here is the first article which seems pretty well written:

http://www.rqriley.com/suspensn.htm

Here is a snippet from that


Also wikipedia article on unsprung mass actually makes it a bit clearer:

http://en.wikipedia.org/wiki/Unsprung_mass

It appears that lower unsprung mass (like lighter wheels) results in a less bumpy ride over poor roads. However, the downside is that it could increase the "fine vibrations" that you feel over some "relatively smooth surfaces" such as concrete roads or an asphalt road with gravels.

Based on the above, I would like to conclude that tirerack was actually wrong in making that statement. Light weight wheels do make for a less bumpy ride.

Anyone care to agree/disagree?

I agree, just never knew why. Great info. Thanks for posting!

[drob23]

I'll throw you a curveball and just go super theory route - The suspension system can be approximately modeled with mass damper systems, with the main purpose being to attenuate any "disturbance" from the road. Be it jittery road noise or big road dips. A perfect suspension system would make the car feel like it's floating on air.

The suspension can be approximated as 2 spring/mass systems in series - the pneumatic tire makes one spring, and the suspension the second spring. My background is in EE, where some interesting math theory can be used to say any force input over time (we'll call the road noise a "disturbance" force input from now on), can be approximated as the superposition of sinusoidal signals (see Fourier Transform). There are nice tools that then show how the suspension reacts to these "disturbance" signals as a function of *frequency*, usually described as a gain (amplification or attenuation). So if you were to ride over a periodic dipping road, then the amount of up/down movement you would feel would depend on the *frequency* of the dip (assuming the magnitude of the bump is fixed). Point is, the suspension will work really well for some frequencies, but maybe not so well for others. What I'm talking about is also a strong approximation, you can read full on research papers going into suspension design here.

In practice, there is a lot of tuning that is done to set everything from springs, dampers, suspension geometry, and the most disliked item by chassis engineering ---- big wheels.

To specifically answer your original question, I'd argue that both assertions were correct. The lighter wheels (less un-sprung mass) will allow the tires to easily travel up/down when hitting bigger dips, since now they have less inertia restricting suspension travel. On the other hand, lighter wheels will have a more negative affect with little high frequency jittery type road noise, since suspension travel will be easier.

When thinking of stuff like this, I just take things to the limit, assume that the wheels are 5000 lbs...you'd be great until you hit a pot hole, likewise with feathers on the wheel you'd feel every bump. So my opinion is you'll have less floatyness but jittery rock like bumps felt. But I will concede that these types of questions can require answers that are much more complex than one could dream. It might be that everything is fine until you hit a bump at 50 MPH for example. I think the most direct route to road comfort is a large tire sidewall, and non-stiff springs. Just my .02

Edit - reread what you wrote (which was long but so was mine) and I think I just repeated your second conclusion. After re-reading the tirerack statement (which I'm guessing they actually tested), I think the situation is if driving at high speed and you have a single pot hole, the larger wheel will resist the suspension travel and result in the other side of the hole hitting the tire closer to the contact patch (not high up). I think this points out that it's hard to generalize much of anything on a car to "this makes the car ride better in all situations."

Interestingly enough (nerd alert), the single pot hole actually shares more in common with high frequency road noise then with a repeated low frequency series of dips. Fourier transform of a square pulse will have a lot of high frequency spectral content, which in this case would be amplified by the lighter wheels.

[grimlock]

Without spending too much time on it, I disagree with the positive effects of heavier wheel mass; from wiki:

As mentioned above, there is a positive effect of unsprung mass. High frequency road irregularities, such as the gravel in an asphalt or concrete road surface, are isolated from the body more completely because the tires and springs act as separate filter stages, with the unsprung weight tending to uncouple them. Likewise, sound and vibration isolation is improved (at the expense of handling), in production automobiles, by the use of rubber bushings between the frame and suspension, by any flexibility in the frame or body work, and by the flexibility of the seats.

Unless the road irregularites are loose objects that can be displaced by the tire, when the mass would be relevent, otherwise all forces - 'road irregularities' - are immovable (ie. potholes, speed bumps are FIXED and effectively immovable vs. the car).
Thus wheel mass has no effect on road comfort wrt road irregularities. (excluding effects on handling etc.)

[grimlock]

This is for the first impact only, a heavier wheel that is off the road and exerts a force on the body of the car naturally with move the car more (harder to damp).
But if the wheel stays in contact with the road effectively its mass should not matter, the dampening done is on the body of the car wrt to the fixed wheels/road.

[rallybull]

Originally Posted by drob23

I'll throw you a curveball and just go super theory route - The suspension system can be approximately modeled with mass damper systems, with the main purpose being to attenuate any "disturbance" from the road. Be it jittery road noise or big road dips. A perfect suspension system would make the car feel like it's floating on air.

The suspension can be approximated as 2 spring/mass systems in series - the pneumatic tire makes one spring, and the suspension the second spring. My background is in EE, where some interesting math theory can be used to say any force input over time (we'll call the road noise a "disturbance" force input from now on), can be approximated as the superposition of sinusoidal signals (see Fourier Transform). There are nice tools that then show how the suspension reacts to these "disturbance" signals as a function of *frequency*, usually described as a gain (amplification or attenuation). So if you were to ride over a periodic dipping road, then the amount of up/down movement you would feel would depend on the *frequency* of the dip (assuming the magnitude of the bump is fixed). Point is, the suspension will work really well for some frequencies, but maybe not so well for others. What I'm talking about is also a strong approximation, you can read full on research papers going into suspension design here.

In practice, there is a lot of tuning that is done to set everything from springs, dampers, suspension geometry, and the most disliked item by chassis engineering ---- big wheels.

To specifically answer your original question, I'd argue that both assertions were correct. The lighter wheels (less un-sprung mass) will allow the tires to easily travel up/down when hitting bigger dips, since now they have less inertia restricting suspension travel. On the other hand, lighter wheels will have a more negative affect with little high frequency jittery type road noise, since suspension travel will be easier.

When thinking of stuff like this, I just take things to the limit, assume that the wheels are 5000 lbs...you'd be great until you hit a pot hole, likewise with feathers on the wheel you'd feel every bump. So my opinion is you'll have less floatyness but jittery rock like bumps felt. But I will concede that these types of questions can require answers that are much more complex than one could dream. It might be that everything is fine until you hit a bump at 50 MPH for example. I think the most direct route to road comfort is a large tire sidewall, and non-stiff springs. Just my .02

Edit - reread what you wrote (which was long but so was mine) and I think I just repeated your second conclusion. After re-reading the tirerack statement (which I'm guessing they actually tested), I think the situation is if driving at high speed and you have a single pot hole, the larger wheel will resist the suspension travel and result in the other side of the hole hitting the tire closer to the contact patch (not high up). I think this points out that it's hard to generalize much of anything on a car to "this makes the car ride better in all situations."

Interestingly enough (nerd alert), the single pot hole actually shares more in common with high frequency road noise then with a repeated low frequency series of dips. Fourier transform of a square pulse will have a lot of high frequency spectral content, which in this case would be amplified by the lighter wheels.

I'm still trying to fully understand what you wrote It's been a long time since I learnt FT (and all the other transforms you learn in signal processing) and its all out of my memory now

Originally Posted by grimlock

This is for the first impact only, a heavier wheel that is off the road and exerts a force on the body of the car naturally with move the car more (harder to damp).
But if the wheel stays in contact with the road effectively its mass should not matter, the dampening done is on the body of the car wrt to the fixed wheels/road.

What you say makes sense if you assume that the wheels directly touch the ground. In that case the wheels and the ground form one effective body of infinite weight. However, bring tires into the picture and the equation is something like this:

EARTH --> Spring(tire) --> Wheel weight --> Spring (suspension) --> Car weight

It is going to be an interesting theoretical exercise to figure out how (and if) changes in the wheel weight will effect motion in the car.

At least once person in this thread(ronin) has confirmed from real life experience that bumps decreased after moving to a lighter wheel (but then again that could have been due to the non-RFTs too).

[grimlock]

Originally Posted by rallybull

What you say makes sense if you assume that the wheels directly touch the ground. In that case the wheels and the ground form one effective body of infinite weight. However, bring tires into the picture and the equation is something like this:

EARTH --> Spring(tire) --> Wheel weight --> Spring (suspension) --> Car weight

It is going to be an interesting theoretical exercise to figure out how (and if) changes in the wheel weight will effect motion in the car.

At least once person in this thread(ronin) has confirmed from real life experience that bumps decreased after moving to a lighter wheel (but then again that could have been due to the non-RFTs too).

Good pt., if you think of the wheel ex. tire as a intermediate pendulum-like object, then any mass above zero is negative as it needs to be damped against the car body

[rjm98119]

Can I tell you what my personal experience with this topic was?

I had a 2013 F10 535 from Nov '12 through June of this year (I downsized to a '13 335 in June... wanted something more sporty, and missed my 2008 335).

The stock 19" M-Sport rims weighed about 32lbs and the heavy RFT tires themselves weighed on average about 26lbs, so averaging 59-60lbs per wheel (I actually had 1 wheel/tire combo max out at 62lbs.!)

The F10 felt lumbering and sluggish (IMO) and I'd read many similar articles, so I bought some 19" spun forged rims weighing a mere ~20lbs each and new tires (Conti DWS non-rft) running around 23-24lbs. I reduced my unsprung weight by an avg of 15lbs per corner.

There were a NUMBER of factors contributing to the difference (not the least of which was ditching the RFT's), but I can say it was immediate and obviously apparent on all roads in the following ways:

1. The car just felt notably lighter and more nimble. It reacted quicker to everything from throttle input to sharp turns.

2. At lower speeds on modestly crappy pavement, little bumps were more obvious, but in that car, it was in a way that made me feel like I was in a smaller, lighter car. In a 335, it might be objectionable, I'm not sure.

3. On the highway on relatively even roads, I still felt more undulations in the road, however, big bumps or expansion joints actually felt like they were swallowed more readily (didn't send as big of a reverberation through the cabin), so the overall effect was one of feeling more connected and in control during fast maneuvers (whereas before I felt in control but slower and less connected).

4. #1 having been said, somehow the car actually felt *softer* overall. Hard to explain. I think it might have to do with #3 where the bigger bumps sort of felt like punching a pillow, instead of the harder impact on the heavier RFT-clad wheels.

So... I dunno... does that help anyone with anything? How does that hold up to the theories above? Seems about right though I may not have read carefully enough...

[rallybull]

Originally Posted by rjm98119

Can I tell you what my personal experience with this topic was?

I had a 2013 F10 535 from Nov '12 through June of this year (I downsized to a '13 335 in June... wanted something more sporty, and missed my 2008 335).

The stock 19" M-Sport rims weighed about 32lbs and the heavy RFT tires themselves weighed on average about 26lbs, so averaging 59-60lbs per wheel (I actually had 1 wheel/tire combo max out at 62lbs.!)

The F10 felt lumbering and sluggish (IMO) and I'd read many similar articles, so I bought some 19" spun forged rims weighing a mere ~20lbs each and new tires (Conti DWS non-rft) running around 23-24lbs. I reduced my unsprung weight by an avg of 15lbs per corner.

There were a NUMBER of factors contributing to the difference (not the least of which was ditching the RFT's), but I can say it was immediate and obviously apparent on all roads in the following ways:

1. The car just felt notably lighter and more nimble. It reacted quicker to everything from throttle input to sharp turns.

2. At lower speeds on modestly crappy pavement, little bumps were more obvious, but in that car, it was in a way that made me feel like I was in a smaller, lighter car. In a 335, it might be objectionable, I'm not sure.

3. On the highway on relatively even roads, I still felt more undulations in the road, however, big bumps or expansion joints actually felt like they were swallowed more readily (didn't send as big of a reverberation through the cabin), so the overall effect was one of feeling more connected and in control during fast maneuvers (whereas before I felt in control but slower and less connected).

4. #1 having been said, somehow the car actually felt *softer* overall. Hard to explain. I think it might have to do with #3 where the bigger bumps sort of felt like punching a pillow, instead of the harder impact on the heavier RFT-clad wheels.

So... I dunno... does that help anyone with anything? How does that hold up to the theories above? Seems about right though I may not have read carefully enough...

I think your observations are exactly in line with the statements above.

It is funny you mentioned the Conti DWS. I've been seriously considering these tires to replace my current Pirelli A/S RFTs. The DWS is known to have a soft side wall. Most people say that these tires soften the ride tremendously (that you hardly feel bumps) but then makes cornering a bit squishy (due to the soft side walls). Did you notice the "fun factor" being reduced due to this tire? Also are you running a higher (than normal) psi on these tires to gain back a bit of handling?

[OC40]

Originally Posted by rallybull

I think your observations are exactly in line with the statements above.

It is funny you mentioned the Conti DWS. I've been seriously considering these tires to replace my current Pirelli A/S RFTs. The DWS is known to have a soft side wall. Most people say that these tires soften the ride tremendously (that you hardly feel bumps) but then makes cornering a bit squishy (due to the soft side walls). Did you notice the "fun factor" being reduced due to this tire? Also are you running a higher (than normal) psi on these tires to gain back a bit of handling?

Speaking of. What psi do run in your Pirelli's?

[rallybull]

Originally Posted by OC40

Speaking of. What psi do run in your Pirelli's?

the "regular sticker" psi - 32/38

[Presario]

I believe tirerack is right. Heavier wheels are bad for performance but good for comfort. Because heavier wheels have more mass, it'll absorb more of the bumps and put less strain on the springs and dampeners. That's why most OEM setups are heavy.

[mikew2069]

Light wheels, ftw!

I too switched my stock wheels and tires and saved about 10lbs/corner. This was some time ago so I can't really comment on all of the road situations but in general the change was night and day. I know some of this has to do with going from rfts to PSS but the steering, ride quality, and everything in between improved.

[rjm98119]

Originally Posted by rallybull

I think your observations are exactly in line with the statements above.

It is funny you mentioned the Conti DWS. I've been seriously considering these tires to replace my current Pirelli A/S RFTs. The DWS is known to have a soft side wall. Most people say that these tires soften the ride tremendously (that you hardly feel bumps) but then makes cornering a bit squishy (due to the soft side walls). Did you notice the "fun factor" being reduced due to this tire? Also are you running a higher (than normal) psi on these tires to gain back a bit of handling?

Well, being that it was an f10, there was comparatively little fun factor to begin with, so the overall experience was so much better that I wasn't disappointed. It did at times feel squishy, and I ran them about 1-2 psi above recommended (though it didn't change all that much).

They worked great in snow, though . Not sure I would go there for the f30 though.

[oceanview]

Unsprung weight, is an agility killer.
The more weight a car has on the corners, the worse a car reacts on turns.
Reducing wheel weight, brake weight and other weight on the corners of a car (also sprung weight) does make sense.

Also, unsprung weight is "bad", because of mass inertia. (manufacturers try compensating that with "intelligent" suspension systems.)

Just have a look, how fast cars are built (also fast around corners ):
- As much weight as possible in the center of the car. (Driver, engine, gear box)
- lightweight brakes (carbon)
- lightweight wheels/tires

If you now have a car where the suspension system has a setup for "high" unsprung weight and you reduce a lot of unsprung weight, it could be, that the suspension is to hard, causing the car to "jump".

A suspenison system is a "system" that needs to be setup to various conditions, like car mass, but also road conditions.

[nicknaz]

That is such a vehicle specific question I think the best way is to test fit the wheel and tire setup you are interested in a go from there.

In general I think lighter wheels and tires are always better (for the performance reasons mentioned above) and any "negatives" associated fall under the WGAS category.

[rallybull]

Originally Posted by nicknaz

That is such a vehicle specific question I think the best way is to test fit the wheel and tire setup you are interested in a go from there.

In general I think lighter wheels and tires are always better (for the performance reasons mentioned above) and any "negatives" associated fall under the WGAS category.

Sorry... WGAS??

[ronin951]

^

[inChargeOfIT]

I am assuming urban slang for 'Who Gives A S***'