Unsprung Weight & Handling (split thread)

MiddleAgeSubie

Forum Member
Joined
Dec 15, 2013
Messages
990
Location
AZ
Car Year
2018 / 2008
Car Model
4Runner / Tribeca
Transmission
5EAT
For a year I have been operating under the assumption that I will be replacing the ATS with a milder AT while keeping the KO2s only for offroad but I am starting to think that I will just move to KO2s full time, getting a new set when the current one falls under 6-7/32nds and relegating the old one to town/weekend trips.

After all, the combined weight of even stock wheels and KO2s does not exceed that of steel wheels and Geolanders so I hope the suspension will be ok with it. What do you think?
 
Why would you have problems with the suspension with heavier wheels ?

More unsprung mass is not so good for acceleration, braking, cornering and MPG but I dont know any side effect related to suspension.

My next set of tyres will probably be Nokian Rotiiva AT's as they have the mountain flake symbol, won't need to change to winter tyres anymore (legislation)
 
Good to know! I had no idea, hence the question, thanks.
 
More unsprung weight is a bad thing. It makes life very hard on dampers. The weight of unsprung weight is working against the car, forcing it upwards on bump and this both unsettles the car and affects both ride and handling.
 
Rotational mass of tyres/wheels is bad for acceleration, braking and fuel economy.

Unsprung mass of the tyres/wheels/hub,etc affects handling/grip,, struts, ball joints, bearings, steering components, etc as it makes all of these work harder on bumps

This is why you see custom billet alloy hubs on some race cars, also alloy 3 piece brake discs, anything they can do to reduce both unsprung weight & rotational weight
 
Okay but if Subarus have done alright on steel wheels + AT tires, then I can drive around town on the stock wheels, which are pretty light at under 22lb plus aged KO2 tires. Right?
 
I'm not a big fan of OEM steelies, they're pretty heavy compared to OEM alloys. Then there's aftermarket alloys which are lighter again. The lightest I've come across are the Audi Pepperpots which are only 5.8kg (12.8lb) each.

Running steelies & KO2s dosn't mean the bearings are going to fall out lol, but it will have an effect on all the different aspects
 
Rally said:
More unsprung weight is a bad thing. It makes life very hard on dampers. The weight of unsprung weight is working against the car, forcing it upwards on bump and this both unsettles the car and affects both ride and handling.
Click to expand...

Could you explain this differently ?

Cars with live axles have a huge amount of unsprung weight but do they kill their dampers/suspensions components faster ?
 
jf1sf5 said:
Could you explain this differently ?

Cars with live axles have a huge amount of unsprung weight but do they kill their dampers/suspensions components faster ?
Click to expand...

They have dampers designed for the heavier unsprung mass.

To picture why less unsprung mass is better imagine a small coil spring fixed to the floor, no damper. If you compress or stretch this spring and then release it, it will quickly return to its resting position. Now put a small weight attached to the top of the same spring, and repeat the test. It will take a longer time to return to rest.
Now affix a small damper to help control this and re-test, it will return to the resting position nice and quickly.
Now with the small damper still in place, swap the weight for a heavier one and re-test, even with this small damper, it will take a longer time to come back to rest.

I hope you can understand/visualise what I am trying to convey here?
 
Thank you !

Yes, independant suspension design has 2x higher frequency than live axle design but with your explanation, except if you are only making jumps all the time, the suspension components damp the weight of the car, not the unsprung masses. Or do I miss something ?
 
Bigfoot has described unsprung wight perfectly. That is precisely why Mercedes Benz in the 1950's brought the drum brakes of its racing cars inboard.
 
jf1sf5 said:
Thank you !

Yes, independant suspension design has 2x higher frequency than live axle design but with your explanation, except if you are only making jumps all the time, the suspension components damp the weight of the car, not the unsprung masses. Or do I miss something ?
Click to expand...

If you watch a car travel over a bump in the road, you will see for the most part the wheels move over the obstacle, and the car only moves a small portion of that. The greater the ratio of sprung to unsprung weight, the less the car will move. Therefore lighter wheel/tyre combos will have a better ride.

The springs are rated/sized to control/support the weight of the vehicle, and the dampers are more about controlling the motions of the wheels and keeping them in contact with the ground as much as possible.
 
Bigfoot said:
If you watch a car travel over a bump in the road, you will see for the most part the wheels move over the obstacle, and the car only moves a small portion of that. The greater the ratio of sprung to unsprung weight, the less the car will move. Therefore lighter wheel/tyre combos will have a better ride.

The springs are rated/sized to control/support the weight of the vehicle, and the dampers are more about controlling the motions of the wheels and keeping them in contact with the ground as much as possible.
Click to expand...

Ok, now I understand ! Everything is about compromise and I didn't think of the vertical oscillation of the wheels...so if you put heavier wheels/tyres on your car, you should load it more to keep the same sprung/unsprung ratio and also change the springs and the dampers to make everything work properly.

Can the difference in terms of damper wear for a 2 kg heavier wheel/tyre combo be calculated ? Sure is but by how much ?
 
Prevention is better than cure. Stay light.
 
Very true ! Thus why I drive a Subaru and not a big fourbie :iconwink:
 
jf1sf5 said:
Can the difference in terms of damper wear for a 2 kg heavier wheel/tyre combo be calculated ? Sure is but by how much ?
Click to expand...

Yes it can, but not by me lol :iconwink:

jf1sf5 said:
Cars with live axles have a huge amount of unsprung weight but do they kill their dampers/suspensions components faster ?
Click to expand...

Another thing with this, big 4wds with big heavy steelies & hige 35" muddies kill shocks very quickly in the outback on the tough corrugated roads. Out of 7 Subarus doing 10,000kms each on these roads, not one had a shock/strut failure
 
The shocks probably won't break- they will just wear out more quickly and not do the job they should be doing as well even when new.
 
Bigfoot said:
If you watch a car travel over a bump in the road, you will see for the most part the wheels move over the obstacle, and the car only moves a small portion of that. The greater the ratio of sprung to unsprung weight, the less the car will move. Therefore lighter wheel/tyre combos will have a better ride.

The springs are rated/sized to control/support the weight of the vehicle, and the dampers are more about controlling the motions of the wheels and keeping them in contact with the ground as much as possible.
Click to expand...

Actually, the opposite is true:

https://www.tirerack.com/wheels/tech/techpage.jsp?techid=108
 
Rally said:
The shocks probably won't break- they will just wear out more quickly and not do the job they should be doing as well even when new.
Click to expand...

Beyond a certain point, sure. But who knows what it is?

I would venture a guess that the factory suspension can easily handle at least 150% of the stock wheel/tire weight.

Do not forget that different Subaru models come with different wheels in different trims, that steel used to be common at the lowest price points, and that many people use steel wheels for snow tires. There is no way that each trim's suspension is engineered to handle just the stock tire/wheel combo.

The weight of aftermarket steel wheels and regular AT tires in my case would be 61 lb with Geolander ATS. The weight of stock wheels and new KO2s is...61 lb as well. Stock combined weight is 46 lb. Seems reasonable enough to me.

Guess I answered my own question, lol.

That said, staying light is of course preferable on account of performance. That is why my KO2s are on lightweight wheels.
 
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Maybe I should weigh my wheel and tyre combination and see what I get
 
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