Wheel weight

[Dieselman]

Michelin will supply tyre weights of all of their tyres. Can be surprising to see different brands being 2kg lighter in some instances.

Get some forged or magnesium wheels if you want to see a real difference.

Or some carbon fibre or even the old Michelin resin-composite wheels for the 1968 Citroen SM. 5kg each.

Can you get tyre information.?

 

[Dieselman]

Is there a question in this or did I miss it?

You missed it..

 

[Dieselman]

Would have expected yours to be lighter (less metal and rubber) - could just be the error between different scales.

It could be but also some Merc wheels are really quite light for their size. They mix with the lightweight aftermarket brands.

The wheels in question are multi spoked though so probably have more metal than required.

I'll get the hacksaw out..

 

[Number_Cruncher]

Here are some sums I've done (in proper units!), based on the numbers in Dieselmans 1st post in this thread.

The text after a % sign is just a comment.

>> m_tyre=10;
>> m_wheel=9.3;
>> Rim_Radius=(16*25.3e-3/2);
>> Rolling_Radius=Rim_Radius+(225e-3*55/100);
>>
>> % Modelling the wheel as a flat disc plus
>> % a concentrated mass at the wheel rim
>> % Assume a proportion of the wheel mass is in the disc (P_disc_mass)
>>
>> P_disc_mass=0.5; % Assuming a 50 / 50 split
>>
>> I_disc=0.5*m_wheel*P_disc_mass*(Rim_Radius^2);
>> I_rim=m_wheel*(1-P_disc_mass)*(Rim_Radius^2);
>>
>> % Assume tyre mass is all concentrated at the rolling radius
>> % this should over-estimate the inertia
>>
>> I_tyre=m_wheel*(Rolling_Radius^2);
>> I_TOTAL=I_disc+I_rim+I_tyre

I_TOTAL =

1.2750

>>% This is the rotational inertia in kg m^2
>>
>> % As the total kinetic energy of the wheel is
>> % 0.5 * m * v^2 + 0.5 * I * (omega^2)
>> % and v = omega x r
>>
>> % kinetic energy = (v^2)/2 * (m + I/(r^2))
>> % i.e., the effective mass is m + I / (r^2)
>>
>> Eff_mass=m_tyre+m_wheel+(I_TOTAL/(Rolling_Radius^2))

Eff_mass =

31.2862

>> % This is in kg
>>
>> Effective_Mass_Ratio=Eff_mass/(m_tyre+m_wheel)

Effective_Mass_Ratio =

1.6210


This means that the effect of the rotational inertia of the wheel is to make it seem about 60% heavier. Tweaking the proportion of mass at the rim of the wheel or on the face, via P_disc_mass will change the results a bit, but will not get anywhere near to 4x.

The big error in the spreadsheet is that it does assume that torque at the road wheel and torque at the engine are the same - where of course, there must be the overall gear ratio of the gearbox and final drive to take into account.

 

[stats007]

Ever heard of Hungarian notation .

Your conversion from inches to millimetres is a bit out but fairly irrelevant.

Torque differs from the engine to the wheels due to losses in the drivetrain - the ratios are irrelevant.

On a Merc I'd say the differences would be minimal, but as you reduce the overall mass and the percentage of the unsprung weight increases it will become more apparant. If you've ever driven a Caterham or Dax and swapped from cast to forged wheels you'll know what I mean.

 

[stats007]

Or some carbon fibre or even the old Michelin resin-composite wheels for the 1968 Citroen SM. 5kg each.

Can you get tyre information.?

Carbon fibre wheels are a bit extreme but available on some cars today.

Contact Michelin technical team through their website and they'll tell you.

 

[Dieselman]

My query was originally due to thinking about upstepping wheel size and what effect it would have. There are loads of threads on forum and technical documents that suggest the increases mass and/or larger diameter are bad for handling and performance and can even be for economy on a town car.

Here is a thread covering converting a MINI Cooper from 17" rims to 15" rims. It reduces the 0-60 time by 0.2 seconds.

A poster then calculates the difference and comes up with an effective weight difference of 1.8:1 after revision.

http://www.mini2.com/forum/wheels-t...ified-wheel-physics-garfields-wheel-test.html

Tirerack also test different wheel weights and concur that the effect is noticeable.

 

[Number_Cruncher]

>>Torque differs from the engine to the wheels due to losses in the drivetrain - the ratios are irrelevant.

Stats - you're right about the mm conversion, that was a typo, 25.3 instead of 25.4 - however, on your other point torque *does* vary because of gearing - it's power that remains the same throughout the drivetrain but for the losses. Stepping up the torque is exactly what the gears are there for!

 

[pete vee rod]

Can you remeber Top Gear testing a Renault Avantime performance and everything that did caused it to reduce.
The first mod was bigger brakes and wheels. That cost them over 2 seconds per lap, which they had to spend thousands to recoup.

I remember that programme, in fact, the best mods was air filter/new plugs etc..in other words a good service, which gained back 30-40hp that had been lost and was by far the cheapest benefit gained.

 

[stats007]

however, on your other point torque *does* vary because of gearing - it's power that remains the same throughout the drivetrain but for the losses. Stepping up the torque is exactly what the gears are there for!

Sorry I probably wasn't making my point clear - obviously moment changes with gear ratios but this is irrelevant if you're comparing different wheel masses.

 

[Number_Cruncher]

My point was that in the spreadsheet, the effect on vehicle performance was estimated by calculating the effect of the inertia at the wheel, and then taking that from the torque at the engine. Which would have been OK had the gear ratios been used, and I think this error is why that spreadsheet misleads people into thinking the effect is as large as 4x.

Depending upon what gear you're in, the speed ratio (and hence the inverse of the torque ratio) can be as high as 10:1, dropping to perhaps 3:1 in top gear - that's a significant error.

This erroneous spreadsheet did mislead some people on the forum which DM posted, as well as on here - I provided the code snippet to clarify the method to find the effective mass, which might enable people to calculate their own if they wanted to - although that would mean that they had to fight their way through my rather free-form notation.

 

[Will]

Will

 

[miro]

At the end of the day there is TUNED SUSPENSION and standard suspension. What we are all riding around on is standard suspension. Changing a few minor components makes the ride alter but I would say that air pressure, grip and road surface have a larger role to play than the rim.

Really high quality components in isolation are nothing special. When combined the overall effect is amazing. However, with all those fancy rims people usually get bigger brakes which weigh quite a bit more.

 

[Mactech]

Will

What's the weight of a 'bear' wheel? I'm not sure, but when I changed the wheels on the ML from the standard Sport 19" to the 'Y' spoke 20" the combined weight of the wheels and tyres was exactly the same at 31kgs per corner.
On the S class the 18" 9 spoke with Michelin are exactly the same as the 19" 5 spoke with Dunlop at 26.5kgs per corner.
The Kahn 20" and 21" I have now got on the S are heavier because of the very butch wheel construction. The tyres, having a big hole in the centre, are actually lighter.
The wheel control is not quite as good due to the weight, but general ride quality is still pretty good with the Airmatic. The handling and performance appears unaltered (despite the calculations) but we have gained grip, which was my intention.