Altering unsprung mass - Tyre grip VS. Suspension efficiency

[Kronos1]

Hi everyone,

I have the option of using 2 different sizes of bearings in the 4 wheel hubs of a racing car, through the use of hub inserts.

However, I am unsure which of the situations below will results in a greater benefit to the car.

1) The smaller bearings (less unsprung mass) will allow the shock absorbers to control the sprung mass more efficiently. (I believe this is correct, but please correct me if I'm wrong.)

2) The larger bearings (greater unsprung mass) means that the normal/reaction force at the tyre is larger (Since Force = mass X gravity, and the mass is larger).
Since the friction force, and thus the "grip" the tyre produces is a product of the normal force and the coefficient of friction, the friction force will increase and there will be more "grip".

The smaller bearings (which have smaller balls) should be beneficial from the point of view of reduced rotating mass (to what extent I don't know) but the question above is still one I'm stuck on.

Any advice is really appreciated.

- Kronos1

 

[GregLocock]

Use the big ones. The difference in unsprung mass is small, sleeping at night is good.

Cheers

Greg Locock

I rarely exceed 1.79 x 10^12 furlongs per fortnight

 

[cibachrome]

Wheel compliance (MZ and MX) will be lower with bigger bearings. It always takes bigger balls to be successful in racing. That's why Danica will probably strike out...

 

[Kronos1]

Sorry GregLocock.. I don't understand, why are you saying the bigger ones are better?

cibachrome - Ha, seems like she's beginning pretty well in ARCA though. I didn't consider wheel compliance - I've no experience with it. Thank you.

I thought it would be favourable to use the smaller bearings..
- Reduced ball mass
- Reduced polar moment of inertia
- Reduced unsprung mass

Are these outweighed by something the larger bearings provide?

 

[GregLocock]

Bigger load capacity, bigger safety factor.

Cheers

Greg Locock

I rarely exceed 1.79 x 10^12 furlongs per fortnight

 

[Kronos1]

Sorry, I should have said - The application is a Radio Control competition racing car. Safety isn't the concern it is in full-scale racing!

The smaller bearings replace the larger ones with inserts inserted into the hub as in the link attached.

 

[FeX32]

lol. I used to build those things when I was in grade 8. The nitro ones are really "shooters" if you know what I mean.
Honestly, you are not going to get any benefit by reducing the mass be say 0.5 grams. (but of course reducing mass everywhere and anywhere has benefits)
For reference, Mugen has suspension geometry/efficiency optimized for racing, not servo saving.


Fe

 

[Kronos1]

Just to be sure we're talking about the same thing FeX32 - the cars are in the link below. I used to build them at that age too - I still do!

You're right about Mugen, the first car in that video is one of theirs. For these cars the servo saver system in the steering linkage does the servo saving by the way. It's a spring loaded pivoting pivoting arm which is also used to adjust the car's ackermann.

The difference is 6 grams per hub and 24 grams for the whole car obviously. That's about 1% of the cars total mass. I don't know the magnitude of the unsprung mass alone unfortunately.

Thanks for your replies.

 

[FeX32]

Yes, we are talking about the same thing. I used to race 1/10th scale so not quite as crazy fast as those, but still challenging in terms of the motor skills you needed to be good and not destroy the 3 grand car.
What I can tell you from my experience, is that if you can manage to reduce the inertial mass (or the rotating mass as you called it) it is equivalency of about 10 time that if it were just weight reduction. This was the most basic but very effective performance enhancer.
I usually started with the flywheel (this was about 10 years ago when I was in 8th or 9th grade)
P.S. a 9th grader who has yet to find out about stress concentrations can be really surprised when their car blows to pieces on a straight away. (I am not implying anything here, just ranting about myself)
The first time this happened was when I superseded the fatigue life on my connecting rod. The rod broke at the base while it was revving at maximum rpm. The result was not good.
Have fun,


Fe

 

[thirty35five]

First post here...hello everyone.

Kronos1, you are correct in your assumption that reducing unsprung mass is a good thing - for many reasons.

The assumptions you make in number 2 lead me to believe you don't have a firm understanding of vehicle dynamics. Research tire load sensitivity. Unsprung/sprung mass. Elastic/inelastic weight transfer. Etc...

In the grand scheme of things, I doubt you will notice ANY difference in your RC car. And with respect to bearing/hub compliance, I doubt the difference is substantial compared to total system compliance which, in the case of an RC car, is astronomical.

Hope this helps.

 

[Kronos1]

FeX32 and thirty35five,

Thanks for your help.

Could you please suggest some online resources or textbooks with which I could study tire load sensitivity, elastic/inelastic weight transfer, etc?

Is "Race Car Vehicle Dynamics" by Milliken & Milliken appropriate, or perhaps some of Carroll Smith's texts?

Also, I would greatly appreciate it if someone could state the "many reasons" that lower unsprung mass is beneficial.

- Regards, Kronos1.

 

[thirty35five]

The books you listed are great references. The Carroll Smith books (Tune to Win, Engineer to Win, etc.) are very easy to read and cover the basics in a qualitative way non-engineers can understand. The Milliken book lies on the other end of the spectrum as it is a much more thorough and quantitative treatment of vehicle dynamics. Don't forget the Internet.

A few reasons... Lowering unsprung mass can reduce tire load variation. Lowering it also shifts the distribution between elastic WT (which can be manipulated with your spring/damper system) and inelastic WT (which is a function of unsprung mass, among other things). Controlling, adjusting, tuning your elastic WT is easier to do than controlling your inelastic WT - of which would you rather have more?

Obviously, lowering mass ANYWHERE on your car is a good thing, provided the weight savings doesn't have a significantly negative effect on other parameters (camber compliance being a specific example). If it does have a significant effect, how do you analytically determine the best compromise?

A disclaimer - I am by no means an expert on the subject. Just my $0.02.