How is the roll centre used during braking?

[nb89]

I'm calculating the weight transfer experienced by a car during braking, accelerating and cornering. So far i have been using moment equations with the centre of gravity height. I've used some suspension software to determine the roll centre and was wondering if this is meant to be used when calculating weight transfer? I've read about the roll centre but have not found it being used in any formulas anywhere. Is the roll centre mainly found to make sure it is not far off from the centre of gravity height? Thanks.

 

[Ranger Mike]

i recommend you look at post in mech engineering forum
Jul22-09, 11:03 AM titled
Race car suspension Class
and get back with me..

 

[nb89]

i recommend you look at post in mech engineering forum
Jul22-09, 11:03 AM titled
Race car suspension Class
and get back with me..

Thanks for that, I read through the roll centre section and saw this:

'Roll center height determines what percentage of the overturning moment (inside to outside weight transfer) will be distributed onto the tire contact patch a downforce, and what percentage is received as lateral loading against the tires tread face'

Does this mean the roll centre height should be used when calculating the weight transfer during a corner? I have been using Weight transfer = (Lateral acceleration x Weight x Height of CG) / Track width.

I am actually assessing the stability of a three wheeled car (1front, 2 rear wheels). I am wondering if during the weight transfer, would only the outer rear wheel receive the transferred weight because the single front wheel is located at the centre? I assume the front of the car would not have a roll centre?

 

[Ranger Mike]

Roll center and CG are two different points and can not be interchanged in the formula...
a percnetage of the sprung weight is transferred thru the roll center..the amount is a function of the length between the CG and the RC and acts as a moment or lever in this case
i am sure motorcycles have roll centers as they have suspensions...right?? You need to find out how to calculate the RC of a motorcycle..i have no desire to do this my self...also you should be able to find RC of a Tryke as well..good luck

 

[nb89]

Ok i think I'm kind of getting it. Would you have to use the weight at the rear of the car multiplied by the length between CG and the rear RC to find the % of weight transferred through the roll centre? And then same with the front weight and front RC? Is the RC also used to determine the angle the body has rolled by?

 

[Ranger Mike]

good questions all.. regarding this post..are you designing a vehicle from ground up? or does the vehicle already exist?
I can address the amount of weight transferred with regard to a four wheel vehicle but i can not address motorcycle questions..
i suggest yo u wait for other posters who know bikes in detail to assist you on this matter...

 

[nb89]

The vehicle already exists. Could you answer the question with regards to a four wheeled vehicle then, just so i can make sense of it. Thanks

 

[Ranger Mike]

Ok We can work this out...but..let us agree on the objective you desire which is..i believe..not the amount of weight transfered..but, the
What is the correct size SPRING required to properly handle maximum weight transferred when cornering...
correct??

 

[nb89]

Yes that is one of my objectives, I've done some calculations already on weight transfer, but how would i select the right spring size? Would i have to use F=kx?

 

[Ranger Mike]

I think it will be beneficial to all if we move this post to Mechanical Engineering forum
Race car set up help