Creating a generic tire envelope

[jz2hs6]

I'm creating an editable tire envelope (CAD) and would like to hear some suggestions to improve it. It is used as a visualization tool and for rough packaging. I'm assuming all the characteristics of a radial tire for now.

The user has to enter the following inputs;

Tread Width (mm); 185 for exaple
Aspect Ratio (mm); 55 for example
Rim Diameter (in); 16 for example
Rim Width (in); 5.5 for example
MPH (optional); Static Loaded Radius would be used if 0 is entered.

From there, two profiles are generated. The top profile starts by using the tread width at the top, the rim width at the bottom, and uses a max grown in the middle. I use some rule of thumb I found that states that the tire width changes 2/10" for every 1/2" of rim change. The top profile side wall height is exactly the aspect ratio for now.

The bottom profile tries to account for being loaded. I'm saying the loaded radius is 44% of the nominal diameter. It increases 1% for every 10MPH. The area of the top porfile is calculated and then using that to set the width of the bottom profile.

I would also like to show somthing for snow chains. I'm not sure if my "max grown" is correct. Any suggestions would be greatly appreciated. I will share this model if anyone want's it. It's CATIA V5.

One last thing, a little unrelated. If I want zero scrub radius for manual steering, should that be set up at static or at 60MPH? I think static because that's when it would be hardest to turn the wheel and when you would want no scrub.

Thanks for everyones help.

 

[GregLocock]

Why do you really want zero scrub? Are you trying to park with your brakes on (hint).

T&RA has a pretty realistic looking tire envelope definition that sounds a little more complex than yours, in shape, even if it doesn't have the various modifiers.

http://tirechain.com/sae.htm

google is your friend



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[GregLocock]

The wiki article on snow chains gives the other classes as well

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[jz2hs6]

Hmmmm, I'm going to have to do a little research to figure out what you mean by that braking comment. I also went to your link for forum policies and saw that I wasn't supposed to ask that question in the thread I just started. If I can't figure it out, I'll have to start another thread but I'm sure I'll get it.

Ok, so I'll keep this thread realated to the tire envelope. Thanks for the tire chain links. I talked to TRA last week. They said that their yearbook has all the dimensions but if I want their formulas, I would have to get their engineering book. I can't find either one at my local libraries. Do you know if their tire envelope definition is included in their yearbook?

 

[GregLocock]

That's what I used, just the yearbook.

So far as scrub radius goes, large old manual steer cars had lots of scrub radius, because that way the wheel rolls when you turn the steering wheel. If you have no scrub then you literally have to scrub the contact patch in place.

But, if you put the brakes on then the tire can't roll, and so the efforts are less with zero scrub.

By the way my sig isn't aimed at you in particular, tho it's never a bad idea to read the faqs.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[CapriRacer]

I'm a little unclear on what you are trying to accomplish.

Other comments:

I noticed that you had the user input tread width, but didn't require him to input section wdith. ?????

Yes, the TRA yearbook has a page showing the envelope - Just be aware that it's pretty generic and in real life, tires vary quite a bit.

I'll post that one page for you some time in the next few days.

 

[jz2hs6]

It is used as a visualization tool and for rough packaging.

You are right. The input is supposed to be the nominal section width, not the tread width. Also, the aspect ratio is supposed to be %, not mm.

I'm trying to come up with a formula that outputs a relatively accurate max grown. If accurate is unrealistic, then I would go for worst case. I would greatly appreciate seeing the TRA envelope.

 

[CapriRacer]

Here it is:

http://www.geocities.com/barrystiretech/traenvelope.jpg

 

[jz2hs6]

Thanks so much. I've read through four automotive and chassis/suspension books, attended seminars, spent countless hours on the internet for exactly that.

Another thing I'm looking for is a loaded radius. If I'm setting up a suspension,I can't be expected to base it on one company's paticular product. Is there an accepted design SLR? How about SLR = 44% Nominal Diameter?

 

[NormPeterson]

Why wouldn't you base loaded radius on a fixed wheel radius (8" or whatever), with only the loaded tire section height being variable (itself some function of load, tire "spring rate", etc.)? The wheel contributes negligibly to changes in SLR.


Norm

 

[GregLocock]

Yup, just use a rate of 200-250 N/mm from the nominal SLR.




Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[jz2hs6]

Sory but I don't get what you guys are saying. (Norm or Greg).

Here an example using the 44% rule of thumb for the SLR;

Given: 205/55R16

The side wall would be 112.75 mm (205 x .55)
The rim diameter would be 406.4mm (16 x 25.4)
The nominal diameter would be 631.9 (112.75 + 112.75 + 406.4)

SLR = 278 (631.9 x .44)

Is than an acceptable approach? I am using a fixed wheel radius unless I'm misunderstanding what Norm is saying.

How does "200-250 N/mm" come into play? That sounds like it could work for me if that's the spring rate of the tire. Then I could determin the SLR based on mass. A light just went off in my head. I like that better that 44% rule. Am I correctly picking up what you're laying down?

 

[CapriRacer]

JZ2HS6,

I just realized there is an error on the page I posted:

Under the "Notes" #5 says: D = Y% of Max. Overall Width. Instead of "D" they should have said "R". This has been fixed in later Yearbooks - in fact they used different letters for these dimensions and added a note about rim flange protectors.

OK, I just realized I should replace the post with the latest version.

Here's the address of the new one:

http://www.geocities.com/barrystiretech/TireEnvelope.pdf

Second, For practical purposes, the differences in SLR between tires is small compared to the effect load and inflation pressure have. This is what both Norm and Greg are alluding to.

Perhaps it would be better if Norm or Greg answer this part of the question, as my experience is limited in this area, but I think they are saying to use 200 to 250 N/mm spring rate to get the amount of deflection based on the load on the tire.

 

[GregLocock]

That's what I meant, I hope that's what Norm meant!

I don't know about radial growth due to speed.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[NormPeterson]

Yup.

I never thought too much about the loaded radius varying with speed. Very indirectly watching top fuel drag car burnout video is about the extent of it.


Norm

 

[jz2hs6]

Thanks Guys! Very helpful.