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tire to body clearance envelope

tire to body clearance envelope

(OP)
Over on a non-Eng-tips forum there is a discussion about reduced fender etc clearance after fitting wider tires and wheels.
Folks are just checking with cars at cars parked and wheels straight ahead.

1 - is this really likely to the the condition with tightest clearance?
2 - if not, (or regardless) does anyone know of a cool image showing the general envelope of tire motion during suspension bounce, and when turning?

thanks,

Dan T

RE: tire to body clearance envelope

Establishing the wheel clearance envelope in CAD needs two separate studies

1) what is the maximum size and shape of the tire? Allow for production tolerances, growth due to age, growth due to tire pressures, and possibly deflection due to loading. Also allow for snow chains etc.

2) what shape does the maximum tire shape sweep out when the suspension is exercised through its full travel, deflections, and steer angles?

Both are rather perfectionist statements, jn practice the extra complexities of 1) are often ignored, and the full potential operating envelope of the suspension in 2) is trimmed somewhat, for example full lock AND full droop is not really a likely combination.

As you can see it isn't very difficult in theory to get the thing roughly right, in CAD, but no I have never looked for an on-line picture of one for a MacPherson.

A physical test for this is difficult on a MacPherson, as you can't easily remove the spring and exercise the suspension manually on a 2 post hoist.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: tire to body clearance envelope

(OP)
Hi Greg,

Do you have any feel whether sitting in the garage is the tightest condition, so if it's OK there it's OK hitting a big bump with the wheels turned pretty far?

The purpose of the picture/rendered image would just be to show the sidewall and tread moving outward toward the fender when turning and bumping, if that is what >>can<< happen.

thanks

RE: tire to body clearance envelope

Depends on the fender shape. I don't have any feel for it, all I know is that BMW use MacP quite a lot and if you order some tire sizes they manually trim the fender flanges at the dealers. Gut feel is that lock is more of an issue than vertical travel, and jounce more than rebound. So bounce up and down on the corner while cycling the steering.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: tire to body clearance envelope

Maybe a floor jack under the A-arm- but that doesn't necessarily get more jounce travel.
Spring compressors on one corner to let it move into jounce, then cycle steering?

Jay Maechtlen
http://www.laserpubs.com/techcomm

RE: tire to body clearance envelope

Understand a few of real issues, circumstances and boundary conditions:

There is a fender inner liner and the outer wheel 'lips' (flanges) where contact can occur. It is also common practice to make the front and rear wheelhouse the same contour. No kidding. There are usually several wheels made of various materials and 2 or even 3 different tire sizes provided for. There must be friendly contact with any and all wheelhouse area parts. There must be tire chain clearance because chains are REQUITRED in many mountain states on certain roads in snow conditions. There are shipping tie down requirements and washrack (car wash) specs, too. AND Aftermarket tire dimensional musical chairs.

A max pothole strike, a curb pushaway and a double elevation change railroad crossing test are usually the most severe tire clearance issue conditions. Everything flexes. And, you still sometimes need to clay the sub-frame, springs, brake hoses and steering parts to check for contact.

As know should also know, the Tire & Rim Association 'Bible' for tire dimensions allows for quite a large variation/overlap in dimensional range for all listed sizes, pressures, rim width and tread depth.

This means even the brand of tire in exactly the same size labeled designation and on the same wheel can have considerable (as in a LOT) of dimensional range in width, spring rate, revs per mile, rolling radius and circumference. Brand 'A' may fit, but Brand 'C' may hit or rub.

Owners will put into the vehicle what fits, not what weight they are allowed. (This is called "foreseeable misuse" in Court. If Grandma spends a lot of time at the Waffle House, you may be well over GAWR with a full tank of gas (Hers and Yours).

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