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New tires to the rear?; Just saw this
Topic Started: Feb 5 2015, 03:05 PM (4,126 Views)
perfesser
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Elite Member - Former Metro owner

In God we trust - all others bring data!

Thanks for posting this information. Real world numbers outrank personal opinions every day of the week.
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Coche Blanco
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myredvert
Feb 9 2015, 03:17 PM
Several month ago I had done a rough weight and balance spreadsheet to calculate the longitudinal center of gravity location, if that would help perfesser?
Assuming you have 3x 200lbs (men, I hope) in your car, along with a full tank, on a daily basis is kind of an extreme...and to use an example like that is kind of pointless.

I figure most people (185lbs)ish are running around, alone, with half a tank of gas or less a good portion of the time.
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Coche Blanco
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Silver2K
Feb 9 2015, 03:05 PM
I bought a replacement set 10 years ago but the shoes on the car are still as thick as the replacements. So I've never changed them. From this I conclude the rear wheels provide very little braking.
:gp
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myredvert
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myredvert

Quote:
 
Assuming you have 3x 200lbs (men, I hope) in your car, along with a full tank, on a daily basis is kind of an extreme...and to use an example like that is kind of pointless.
The real pointless part was apparently providing two "extreme" opposite loading examples to allow pretty simple interpolation in between them, as well as rough adjustments for variations in passengers and fuel so anyone with a different idea of a 'typical" loading could still get an idea of how it would be affected.

Next time I share data with the forum I'll think about consulting you first to make sure I use a very specific example that you consider relevant, and then everyone else with different 'norms' can think through the adjustments but you don't have to.

Were you actually interested in what the center of gravity would come out to using your example, or were just taking an opportunity to take a shot at something you yourself wouldn't have put the effort into doing in order to share with others here? ;)

If it just isn't something that interests you or you don't understand, that's fine too, but there are at least a few people here that I know who would be interested, whcih is why i took the time to dig the information up.
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Coche Blanco
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myredvert
Feb 9 2015, 06:34 PM
Next time I share data with the forum I'll think about consulting you first to make sure I use a very specific example that you consider relevant, and then everyone else with different 'norms' can think through the adjustments but you don't have to.
Please do. That will save us both some time.
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fireflyin


u don't need really good tires on back of a car its no fun when u can turn a corner and let the back end slide tokyo drifting lol but if your in the city u mite wanna get 4 winters lol where i em i run 4 winters but i could run 2 winter at front n 2 semi bald tires at the back its just more fun haha
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Woodie
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perfesser
Feb 9 2015, 10:43 AM
Woodie
Feb 9 2015, 05:53 AM
100% of the traction, 90% of the braking, and about 70% of the cornering are handled by the fronts.
Could you post links to where that information is found, please?
In my head. There's not much else in there, so it's not hard to find.

First one is obvious and set in stone.

Taking myredvert's calculations that 66% of the weight is on the fronts sitting still, then think of how much that changes when you do a panic stop and really stand the thing up on its nose. Has to be 80% in that case. Now add in the fact that you STILL can't get the rears to lock, that bumps the estimate up a little more. In the interest of harmony (and knowing how little things like this can easily turn into a pissing match) I'll temper that estimate down to 80 - 90% of the braking.

Cornering was just made up, from whole cloth. A SWAG if you will.


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sphenicie


Silver2K
Feb 9 2015, 03:05 PM
As far as the braking, my car has 400K. In that time I've changed the fornt brake pads 6-7 times. The rear shoes are still original and look new. I bought a replacement set 10 years ago but the shoes on the car are still as thick as the replacements. So I've never changed them. From this I conclude the rear wheels provide very little braking.



From this I conclude that you have a problem with your braking system !!!!


1) my first thought is that your rear brakes are not properly adjusted. if you have 400k on the original shoes, they have probably NEVER made contact with the drums.

2) faulty proportioning valve.

3) frozen wheel cylinders.

your front pads are much thicker than the shoes. Hmmm, almost exactly 2 1/2 times thicker. Hmmm, just right to wear out at the same time as the shoes, when you consider that the front should provide 70% of your braking.
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perfesser
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Elite Member - Former Metro owner

I entered the information for my Metro into my Braking Force Calculator, and assumed 80 & 90% load on front axle during hard braking. I think the calculations are generous in over-stating the results, but ya' gotta start somewhere!

Here is 80%.
Posted Image

Here is 90%.
Posted Image

The difference between 80 and 90 % shows up in the weight on the rear. The front brakes generate only the maximum force they can according to the design of the braking system, but if we assume 90% (which, based on this, I don't think we can physically reach) the rear tires are getting really light. The braking force on the front tires exceeds the traction available, and even at 80% they are sliding on dry pavement, reducing the real-world (as opposed to calculated) braking force by roughly 15%. The Metro appears to be traction-limited.

80% weight on the front axle under maximal braking gives you more total vehicle braking force, more deceleration, and a shorter stopping distance than 90%. Think of how much braking force a motorcycle rider gives up when he does a "stoppie" and his rear wheel is off the ground. Looking at the weight on the rear axle, that's where this is going. I did some simulation at lower front axle loads, and as more weight was left on the rear axle under hard braking, performance just kept getting better and better. The more braking force the rears are providing, the higher the deceleration forces and the shorter the stopping distances.

The Metro is limited by the traction available at the rear axle. Having poorer, worn tires on the rear axle reduces rear traction, increasing stopping distances and reducing total vehicle braking forces. There is no problem with front traction. The problem's at the rear. Putting the newer tires on the axle that needs them just makes a lot of sense to me, and I have good data to back me up.

Once you can lock the front brakes, that's it. Understeer city. Beefing up the brakes without going to high-performance rubber will only help you lock 'em up faster. Changing to better rubber without doing anything to the brakes will allow you to utilize more of the potential braking force, but only on the axle that needs the help.
Edited by perfesser, Feb 10 2015, 02:11 PM.
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perfesser
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I just came this blog posting from Allstate (the good-hands people). It talks about where the new tires should go, and why.

Put the new tires on the rear axle!
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nineoneandblue
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With all due respect perfessor...... that blog was from the op at the start of this topic( see post #1)
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perfesser
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Oh well. At my age I'm entitled to have a "senior moment" every now and then! Thanks for keeping track.
Edited by perfesser, Apr 10 2015, 11:45 PM.
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wahoospa
Advanced Member
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I believe 18 wheelers are required to use new tires on the steering and can use recaps on the rear. But I have always heard to put new tires on the front because there is less chance of a blow out with a new tire compared to a used tire.
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Metromightymouse
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Powdercoat Wizard

That law was/is based on the mistaken belief that the tires that come apart and leave alligators all over the road are from failed recaps. Turns out that when they actually did a study, the tires come apart from under inflation and non recap tires were more susceptible. Of course it is possible the study was financed by the recap industry, since I was a company driver I didn't care enough to research further.
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wahoospa
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I was at a tire store today and on the wall was a brochure for Bridgestone Tires and the message was "putting on just 2 new tires?" "New tires go on the rear". Something to do about control in wet situations.
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