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stopping distance - SCARY
Topic Started: Jul 5 2015, 11:43 PM (1,685 Views)
Woodie
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Those Kumhos have a UTQG of 700, that's practically plastic, not rubber. Will probably last a long time, but don't expect much traction. I never buy any tire with a UTQG below ABOVE 400, but that's just me.

On the best day the rears do about 20% of the braking. The rear adjusters are very well known failure points. How far up does your emergency brake handle go? Around 45° is normal, if you need to pull it up pointing at the roof, then your rear adjusters have failed and the rears are doing nothing.
Edited by Woodie, Jul 18 2015, 03:08 AM.
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Dattman
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rmcelwee
Jul 16 2015, 01:34 PM
Skinny tires and fat tires under the same PSI and rubber will have the same contact patch. In auto-x where cornering is extremely important you increase, rather than decrease your tire pressure before a race.
:shake a wider tire will have a wider contact patch and a bigger OD tire will have a longer contact patch, either way the contact patch area will increase, conversely the weight per square cm will decrease and this can be an issue on really big tires on lightweight cars, especially in the wet.

Try it at home, jack the front of the car up, paint the front tires, rotate so the painted part is facing down and stick a sheet of newsprint under each wheel, drop the jack quickly, raise the car and look at the tread patterns on the paper, try it again with wider tires and measure the difference, it's quite obvious, dropping the jack quickly will simulate the tires loading up under braking.

Increasing the tire pressure for hard cornering is basically to prevent the tire carcass from deforming from sideways load, too much deformation will result in the shoulder becoming the only contact patch which will then very quickly overheat.

"Those Kumhos have a UTQG of 700, that's practically plastic, not rubber." keep that up Woodie and I'll have to change my sig, oh I presume you mean you'll never buy anything over 400? most of my DOT road legal race tires are 40 or 50.
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orfeo
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Woodie
Jul 17 2015, 03:32 AM
Those Kumhos have a UTQG of 700, that's practically plastic, not rubber. Will probably last a long time, but don't expect much traction. I never buy any tire with a UTQG below 400, but that's just me.

On the best day the rears do about 20% of the braking. The rear adjusters are very well known failure points. How far up does your emergency brake handle go? Around 45° is normal, if you need to pull it up pointing at the roof, then your rear adjusters have failed and the rears are doing nothing.
Thanks for the good info. . . I have noticed over the years that I thought the e-brake was going higher and higher. Figured the cable had stretched or something like that. Never occurred to me that the rear adjusters could be the culprit. How do I check (and fix) the adjusters? Is it a simple swap out? Or is it very involved and difficult? I really appreciate your input
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Woodie
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Rear brakes need to be disassembled and the adjusters replaced. They've got very fine teeth on them, and once they strip the teeth they're done. Don't know if they're available anymore from the dealer, last time I bought one was ten years ago, think it was $22. Here's an old thread about the same thing, with pictures and part #s. http://geometroforum.com/topic/4571492 You may as well do the shoes and drums as long as you're in there, it's all got to come apart to get to the adjuster, and the parts are cheap. It's possible that new shoes will put the adjuster in a position that works again, but eventually it will walk it's way back to the bad spot and slip again.

Thanks to Dattman for catching my silly mistake. I never buy tires ABOVE 400 UTQG, try to stay in the 300's.
Edited by Woodie, Jul 18 2015, 03:38 AM.
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orfeo
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Great info Woodie, and great thread you linked to.

Question:

1. Does my '95 1.0 LSi hatchback have the same part shown in post #6 by snowfish in the thread you linked?

2. If the rear shoes are adjusted by the e-brake, does it also adjust by braking while backing-up the car?

Thank you for your knowledge and your patience.
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Woodie
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Yes, that's the part in question. Don't know what happened to the "unclean, disgusting" picture, it was there yesterday.

As for how it works, I really don't know for sure. I thought it just took up all the slack each time you pressed the brake pedal, but Snowfish says it adjusts when you use the parking brake. Seems if that was the case, then most automatics would never adjust, and we would have a lot more questions of this nature on here.
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Woodie
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There are people parting out cars all the time, check with them for new adjusters. Fogger and Johnny Mullet pop to mind right away:
http://geometroforum.com/topic/5784662
http://geometroforum.com/topic/5843252


Edited by Woodie, Jul 19 2015, 06:35 AM.
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Cardinal Grammeter
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Interesting thread.

Have noticed that while I need way more pedal pressure to lock the fronts than what the booster provides, when I do lock them, it is surprising how "poorly" the car stops.

My initial thought in this cases was that there were "crumbs" on the road - like you could almost hear them as the tires were sliding along.

I also thought there are aspects of small diameter tires that can come into play. For one thing, small diameter kills traction - just look at how well some "little" farm tractor with huge diameter wheels can pull out a stuck 4WD truck with tons of tread and width (but no diameter.) I read somewhere that NHTSA outlawed 12" wheels due to "safety" reasons.

Agree that while back brakes contribute little to overall braking, they get the braking level up to where you expect it. I drove a van around for I don't know how long with totally frozen rear wheel cylinders. I didn't notice it, but when I replaced them, the improvement was quite noticeable.

Edited by Cardinal Grammeter, Aug 7 2015, 04:58 PM.
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