FE Power Forums
FE Power Forums => Non-FE Discussion Forum => Topic started by: Tom Gahman on October 21, 2021, 06:17:06 PM
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What would be an allowable difference in tire circumference?
My car has a Detroit locker and its not happy with the new American racer tires I put on the back.
At first I thought something came loose or broke, but nothing did. It would power steer on throttle and lurch the other way when I let off. I put a set of BF Goodrich on and problem solved. I remeasured the tires tonight and the difference seems less than before. (heavy 1/4-3/8 now ). I don't know, maybe I'll try them again. I've been working on a cooling issue with my new engine and I haven't been able to dial in the tune or other details till the cooling issue is fixed. Thanks for any advice or suggestions.
Tom
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Detroit lockers are notorious for exactly what you describe.
Tires have no effect on this issue, in my experience.
Street cars don't need a locker, limited slip is the best for street cars.
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I've had the locker over 25 years and I am aware of their quirks and the car is more strip than street.This is the first time that it was so pronounced and I was thinking the difference in circumference might be the cause. I guess I'll measure some other sets of tires and see what happens. Thanks
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maybe play with off setting the air pressure between the two tires a bit.
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....... play with off setting the air pressure between the two tires.......
Yes, this, and with tires differing in diameter also swap left for right and see what happens; as cars don't always carry their weight evenly distributed as applied to the tires. ;)
In the old days, a good tire store with capable attendants understood that tires (particularly 'back then') were not all exactly the same size (even if indicated as such on the sidewall ::) ) and you could explain your concerns/requirements (oh, BTW I have a Detroit-Locker! 8) ) and they would mount and air a tire and then with a handy dress-makers (measuring) tape (yeah, see you can find one in a tire shop today! ??? ) measure the circumference, making a comparison to your other mounted tire. If the tires were unreasonably different in dimension, and being of experience, the attendant would suspect which tire was further from the norm, and swap it for another, marking the less desirable unit as + or - for next time, or just for a less demanding customer. :)
But, everything is so much better now, now that the internet IS the smart place to buy stuff, cheap; and the local quality brick and mortar store is closed up, leaving only the national chain stores, operating a such low margins that nobody works there who knows which end is up! Next time just ask one of the tire changers to check the overall diameter of two tires, the response will probably be: "why would you need to do that?", and then: "how would you do it?"; and then imagine the look when you tried to explain what a dress-makers tape was? ::)
Scott.
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Interesting problem. I have only read about the lockers wanting to change lanes on their own but never had any real life examples. I'd compensate with air pressure if possible.
I'm thinking about the Detroit Locker or Truetrac. The latter is more money but perhaps worth it. I want to avoid clutch based differentials if possible.
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This is all I could find in the Eaton manual
"Maximum delta wheel speed 50 rpm for
engagement"
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Thanks everyone, I have it down to a 5/16 difference with a 5 lb. air adjustment.I have way to many life chores at this time, but I'll do some more testing this weekend. The weather is iffy though, I'll swap side to side also. If this doesn't work out I'll call the man I bought them from and see if I can take a wheel up and mount and measure tires and see if we can match up a couple. Thanks again I am looking up the calculator for wheel speed to see where I'm at for the delta.
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This site has a couple calculators that are impressive with good visuals.
https://www.blocklayer.com/tire-size-calculator.aspx
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Gaugster, thanks for that site, I adjusted the circ. to a difference like my tires and it said it would be the same mph. Another learning experience.
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I have a 31 spline Detroit Locker in my 59s 9", unfortunately, there are no good clutch type limited slips available for a 9".As the rear pinion support bearing takes up too much room, so unlike most other rearends, a 9" Traction Lok only has a clutch pack on 1 side, and even so, that side has fewer clutch plates than the clutch style limited slips most other rear ends utilize. Plus, to squeeze those few clutches in the 1 side of the carrier, Ford had to carve away a lot of material, so cracked side covers have also been a major concern with Traction Loks. They are borderline OK on a street only car, but a poor choice for street strip cars, especially with a stick trans and slicks. I have not tried a Tru Trac, but my understanding is those small gears inside are not very durable in high shock load situations, like a stick car on slicks. So it seems like for a 9", the only decent choices for a street strip stick car that gets launched hard, is a Detroit Locker, and a spool is the best choice for a drag only car. Yes, the Detroit Locker has some quirks, like the occasional snapping or clunking , and will tend to pull to one side if the unit is not fully engaged, or the tire diameters don`t match. But if want something strong, that will 100% lock up both tires every time, the Detroit Locker is it.
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Never to old to learn something new. I did not know tire circumference variation could cause a Detroit locker to act up. Thanks Jay, this site is great.
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I put the tires back on last night and was able to take it out today, big difference!
I don't know what the difference is but its acting like it used to (much better, predictable and controllable)
Maybe the tire wasn't seated fully. Also when the mounted tires were in my shop the shape from side to side didn't look right or even, but last night when I went to put them back on, they looked more even.
The tires are lite weight stock car DOT tires and when they store them they get all squished out of shape.
Maybe they just needed some heat, pressure or time to get where they needed to be.
I did fine tune one tire with a 5 lb. adjustment. When they were first mounted , tire pressure couldn't get them close.
Thanks again for all the input and anything else anyone has to add.
Tom
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i drive my hotrod a lot, in the winter also.i noticed in cold weather when i get my car out the first few miles my street slicks act up till they get some heat in them.i have took off many times wondering if something was wrong back there
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Related 21st century problem tire story - friend was a master Ford mechanic at a local Lincoln dealer (since retired). They had a Lincoln come in with an intermittent problem. At some speed, the car would start trying to shut it's self down, started acting really weird. A test drive confirmed the issue. After a few phone calls to Ford and some head scratching they finally figured out that one of the rear tires was a little different. The owner had lost a tire on a recent trip and had to buy what was available. The slightly smaller tire diameter at some forward speed - around 65 or so IIRC - set off the traction control system, tricking it into thinking the car was in a skid so it started applying the brakes to slow and control the "skid".
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I sounds like another good reason to stick with FE's and avoid the newfangled stuff. I was speaking wih a guy who crashed his new Chevy... or it crashed itself for him. Going across the bridge there is a utility trailer with a tarp flapping slowing ahead of him. He changes lanes and the tarp comes off and flies across in front of him and over the railing into the river... his truck automatically nails the brakes hard sending his Tim Horton's coffee first all over the dash then all over him when he gets rear ended by a gravel truck. Not really the fault of the gravel truck driver and the insurance company is still working on who to ding for the eighty grand "worth" of pickup truck.
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Related 21st century problem tire story - friend was a master Ford mechanic at a local Lincoln dealer (since retired). They had a Lincoln come in with an intermittent problem. At some speed, the car would start trying to shut it's self down, started acting really weird. A test drive confirmed the issue. After a few phone calls to Ford and some head scratching they finally figured out that one of the rear tires was a little different. The owner had lost a tire on a recent trip and had to buy what was available. The slightly smaller tire diameter at some forward speed - around 65 or so IIRC - set off the traction control system, tricking it into thinking the car was in a skid so it started applying the brakes to slow and control the "skid".
Great story Chris.
I've been told that one should replace all 4 tires at once with all-wheel drive cars, like my '18 Audi S5 turbo car for the same reasons: mismatched O.D.'s.
Seems with fast cars and guys that burn up the rears (mainly RWD versions) the sensors can be tricked since the new ones in the rear have a bigger O.D. than the older fronts. Same goes for running a mini-spare for any distance where the mini is a lot shorter than the other 3 tires.