are new tires causing my rear to act squirrely at speed? - Page 2 - Toyota GR86, 86, FR-S and Subaru BRZ Forum & Owners Community

Razorlor · 07-17-2018, 04:51 PM

As mentioned many times already, sidewall stiffness greatly dictates how the car feels.

If the rear tires have a little more give than the fronts, it will feel as though the rear is sort of floating around. Its a weird sensation, but i have seen it on many cars that I have serviced.

TOPEC · 07-18-2018, 03:24 AM

how many miles do u have on the new tires? dont forget brand new tires need a bit of wearing down before it gives max grip as the top layer is usually greasy from the tire mold.

re-animator · 07-18-2018, 02:48 PM

Quote:

Originally Posted by TOPEC

how many miles do u have on the new tires? dont forget brand new tires need a bit of wearing down before it gives max grip as the top layer is usually greasy from the tire mold.

a few hundred at this point.

i did check the tire pressures and the rears were just a touch under inflated. When moderately warm the fronts were about 35 and the rears were only at 30. I evened it up, but the traction control was still pretty active re: liftoff oversteer this morning, and the rear still felt pretty floaty at freeway speeds.

re-animator · 07-19-2018, 12:55 PM

i've got a new theory:

The previous tires had worn into the ~1-1.5 degrees of negative camber in the back. With newer tires without any wear, there is less of a contact patch. That is also why it is more noticeable at high speeds, because the contact patch is greater off the line or at low speeds. When the car is at 50-70mph there is less weight over the back end so the inner part of the tire has less contact than when it is at low speed. When there is weight shift due to liftoff or anything else that compresses the sidewalls of the front michelins, the effect is made greater. Additionally, because I'm lowered on stock shocks, there is less shock travel than a properly damped suspension, which can also lead to loss of grip. The tires are at roughly ~500 miles or so right now, which is probably enough to wear down the grease that new tires have, but not enough time for the innermost part of the tire to wear into the camber.

Does this sound right? If that's the case the best thing to do would be just give it a little more time or zero out the camber which i'm not going to do. Obviously I will still be upgrading the remaining tires and getting an alignment done soon.

07-17-2018, 04:51 PM	#15
Razorlor Member Join Date: Dec 2017 Drives: 2017 BRZ Limited, Perf Package Location: STL Posts: 50 Thanks: 5 Thanked 13 Times in 8 Posts Mentioned: 1 Post(s) Tagged: 0 Thread(s)	As mentioned many times already, sidewall stiffness greatly dictates how the car feels. If the rear tires have a little more give than the fronts, it will feel as though the rear is sort of floating around. Its a weird sensation, but i have seen it on many cars that I have serviced. Last edited by Razorlor; 07-17-2018 at 04:52 PM. Reason: Grammar

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07-18-2018, 03:24 AM	#16
TOPEC Member Join Date: Dec 2017 Drives: 2013 Scion FR-S Location: Vancouver B.C. Posts: 10 Thanks: 0 Thanked 7 Times in 5 Posts Mentioned: 0 Post(s) Tagged: 0 Thread(s)	how many miles do u have on the new tires? dont forget brand new tires need a bit of wearing down before it gives max grip as the top layer is usually greasy from the tire mold.

07-19-2018, 12:55 PM	#18
re-animator Senior Member Join Date: Jul 2017 Drives: 2013 BRZ Location: San Diego Posts: 213 Thanks: 135 Thanked 83 Times in 45 Posts Mentioned: 2 Post(s) Tagged: 0 Thread(s)	i've got a new theory: The previous tires had worn into the ~1-1.5 degrees of negative camber in the back. With newer tires without any wear, there is less of a contact patch. That is also why it is more noticeable at high speeds, because the contact patch is greater off the line or at low speeds. When the car is at 50-70mph there is less weight over the back end so the inner part of the tire has less contact than when it is at low speed. When there is weight shift due to liftoff or anything else that compresses the sidewalls of the front michelins, the effect is made greater. Additionally, because I'm lowered on stock shocks, there is less shock travel than a properly damped suspension, which can also lead to loss of grip. The tires are at roughly ~500 miles or so right now, which is probably enough to wear down the grease that new tires have, but not enough time for the innermost part of the tire to wear into the camber. Does this sound right? If that's the case the best thing to do would be just give it a little more time or zero out the camber which i'm not going to do. Obviously I will still be upgrading the remaining tires and getting an alignment done soon.

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