16 replies to this topic

[gtup1]

I installed a set of Shelby 20 inch rims and now my car feels really unstable at speeds. Normal cruise feels just fine but if I'm running it and hit 100+ it feels like its going to come of the road. As you all know with the factory 18's it felt stabile and inspired confidence. What could be wrong? I have not changed anything else in the suspension.
Any of you handling gurus have any ideas you can share?

[ChicagoChris]

Aside from the obvious concerns of front end alighment or bushings, I would do a Rim Out test where the dealer verifys the rims are balanced.

[shelbymotorsports]

While I would not call myself a handling guru, I will say that what you are describing is unusual. Many SGT owners have upgraded to 20" wheels. If you would provide more info, like wheel style & offset along with the size & brand of tires perhaps we could figure out whats going on.

Steve

[gtup1]

shelbymotorsports, on 13 December 2012 - 08:19 AM, said:

While I would not call myself a handling guru, I will say that what you are describing is unusual. Many SGT owners have upgraded to 20" wheels. If you would provide more info, like wheel style & offset along with the size & brand of tires perhaps we could figure out whats going on.

Steve

CS66 Rims with Sumitoma HTRZ's staggered fit 245 front 275 rear. Not sure of the back space or offset. I assume whatever is standard for Shelby wheels. They do fit the big brake kit, that's why I installed them to clear the rotors and calipers.
The car feels fine until I hit that triple digit mark. No wander, unless there's a really bad crown on the road. Feels very stabile at normal speeds. The only other thing I noticed but just assumed was normal when going from 18's to 20's is very quick turn in compared to stock.
Hope that info helps, I appreciate the input. I don't hit triple digits often but when and if I do I don't want to feel like I'm taking my life into my hands. Of course maybe I am but I try to do it on the track not on the street.  

Thanks

[gtup1]

ChicagoChris, on 13 December 2012 - 08:18 AM, said:

Aside from the obvious concerns of front end alighment or bushings, I would do a Rim Out test where the dealer verifys the rims are balanced.

Thanks Chris, I haven't thought about balance because I have no vibration or wobble in the steering wheel. I may try that along with alignment. I did not realign when I went to the new wheels and you can tell when the wheel is turned Caster Camber plates would probably flatten out the tire contact patch. I didn't do anything because it seemed to drive fine under normal conditions. I don't hit triple digits often and the tires and rims have been on for probably 6 months before I ever hit those speeds to notice it.

[Albino500]

Couple of things to check - Wheel balance and load force variation ( a good tire shop with a Hunter GSP9700 balancer should be able to handle that ) , Check the wheel for being "hubcentric" in other words - is the hole size on the wheel the same as the hub on the axle - if it is larger, then the wheel may not be centered to the axle centerline - you may have to use a hub adapter ring ( not a wheel spacer ) , there are assembly clips on the rotors make sure that these have been removed so the wheel is sitting flush to the rotor. Also , vibration felt through the steering wheel is usually from a front end issue , a vibration felt through the body ( floor/seat ) usually indicates an issue from the rear axle/wheels/driveline.

[07SGT]

Does the 20 inch wheel and tire with shorter side wall have any effect on handling and ride?  There is less side wall to flex.  Thoughts?

[carnut12]

I have had 20's for years and no issues what so ever, I do not hit over 100 often, but have hit 130-140 with these and they were as smooth as can be.

Something is wrong, it is not because they are 20" wheels

[slygray1]

I think the main reason some of the folks go to 20" wheels is they like the looks of them. I can't imagine the handling would be better with less side. wall. I just installed a new set of Shelby Razors eand stuck with the 18"s. You don't see nascar running around on skinny sidewalls. Just my thoughts. There are probably others that will disagree.

[Albino500]

07SGT, on 13 December 2012 - 02:53 PM, said:

Does the 20 inch wheel and tire with shorter side wall have any effect on handling and ride?  There is less side wall to flex.  Thoughts?

Ride quality is much harsher with a shorter sidewall and because it is shorter and does not flex as much you will not need as much negative camber to compensate for tire roll over ( other factors are involved with concerns pending the amount of grip )

carnut12, on 13 December 2012 - 04:03 PM, said:

I have had 20's for years and no issues what so ever, I do not hit over 100 often, but have hit 130-140 with these and they were as smooth as can be.

Something is wrong, it is not because they are 20" wheels

Agreed

[RACERX009]

Never had any problems with 20's at any speed as far as handling  no issues on any of my past cars M3 20's handled like an animal

[RACERX009]

Albino500, on 14 December 2012 - 07:13 PM, said:

Ride quality is much harsher with a shorter sidewall and because it is shorter and does not flex as much you will not need as much negative camber to compensate for tire roll over ( other factors are involved with concerns pending the amount of grip )
Agreed

My ride quality on 20's is not harsh and thats with 255/35/20 front and 275/35/20 you do feel small bumps but highway its a nice ride

[JeffJ]

I've been 165 on my 20's.  I would lean more to the tires then the wheel size as far as feel.  Ride harshness on the street will certainly be increased with the 20's, but handling at 100 mph should not be effected.  I do not have any experience with the Sumitoma's, but the construction of the tire can make a huge difference in high speed handling.  I think there has been some good advise given above as far as making sure everything is in balance.  These Mustangs are designed to be lug centric and I'm sure your wheels have divits in them for the lug holes and also coned lug bolts to make sure that the wheel is properly located.  That is one thing to verify that I didn't see mentioned above, are the lugs properly torqued?  I would imagine yes, but just did not see that mentioned.  20's have nothing to do with high speed handling.  It's all in the tire and suspension design.  Somebody above said that you don't see Nascar running skinny sidewalls, well in Cup, that is true, but the Nascar road racing cars (Grand-Am Rolex) do run skinny side walls with 20" rims and run over 170 mph.  It's all in the tire construction.

P.S. on the street I run BFG GForce 20's., on the track, I run BFG R1's with 18" wheels.  More because 18" race tires and wheels are way cheaper then 20" combinations.

Edited by JeffJ, 21 December 2012 - 04:25 PM.

[Jer]

JeffJ, on 21 December 2012 - 04:21 PM, said:

on the track, I run BFG R1's with 18" wheels.  More because 18" race tires and wheels are way cheaper then 20" combinations.

Even on his rental cars... cough, cough... right, Jeff?


Jer

[JeffJ]

Jer, on 21 December 2012 - 06:04 PM, said:

Even on his rental cars... cough, cough... right, Jeff?


Jer

I know nothing.... I wasn't there.......  It wasn't me......

[Albino500]

JeffJ rent a car ? Really ? LOL . A shorter sidewall ( in reference to aspect ratio ) will enhance the feel of turn in but toe setting effect it more as far as corner entry . One thing to mention is that when changing tire/wheel combinations or sizes ( and use of wheel spacers ) that one has to be aware of the effect that it can create due to the change in scrub radius in which stability is subject to change .

Edited by Albino500, 21 December 2012 - 06:29 PM.

[Jer]

JeffJ, on 21 December 2012 - 06:22 PM, said:

I know nothing.... I wasn't there.......  It wasn't me......

Yeah, the Terlingua sticker didn't fool anyone....LOL... but it was COOL.


Jer