Ride comfort: IRS w/17's vs. 3 link w/15's

[Real Time]

What do you guys think?

I've got several different scenarios bouncing around in my head.

Would anyone be able to give a comparison of ride comfort for IRS w/17's vs. 3 link w/15's ?

I've got some time before my Jan build date so trying to makes some good use of it now...lol

Thanks!

[GTBradley]

The only input I can contribute is that with the IRS and 17s the ride is firm but never jarring. I consider my suspension choices to be a success because my wife has never complained about the ride comfort and she would if it were at all uncomfortable.

[CraigS]

I have had both but they were 10 years apart. Overall I'd say they are similar. But the 15 in restrictions on brakes and tire choices I can't handle so would never even consider 15s.

[David Williamson]

My Coupe with 18's and IRS rides a little harder than my BMW 335 with sport suspension and the Mkll with 17's and 3 link is much harsher. I am with Craig, I would never go with 15's because of the brake and tire limits with the small wheels. Some like the look af 15's but for me I want the performance of the better tires that are available in 17 & 18 inches.
if ride is a concern go IRS, the difference is huge, much bigger than I expected when I built the Coupe with IRS.
David W

[J R Jones]

RT, IMO it comes down to mass (which is similar, sprung and unsprung) spring rate/wheel rate and damping which all contribute to adsorbing road bumps or transfering road bumps into the chassis and passengers..
Not as obvious is that the tire has a spring rate. The 15s likely have a larger aspect ratio or cross section that allows them to deflect on bumps and adsorb some of the displacement.

When the tire has deflected the additional displacement is resisted by the unsprung mass, the springs and dampers.
Less unsprung mass displaces more easilly that greater unsprung mass.

The higher rate of the springs and the stiffer (jounce) of the shock damping, the less displacement of the wheel/tire and the more the chassis and passengers are lifted.

With the variables I listed here, you can tune IRS and live axle for a soft ride or a harsh ride.
15 in (tall) sidewalls will always ride better than 17 in (short) sidewalls.
jim

[Scott Zackowski]

IMHO ride has a lot to do with your shock adjustments.
I have 15's, 3-Link, and Avon Tires. Now in general IRS will be a smoother ride.
My ride smoothed out significantly after adding rear duel adjusting shocks and adjustable rear LC's (from Breeze).

I have not driven an IRS Roadster, but I have driven 3-Link's with 15 and 17 inch wheels and my set up is significantly smoother. Plus with dual adjustable shocks you can dial them in for comfort and for track.

[edwardb]

I've had solid axle and IRS. Both Roadster and Coupe. No question the IRS is the better ride. I haven't had 15" tires, so can't comment about those. I've had 17's and now 18's. No complaints. The ride is nice and everyone who rides with me comments about the good ride. Stiff for sure, but compliant and not harsh. I've had both the regular red Koni's and the dual action Koni's. I agree the dual ones seem to offer a slightly better ride plus the easier adjustment.

[CraigS]

One other variable is spring rate. I believe the FFR supplied springs for the 3 link are 350#. Personally I prefer 250# which was the standard # back in the day. Point is that springs are relatively cheap $100-150 a pair so experimenting w/ rate isn't all that terrible.

[GoDadGo]

I have no experience with 15" wheels on a Factory Five.

My friend Donny has 18" with the IRS and his MK-4 rides significantly better than my car which sports the 3-Link and 17" wheels...We run similar air pressures (22-ish) and hands down his car rides much nicer; however, he's running no sway bars and I'm running a 3/4" bar on the front only...Neither car has much body roll in the corners so the big takeaway for me is the 3-Link better at planting the tires under hard acceleration than the IRS because he's riding on 315 NT-01's and I'm on 285 Nitto 555's... I can pull away from him when playing the stop light game in spite of my harder tire compound...Engine displacement and horse power are similar (FE 396) vs (SBC 383)

If you want a better ride quality, then definitely consider going with the IRS...It will also help your resale value.

Good Luck & Happy Wrenching!

[RFracer]

I’ve had a 4 link MKI 15”, 3 link MKII 17” Roadster Gen I coupe IRS 17” and now roadster IRS 15”. 15” is the best ride. Not much comfort difference between 3 link & IRS from my experience. The 17’s were stiff to hard. I’m aware of the limited tire options with 15” but still prefer the ride and classic look. Lots of guys put big brake packages on and these cars. At 2,400# the stock Mustang brakes are overkill for their design intent.

[KDubU]

I had 3-link and 15”s and the ride was fine except for the I495 around Boston. The sections that meet any overpass suck and sometimes would literally move the backend and at 70mph, that is a tad scary. Overall though it was very comfortable with any other roads and I enjoyed it.

[CraigS]

I know this thread is about ride but also consider costs. 3 link can be the least expensive if you find an old solid axle and then change it to get disc brakes and a better gear ratio. It becomes quite a bit more expensive if you buy it new from FFR. For the IRS you can save quite a bit by buying a takeout complete rear suspension from a salvage yard. If you be sure to get one that includes the rear brakes you get those brakes pretty much for free. They are excellent brakes so there is an upgrade you don't need to buy.

[J R Jones]

I know this thread is about ride but also consider costs. 3 link can be the least expensive if you find an old solid axle and then change it to get disc brakes and a better gear ratio. It becomes quite a bit more expensive if you buy it new from FFR. For the IRS you can save quite a bit by buying a takeout complete rear suspension from a salvage yard. If you be sure to get one that includes the rear brakes you get those brakes pretty much for free. They are excellent brakes so there is an upgrade you don't need to buy.

If one is using salvage yard parts, consider a national internet search engine if you are in the rust belt as I am. I do not use rust belt salvage brakes, and even other rusty parts are too much trouble to recondition for a specialty vehicle. Car-part.com is a good source but not all part providers are willing to ship. I also avoid donor vehicles from the rust belt, disassembly can be problematic, as is divesting the left-overs. Shipping does add to cost.
jim