17" Momo GT2 wheel holy grail

[Kjeld]

These are the wheels that came on the Momo version of the M100. I saw the wheels on Marie's car, and really love them. Spent 2 years trying to find a set. Finally got them, had them mounted the other day and they are sweet! Took off down a typical San Francisco street and hit a typical San Francisco pot hole. Nearly lost a tooth filling. The jarring is shocking. Other than the potential for blood in my urine, it seems this ride could rattle the car apart.

I am trying to remain brave. The wheels look sooooo great. Maybe I need to get down to 15psi.

[Doug]

cool -- it can't be that different? maybe you'll have to get a mouthgard then -- at least it help will keep your teeth from drying out from the perma-grin

[Kjeld]

I expected a more firm ride going from 16" to 17" + lower profile tires. But this is scary. Anyway, I am going to lower the psi and consider a whoopy cushion on the drivers seat.

[garymb]

These are the wheels that came on the Momo version of the M100. I saw the wheels on Marie's car, and really love them. Spent 2 years trying to find a set. Finally got them, had them mounted the other day and they are sweet! Took off down a typical San Francisco street and hit a typical San Francisco pot hole. Nearly lost a tooth filling. The jarring is shocking. Other than the potential for blood in my urine, it seems this ride could rattle the car apart.

I am trying to remain brave. The wheels look sooooo great. Maybe I need to get down to 15psi.

Could you post a photo?

[nitroman]

Other than the potential for blood in my urine, it seems this ride could rattle the car apart. The wheels look sooooo great. Maybe I need to get down to 15psi.

Kjeld, if the wheels also came in 16" and you had the chance to buy again, would you go 16" or 17"?

[Ekapol]

I expected a more firm ride going from 16" to 17" + lower profile tires. But this is scary....

Hi Kjeld,

I've just switched from 16" to 17". Expecting firm and a lot more bumpy ride, but not bumpy at all. The car just drives a lot more stable with even better control on the steering with lesser effort.

Mainly I think because now I have correct aspect ratio, previously I ran 225/45 on 16".

Now 205/40ZR-17.

Also the OZ wheels are stiffer with less meat on the tires, I can feel the suspension is doing it's job. 28 psi all around.

[tweetdriver63]

You don't necessarily need to go much lower profile on the tires. Since everybody's consensus seems to be that the speedo indicates about 10% faster than the car is really going (compare using a GPS unit), going slightly up on the overall diameter of the tires should make the speedo more accurate (you can use one of many online calculators to compare the effect of different size tires on the speedo), and allow you to keep a little more air between you and the pot-holes.

For example, if you go from 205/55-16 to 205/55-17, you increase the overall diameter by 3.87%, which will decrease the displayed speed on the speedo by 4.02%. Keep the same amount of air between you and the road, and bring your speedo a little closer to accuracy at the same time. Of course, you reduce the clearance inside the wheel well also, so you'd have to consider that.

Just a thought.

[johnny]

These are the wheels that came on the Momo version of the M100. I saw the wheels on Marie's car, and really love them. Spent 2 years trying to find a set. Finally got them, had them mounted the other day and they are sweet! Took off down a typical San Francisco street and hit a typical San Francisco pot hole. Nearly lost a tooth filling. The jarring is shocking. Other than the potential for blood in my urine, it seems this ride could rattle the car apart.

I am trying to remain brave. The wheels look sooooo great. Maybe I need to get down to 15psi.

Could you post a photo?

Yeah photo would be good

I had 17 Momo arrows once, they looked nice.

[L.G.]

Tweetdriver,

Changing the tyre size may affect several other aspects of the car. For example the 225-45 R16's mentioned are 3% different (bigger) than the original 205-45 R16's. One problem you may cause with your theory is ride height (ground clearance). The difference between the two I mentioned is 18 mm, which isn't huge, but considering the 130 mm original height it makes the car 14% higher up. That should be bad in terms of handling as it's always preferred to make it lower. In terms of potholes though you are right...

The other thing you are saying is changing tyres to make the speedo more accurate which is a dilemma. You know the speedo is wrong. If you change the tyres, you'll have both the speedo and tyres wrong so that will make it worse. Don't misunderstand I am not trying to prove you wrong, just my ideas. I think all speedos are wrong, my daily drive is about 7% wrong as well.

Hope I made some sense up there ,

L.G.

[ailwyn]

Just because the speedo reads 10% higher doesn't mean that the ECU receives a speed signal that is 10% higher than actual.

Changing the wheel radius to make the speedo read closer to the actual figure could have a detrimental effect on the ECU reading.

[tweetdriver63]

Just because the speedo reads 10% higher doesn't mean that the ECU receives a speed signal that is 10% higher than actual.

Changing the wheel radius to make the speedo read closer to the actual figure could have a detrimental effect on the ECU reading.

I wasn't suggesting he change the tire size to make the speedo more accurate. He said when he plussed up the wheel size, which requires a lower profile tire to maintain the proper overall diameter, he created a harsher ride, and might need to wear a mouthpiece. I just suggested that keeping the same profile tire to avoid the mouthpiece, would have the side effect of making the speedo more accurate. My understanding has always been that the speedo and odo are the main reasons you need to keep the overall wheel/tire diameter the same when you change wheel size.

Even if the speedo isn't getting a 10% high reading from the sensor, a bigger diameter will definitely result in a lower speed displayed, and that will always be true in any car, unless you recalibrate the speedo.

LG, your comparison of the 205/45-16 tire to the 225/45-16 tire is incorrect. The overall diameter of the 225 tire is 18mm bigger than the 205 tire, which would make the ride height 9mm higher, not 18mm. I can't imagine that would really have any detrimental effect under normal driving conditions. It might on the track, but most people that serious about their track days, probably change the tires/wheels when they track, rather than running on their street tires.

Now, just to help educate me, since I'm not an automotive engineer or anything, how does the vehicle speed actually affect what the ECU does? This is a serious question. Since I read your post, I've been trying to figure out the answer to that, and I can't come up with anything. I've tried some google searches, and gotten nothing. Everything I find says engine speed, throttle position, temperature, atmospheric pressure, oxygen sensors, etc. Nothing I find says vehicle speed has any affect.

[L.G.]

Oops my mistake Sorry about that 9 mm is correct. I agree when you drive normally you wouldn't spot the difference, I still won't do it though.

As for the speed sensor thing I don't know but my VSS isn't working at the moment and ECU does everything quite well; other than when you're going downhill and engine is in decompression.

Will be more careful next time

L.G.

[lotusgirlmarie]

Pic of the way my car used to look with the MOMO GT2's...

Very nayce

[ailwyn]

going slightly up on the overall diameter of the tires should make the speedo more accurate

I read this and it sounded like a recommendation, and it appears that I was not the only one to read it that way:

The other thing you are saying is changing tyres to make the speedo more accurate

Now, just to help educate me since I'm not an automotive engineer or anything

What's the sarcasm in aid of?

how does the vehicle speed actually affect what the ECU does?

No idea, but why would they fit a VSS as it only feeds the ECU if it wasn't required?

Perhaps it uses it as a fail-safe if some of the other sensors fail?

I was just pointing out that I'd rather put up with a speedo that over-reads than risk messing up the signal that is received by the ECU.

[nitroman]

Gents, I've not researched it toward finitum as you two have, but I've not ever gotten a reason why the ECU receives vehicle speed information. Perhaps it goes unused by the Elan's ECU(?). Certainly it does not seem to be critical information, as in the end it does not seem to upset any systems.

For instance, on every stability-controlled car, changing to a DIFFERENT F/R wheel/tire OD will cause the stability system to think one end of the car is losing traction, and the system will either cut power or apply brakes. Thankfully our Elan is not so automated. Obviously on cars with a preset top-speed the ECU will at the preset point cut either/both fuel or ignition. The Elan remains, in this regard, a free spirit!

Off your topic, but another new feature starting to appear is the electronic emergency brake, in lieu of the time-tested mechanical one. Supposedly this is to ensure you both apply & release it "on queue." However, what happens when your battery dies? Does your car suddenly become a careening two-ton projectile?!!

[addict]

the elan ecu does have a cutout speed ....you have just not reached it yet keep trying

[tweetdriver63]

Now, just to help educate me since I'm not an automotive engineer or anything

What's the sarcasm in aid of?

how does the vehicle speed actually affect what the ECU does?

No idea, but why would they fit a VSS as it only feeds the ECU if it wasn't required?

Perhaps it uses it as a fail-safe if some of the other sensors fail?

I was just pointing out that I'd rather put up with a speedo that over-reads than risk messing up the signal that is received by the ECU.

I wasn't trying to be sarcastic. I'm really not an engineer or expert, so I was actually hoping somebody would teach me something. I'm just using my own research and what I hope passes for common sense, where I know there are people here who really DO know what they're talking about.

Again, it wasn't so much a suggestion as an observation about one effect of a slightly larger overall diameter.

On to the VSS. Does the speedo actually get its signal from the VSS electronically, as opposed to mechanically from the speedo cable? I ask this simply because, coincidentally, my speedo decided to go on strike today. Maybe it's just mad at me for using the GPS. Anyway, so as not to totally hijack this thread, I've started a new one.

[nitroman]

the Elan ecu does have a cutout speed ....you have just not reached it yet keep trying

I have only read of (in the factory service manual) & experienced an RPM cutout. Please let me know what page of the manual the speed-cutout is mentioned and I'll believe you.

[Ekapol]

the Elan ecu does have a cutout speed ....you have just not reached it yet keep trying

I have only read of (in the factory service manual) & experienced an RPM cutout. Please let me know what page of the manual the speed-cutout is mentioned and I'll believe you.

Marie went up to 180 mph (on the track ) I guess she would have to try harder for the speed cutout.

[ailwyn]

The only speed cut-out I can think of is hitting the rev limiter in 5th, which would take some doing