Montreal roll center - Alfa Romeo Bulletin Board & Forums
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post #1 of 12 (permalink) Old 07-03-2019, 10:46 PM Thread Starter
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Montreal roll center

Does anyone have information about the front and rear car roll centers?
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post #2 of 12 (permalink) Old 07-04-2019, 05:07 AM
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Without special linkage, such as sliding block or displaced Watts, the rear roll center of a live rear axle is always at the center of the bell housing, exactly at the level of the axles.

Don P
Carson City, NV

Past Alfas...
59 102 Touring (first Alfa $500 running)
65 Sprint GT (2nd Alfa, $500 daily driver)
102 Sprint (never did anything with it, but wish I had)
74 Berlina (first new car - now certainly rusted into oblivion)
61 Giulietta Spider G-Prod Race Car (where is it now?)
84 Spider Veloce (rarely drove it, so sold it)
86 Quadrifoglio (Dull car - no more 115s for me)
1971 Montreal "The Full Monty". Fair winds and following seas

Current Alfas
59 102 Touring Roadster - restoration complete, enough Alfa for any rational man. Or irrational, for that matter
Oops. Add to the "present" list, 10204 01488, 2000 Touring Roadster project

And past...
Two 1946 Stampe SV4C (c/n 294 "Rocinante" - wife's favorite airplane. RIP), and c/n 235 "La Bon Temps Femme" (gone to a new home, but never forgotten)
Zlin 50LA (+9 -6 gees, titanium spar, 1200 lbs, 260HP rumored to now be in Brazil)
1946 Luscombe 8A
Starduster Too (recently spotted at the Nevada City, CA airport - over 20 years and an entire continent separating it from our stewardship in Binghamton, NY)
1955 Cessna 170B (wife taught me to fly tailwheel in this)

And present...
64 Mooney M20E ("Rambo". My faithful steed for over 40 years) Over 55 years old, and just returned from a trip to Argentina in him
Newest in the fleet
1967 Piper Super Cub on Wipline amphibious floats (a true "all terrain vehicle")
2010 Triumph Thunderbird


You can snap roll an Alfa only one time...
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post #3 of 12 (permalink) Old 07-04-2019, 05:47 AM
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Since a Montreal chassis is basically that of a 105, handling comments should be similar:

I respectfully suggest the rear roll center is at the point where the T-Arm attaches to the differential; several inches higher than the center of the bell housing. Then, the front roll center is considerably lower resulting in a roll axis tilted downward toward the front.

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Originally Posted by DPeterson3 View Post
Without special linkage, such as sliding block or displaced Watts, the rear roll center of a live rear axle is always at the center of the bell housing, exactly at the level of the axles.
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1976 Spider (Dedicated Autocrosser, "SPICA, No Carbs")
1991 Spider Veloce (Retirement cruiser)
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post #4 of 12 (permalink) Old 07-04-2019, 06:54 AM
Richard Jemison
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Roll Center

George is correct.

In fact the front roll center on the 105/115 chassis as is on the Montreal the roll center is right at ground level, and cars lowered by using shorter springs are typically below ground level.

Both front and rear can be raised significantly with correct modifications.

Richard Jemison
RJR Racing

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post #5 of 12 (permalink) Old 07-04-2019, 07:03 AM
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Ah... the above is correct. I was referring to a generic live axle. The “tee arm” is a roll-Center displacing element, much like Watts or Slider.

Alfas took a road less traveled with their tee-bar, moving the RC upward. The classic method to achieve superior predictable handling is to have low roll centers front and rear. However, this requires either or both stiff springs and/or roll bars. The high center in the rear allows soft rear springs for a pleasant street ride while minimizing body roll, even without an anti-sway bar.

Don P
Carson City, NV

Past Alfas...
59 102 Touring (first Alfa $500 running)
65 Sprint GT (2nd Alfa, $500 daily driver)
102 Sprint (never did anything with it, but wish I had)
74 Berlina (first new car - now certainly rusted into oblivion)
61 Giulietta Spider G-Prod Race Car (where is it now?)
84 Spider Veloce (rarely drove it, so sold it)
86 Quadrifoglio (Dull car - no more 115s for me)
1971 Montreal "The Full Monty". Fair winds and following seas

Current Alfas
59 102 Touring Roadster - restoration complete, enough Alfa for any rational man. Or irrational, for that matter
Oops. Add to the "present" list, 10204 01488, 2000 Touring Roadster project

And past...
Two 1946 Stampe SV4C (c/n 294 "Rocinante" - wife's favorite airplane. RIP), and c/n 235 "La Bon Temps Femme" (gone to a new home, but never forgotten)
Zlin 50LA (+9 -6 gees, titanium spar, 1200 lbs, 260HP rumored to now be in Brazil)
1946 Luscombe 8A
Starduster Too (recently spotted at the Nevada City, CA airport - over 20 years and an entire continent separating it from our stewardship in Binghamton, NY)
1955 Cessna 170B (wife taught me to fly tailwheel in this)

And present...
64 Mooney M20E ("Rambo". My faithful steed for over 40 years) Over 55 years old, and just returned from a trip to Argentina in him
Newest in the fleet
1967 Piper Super Cub on Wipline amphibious floats (a true "all terrain vehicle")
2010 Triumph Thunderbird


You can snap roll an Alfa only one time...
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post #6 of 12 (permalink) Old 07-04-2019, 05:52 PM Thread Starter
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Roll Center

Ok guys thank you very much! Understood the full concept of an inclined roll axle, low front and up rear. Moving forward - does anyone knows the average weight in each wheel? And how about the approximate CG position? Is it in the firewall section or in the front seat position?
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post #7 of 12 (permalink) Old 07-04-2019, 07:46 PM
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Why would you even think about raising the rear roll center?

Quote:
Originally Posted by Alfar7 View Post
George is correct.

In fact the front roll center on the 105/115 chassis as is on the Montreal the roll center is right at ground level, and cars lowered by using shorter springs are typically below ground level.

Both front and rear can be raised significantly with correct modifications.
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post #8 of 12 (permalink) Old 07-04-2019, 08:42 PM
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Quote:
Originally Posted by DPeterson3 View Post
Ah... the above is correct. I was referring to a generic live axle. The “tee arm” is a roll-Center displacing element, much like Watts or Slider.

Alfas took a road less traveled with their tee-bar, moving the RC upward. The classic method to achieve superior predictable handling is to have low roll centers front and rear. However, this requires either or both stiff springs and/or roll bars. The high center in the rear allows soft rear springs for a pleasant street ride while minimizing body roll, even without an anti-sway bar.
Stiffer sway bars emulate raising the roll center-increasing weight transfer to the other end of the car. The upper arm Alfa uses at the rear was OK back in the 50s. It's a real hindrance to good handling with current tires and increased engine power.
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post #9 of 12 (permalink) Old 07-05-2019, 06:02 AM
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Montreal #8787, a non-air conditioned car. Weighed with full fuel tank, spare tire and toolkit. Total weight 2,873 lbs (2,773 net of fuel). LF-802, RF-807, LR-627, RR-637.
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post #10 of 12 (permalink) Old 07-05-2019, 06:14 AM Thread Starter
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Roll Center

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Originally Posted by DWR46 View Post
Montreal #8787, a non-air conditioned car. Weighed with full fuel tank, spare tire and toolkit. Total weight 2,873 lbs (2,773 net of fuel). LF-802, RF-807, LR-627, RR-637.

That´s it. Thank you!
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post #11 of 12 (permalink) Old 07-05-2019, 06:58 AM
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Roll centers

Quote:
Why would you even think about raising the rear roll center?
You wouldn`t. I should have used "changed or improved" rather than raised relative to the rear.

The target is roll centers front and rear that are close to the same so tire loads have less rear to front weight transfer during cornering to improve lateral grip.

Typically race prep`d (done correctly) has the rear roll center around the center to botton of the axle tubes, easy to achieve with a Panhard set up. The old Sliding Block wasn`t as effective, read Jon Norman`s article in Alfa Owner earlier this year on handling with the Sliding Block Vs a Panhard on the same GTA.

Richard Jemison
RJR Racing

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Last edited by Alfar7; 07-06-2019 at 07:44 AM.
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post #12 of 12 (permalink) Old 07-06-2019, 07:19 AM Thread Starter
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Roll centers

I was looking at a youtube video of a Zagato Alfa where the owner figured out the roll center. Nice vídeo. Also I was looking at my GTV 1750 under restoration rear suspension which is very similar to the Montreal. After driving my Montreal about 100 miles since restoration, mainly in urban streets, I noticed there is something so bad in terms of cornering. It seems I will loose the entire rear axle. The GTV 1750 does not have this effect. Why such big difference between the two cars? Or there is something wrong with my rear suspension? It´s all stock.
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