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Old 12-31-2008, 12:00 PM   #886
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John is 100% correct about the roll center change.
While changing the arm angle does effect camber gain the biggest influence on handling will be through the roll center. 12th cars don't have much travel in the front suspension for camber gain to make much difference. I didn't even know that Yokomo had changed the front end of Naotos' car until after the race. Juho ran his front end stock the entire time.
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Old 12-31-2008, 12:23 PM   #887
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Originally Posted by John St.Amant View Post
I've been doing this for almost 30 years now and changing the ratio of the length or the upper arm and lower arms including the angle of the upper arm effects the ROLL CENTER. What ever effect you may have described is in fact a ROLL CENTER change. Raising the roll center make the wheel camber gain and lowering it does the opposite. And I AM John St.Amant thank you.
I may resemble your remark, but concidering the source....
remember some values are absolute and usually refered to as positive when it is actually and commonly not refered as a negative value.
ok, well in the future maybe you can voice a correction rather than being an @$$hole and jumping on people. it will probably be recieved differently my friend. Example, Crashby is one of my friends and rather than call him a moron I pm'd him and we had a discussion about roll center...
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Old 12-31-2008, 12:25 PM   #888
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Originally Posted by Sean Cochran View Post
John is 100% correct about the roll center change.
While changing the arm angle does effect camber gain the biggest influence on handling will be through the roll center. 12th cars don't have much travel in the front suspension for camber gain to make much difference. I didn't even know that Yokomo had changed the front end of Naotos' car until after the race. Juho ran his front end stock the entire time.
wasn't disputing that wasn't correct. I was just pointing out that you can correct someone or have a discussion without being rude.. it might be different if I called crashby an asshole or something because I also know him lol.. but he isn't a moron or an asshole.. hence why I was a lil confused as to why john was stirring the pot without conversating...
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Old 12-31-2008, 02:29 PM   #889
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Quote:
Originally Posted by John St.Amant View Post
I've been doing this for almost 30 years now and changing the ratio of the length or the upper arm and lower arms including the angle of the upper arm effects the ROLL CENTER. What ever effect you may have described is in fact a ROLL CENTER change. Raising the roll center make the wheel camber gain and lowering it does the opposite. And I AM John St.Amant thank you.
I may resemble your remark, but concidering the source....
remember some values are absolute and usually refered to as positive when it is actually and commonly not refered as a negative value.

First let me apologize for my remark, Mr. John St Amant. That was very immature of me to drop to that childish level. For two individuals to successfully communicate, one has to remain in the adult mode for successful communication.(Deltac Customer Relations) With that being said... The following is a quote from a Circle Track magazine article on roll centers and an interactive software that determines the roll center they where reviewing. I will not publish the whole article here on this forum as it is quite lengthily but I will attach the link so you can read the entire article at your leisure. Instead, I will only post the section about measuring the suspension points and what effect changing those points have on changing the roll center.

"It's important to look at the control-arm angles which will determine the roll center location height and width (both static and dynamic). To reposition the roll centers, we must change these control-arm angles. For example, if we increase the right upper control-arm angle and/or decrease the left upper control-arm angle, we will move the roll center to the right from its original position. If we increase both the upper control-arm angles, we increase the roll center height.

The program calculates the tire camber angles after the chassis dives and rolls, and the kingpin angles of our spindles. Making changes to the upper control-arm lengths affects the amount of tire camber change when the chassis dives and rolls."

As I said in my original post, changing the length of the upper control arm does not change the roll center. It only changes the camber gain. To change the roll centers you must change the angles of the control arms relative to a horizontal flat plain such as the ground or in the case of pan cars, the chassis.

Additionally, since we are sharing experiences, I started racing radio controlled cars in 1979 with an Associated 12E. From 1976 to 1981 in addition to racing RC cars, I was also a part of a Mazda race team. In 1978 we won the SCCA national championships in B sedan at Road Atlanta. Because of that win, Mazda gave us a full factory ride for 1979 and 1980 and gave us two new, full tube chassis RX7’s that where fabricated by Huffaker Engineering. We campaigned these cars in both C production in SCCA and selected IMSA endurance races in GTU (Daytona 24hrs, Sebring, Riverside Times Grand Prix, etc.). My main responsibilities on the team, among others, where for the drive train and rear suspension. I also did fabrication and the fuel catch can for long races.

Mr. Cochran says that the movement of the suspension on a 1/12th car is so little, we common racers would probably not feel the difference of a roll center change. He is correct and I have been told many times by fellow racers that the real world does not apply to toy cars but… I still say that basic geometry and physics apply across all worlds.

So being that we both have considerable knowledge about race cars, we should be able to have adult discussions about said subject. I look forward to future discussions with you.
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Old 12-31-2008, 02:31 PM   #890
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Quote:
Originally Posted by John St.Amant View Post
I've been doing this for almost 30 years now and changing the ratio of the length or the upper arm and lower arms including the angle of the upper arm effects the ROLL CENTER. What ever effect you may have described is in fact a ROLL CENTER change. Raising the roll center make the wheel camber gain and lowering it does the opposite. And I AM John St.Amant thank you.
I may resemble your remark, but concidering the source....
remember some values are absolute and usually refered to as positive when it is actually and commonly not refered as a negative value.

Oops. I forgot to post the link to the article:

http://www.circletrack.com/howto/181...ion/index.html
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Old 12-31-2008, 02:40 PM   #891
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No harm no foul On the other hand I didn't need to call you an idiot. But I guess I didn't feel I needed to type what I had said again. As poitely and suggestive as I could heve been without telling them what was what it was. If that makes any sence. No appologies necessary.
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Old 12-31-2008, 02:47 PM   #892
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Originally Posted by John St.Amant View Post
No harm no foul On the other hand I didn't need to call you an idiot. But I guess I didn't feel I needed to type what I had said again. As poitely and suggestive as I could heve been without telling them what was what it was. If that makes any sence. No appologies necessary.

Maybe we can hook up at one of the national level events, discuss suspensions over a beer, my treat and have a good laugh. We may just discover I am an idiot.
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Old 12-31-2008, 03:15 PM   #893
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Originally Posted by Crashby View Post
Maybe we can hook up at one of the national level events, discuss suspensions over a beer, my treat and have a good laugh. We may just discover I am an idiot.
I wouldn't call you an idiot. Miss guided? yes, idiot no.. lol..
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Old 12-31-2008, 03:58 PM   #894
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One of our local racers has managed to bend the motor mount on is RC125R. Has anyone else had this happen? These seem to be made from a relatively low grade aluminum. I am about to make some of these out of some much better 7050 grade aircraft stuff that we make Lockheeds JSF airplane from. If anyone is interested, send me a PM. I will post some pics when I have some finished.
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Old 12-31-2008, 03:59 PM   #895
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Can't we all just get along?

And for what it's worth you can't use a circle track article to show rollcenter change on a 12th car.

The circle track cars (except pony stock) all use double wishbone front suspensions.

Our 12th cars use a funky McPherson strut front end (it's actually upside down). The rollcenter calculation is totally different and involves the kingpin angle if I remember correctly.

As far as my opinion, the rollcenter change of extending the upper arm is probably about the same for handling as the camber change that comes with it.

John, be nice... Not everyone understands your personality.

Last edited by Fred_B; 12-31-2008 at 04:01 PM. Reason: fat fingers
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Old 12-31-2008, 04:25 PM   #896
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Originally Posted by Fred_B View Post
Can't we all just get along?

And for what it's worth you can't use a circle track article to show rollcenter change on a 12th car.

The circle track cars (except pony stock) all use double wishbone front suspensions.

Our 12th cars use a funky McPherson strut front end (it's actually upside down). The rollcenter calculation is totally different and involves the kingpin angle if I remember correctly.

As far as my opinion, the rollcenter change of extending the upper arm is probably about the same for handling as the camber change that comes with it.

John, be nice... Not everyone understands your personality.
While you are correct about the double wish bone suspension systems, the same holds true for McPherson strut front ends but only upside down as you suggest. It still requires the lower control arm angle be changed on a McPherson strut front end to adjust/move the roll center.

"Roll Centers
Every vehicle has a front roll center and rear roll center. They are independant from each other and are determined by suspension geometry.
The front roll center of a car with the Mustang's MacPherson strut suspension can be found as follows:



Draw a line at an angle of 90 degrees from the top of the front strut
Draw a second line through the lower control arm. The point where these lines intersect is the instantaneous center
Draw a third line from the instantaneous center to the center of the tire contact patch. The point where this third line crosses the car's centerline at the roll center.
Note in the diagram below how lowering the car by installing shorter springs changes the angle of the control arm and moves the roll center closer to (or even below) the ground.

http://www.miracerros.com/mustang/t_rollcenter.htm
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Old 12-31-2008, 05:08 PM   #897
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Yes, it's 90 degrees to the kingpin angle. This is affected by the static camber in our cars.

The whole point here is that it's not the same as a double wishbone.
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Old 01-01-2009, 07:26 AM   #898
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There are other factors as well. Not many people know about ackerman or the untimate steering point. Thats not an endpoint either. Maybe I've just stirred up a bit more internet surfing et Fred?!
Oh yeah ! Ummm beer sounds good Craig!
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Old 01-01-2009, 08:09 AM   #899
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Quote:
Originally Posted by teamtamale View Post
One of our local racers has managed to bend the motor mount on is RC125R. Has anyone else had this happen? These seem to be made from a relatively low grade aluminum. I am about to make some of these out of some much better 7050 grade aircraft stuff that we make Lockheeds JSF airplane from. If anyone is interested, send me a PM. I will post some pics when I have some finished.
Bent one myself. Not even sure when I did it but happened to look at the motor and notice it was skewed. I took at hammer to mine and bent it back into place so that I could race until the new one showed up.
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Old 01-01-2009, 09:44 AM   #900
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Hi all

Has anyone had an issue with their bodyshell rubbing the top corner of the motor pod where the side shock mounts? We mainly have to use the Parma Zytek or Hotbodies Reynard in the UK and even with the shell mounted level with the top of the chassis the corner of the motor pod touches at certain angles.
I tried to heat up a spoon to make a bubble in the shell but it went very wrong.

Any ideas please?

Thanks
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