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Old 11-23-2006, 07:09 PM   #586
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Ok i have only just noticed that the lower arm is fixed. Also I assume that
the spring on the bottom of the hub is supporting the car on the front.

That would then make it a virtual upside down strut.

Finding the roll centre then would be as follows. Make a right angle from the strut (what the hub slides on in the lower arm.) at the point that it is supported inside the lower 'arm'. intercect this line with the upper control arm to get the instant centre and you know the rest.

It appears that this provides a roll centre change that i have not seen before in independant suspension. As the chassis rolls the outside (loaded) wheel gains roll centre hieght and the inside (unloading) wheel loses roll centre hieght which would resist tendancies further roll as it rolls (or rising rate roll stiffness due to roll centre).

Every double a arm setup I have seen does the opposite and promotes body roll as it rolls (falling rate roll stiffness due to roll centre).

As far as I am concerned your last statement is correct.

I was just re - reading this last page and what Pro 10 Holland said seems to re-inforce my theory that perhaps the tyres are too wide for the wieght of the cars. I don't know much about running foam tyres, but maybe you could run a very low profile on the wheel (less tyre to get warm for the engergy input) or if diameter is of concern run higher density foam over the wheel then glue the tyre compound foam over that. just thinking. In V8SC the tread depth (ie sticky rubber) of a slick is only 4mm from new. scale that to 1/10 and you have .4mm which isnt vey thick at all. so maybe if you tried geting the desired diameter with a harder foam over the wheel and then applying say even a 1mm layer of tyre foam (ie sticky stuff) over the top. More than likely though easier said than done.
josh

Edit - just too give you an Idea at a V8 round earlier in the year we had qualifying at 9.00am and track conditions were about 4deg celcius (fucking cold for Australia). we were faster on old tyres with about 2.5mm of tread depth than new (green) tyres, suggesting that the lower tread depth was warming up more due to there being less mass to heat.
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Old 11-23-2006, 10:31 PM   #587
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I don't know the exact theory behind it, but a larger tire always seems to provide more traction than a small one. However, when there's enough traction, a smaller tire will give faster lap times.

I suspect the first is because a bigger tyre is softer all around, whilst the second is because a smaller tire has less rotating mass.
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Old 11-23-2006, 10:38 PM   #588
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Josh-Thanks for the discussion. I'll do some more sketching. I think you have pinned it down for us. I agree that the movement of our front suspension is like an upside down strut and mentioned this earlier in the thread. The roll center changes that you describe are probably good for carpet racing where traction is very high and traction roll overs are possible. I'll add a few experiences on tire diameter.

My car hooks up best with a large tire on the straight. I think it just handles the bumps better as they would seem smaller to the large tire. I get much better speed into the sweeper. Gearing the small tire up does not help much as it is just spinning for part of the time where the large tire stays hooked up.

We have about .3 inches of foam. I use about the first .2 inches. I have found in some sessions about .050 or more of tire wear. A thin layer would not be practical from a cost perspective. Tire wear is highest when grip is high. When there is dust, tire wear is almost non existant. I don't think these foam tires are getting hot or need to be hot to perform like a rubber tire might. The same compound works good at 140 F track temperature or 60 F track temperature. This is different than the sedan rubber tires that I used where I would need to change the grade at least twice for this temperature range. I think the hookup is more mechanical on the irregularities than molecular adhesion. Generally,though, 3 packs on a dusty track with traction compound will clean off most of the dust from the race line and give me pretty good traction. The dust ends up on the chassis. It would be easy to make a narrow foam for a test.

Lower Roll Center
New adjustment. I wanted a lower roll center to run the Peugeot Body which I have mounted for my next test of the shocks. I Dremeled off .030 inches from the top of the steering blocks. This lowered the outer end of the upper arms increasing my radius and possibly lowering the roll center. I'll make some sketches to see and report results later.

Last edited by John Stranahan; 11-23-2006 at 10:49 PM.
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Old 11-24-2006, 06:10 AM   #589
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guys,

losing traction when your tire sauce ran out isnt fun at all. I prefer to find the best tire combination and later work on the minor detail like bodywork, caster, damper and spring.

Once I have found the magic setup, i have a choice on whether to add traction sauce or not. If the car setup is right, the car is still competitive even when the tire sauce is out. The tire sauce will help at the start and the car shouldnt be out of balance once the tire sauce is out.

With 3.5 or 4.5, it's really really hard to preserve the outer 1-2mm of the tire. When the outer 1-2mm is out, so will the traction additive.

With 3.5 or 4.5, buy yourself a bodyshell with really high downforce. It makes a big big difference on the straight, sweeper and chicane.
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Old 11-24-2006, 09:41 AM   #590
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Quote:
Originally Posted by Pro ten Holland
I don't know the exact theory behind it, but a larger tire always seems to provide more traction than a small one. However, when there's enough traction, a smaller tire will give faster lap times.

I suspect the first is because a bigger tyre is softer all around, whilst the second is because a smaller tire has less rotating mass.

Yeah a bigger wheel/tyre will provide a greater moment of inertia making it slower accelerating and acting as a kind of traction control when grip is low.
thats exactly the reason top fuel rails run such tall tyres.

John for what reason did you want to run a lower front RC with the different body. I assume understeer is the problem. is it understeer every were or corner entry or exit that it understeers.
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Old 11-24-2006, 09:52 AM   #591
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Josh-I am running a tall Nissan 300z body at present. The car is supple and very forgiving. Hard to make a mistake. It seems to go faster around the corners.

The Peugeot body is very low to the ground. The car is no longer supple. Grip in the corners is good, but it is easy to make a mistake as the car is too sensitive and too quick to respond.
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Old 11-24-2006, 09:59 AM   #592
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Now for a question of my own.
I am currently in the design stage of builing a pan type car. The class it will
run in is 4wd TC but i am building a RWD live axle car. . Our cell limit is six without exceeding 4300mah and we have a control rubber tyre so traction will be at a premium. I have been considering running 5cell instead of 6 to get rid of weight but don't have a lot of experienc with batteries or motors. I am also weighing up pros and cons of running brushless. our races are 5 minutes.
what do you guys run in your pans?
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Old 11-24-2006, 10:09 AM   #593
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Quote:
Originally Posted by John Stranahan
Josh-I am running a tall Nissan 300z body at present. The car is supple and very forgiving. Hard to make a mistake. It seems to go faster around the corners.

The Peugeot body is very low to the ground. The car is no longer supple. Grip in the corners is good, but it is easy to make a mistake as the car is too sensitive and too quick to respond.
Have you calculated what your rear RC is and compared it to your front?
do you have a weight distribution front to rear?
If the car is too quick to respond lowering the front roll centre may just make things work. I gather it wants to back into corners (ie oversteer) from your statement. If thats the case then there is probbably merit to lowering the rear roll centre a little if its possible to do so. That is one feature I am building
into my chassis that I am building. I am making a 4link trailing arm rear with a watts link to control lateral movement of the axle and Rear roll centre.
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Old 11-24-2006, 10:14 AM   #594
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In a pan, I think brushless is the way to go. 500 minutes of runtime with only a bearing replacement. That is really nice. You can limit speed and rubber tire melting torque by selecting one of the stock type of brushless if the track is shorter. I run a Novak 4.5 R with a Sphere Competition speed control. If the track is oudoors I recommend using some front shocks as well as 3 rear shocks at this point in time. I use a 2S 1P Lipo.
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Old 11-24-2006, 10:20 AM   #595
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Josh-I don't think there is a rear roll center adjustment on this car as the rear pod is a solid piece. There is a low roll center kit offered for sale which lowers the center pivot of the car. It had no effect on cornering in my experiments. I want the low body to handle just like the tall body. This is the reason for the front roll center change. I don't have much else that I can change to accomplish this. If it gets worse I will know straight off. What I want is some more roll and suppleness to the car so the tires are not shocked.

I have thought of using a 4 link rear suspension so that roll center could be adjusted. It might work out nice. If the motor is not driving the rear axle directly, there will be some loss of efficiency, though. The brushless may start to have thermal shutdowns in hot weather.

Rear weight is 2-3 ounces heavier than the front. Ideal for me.
John

Last edited by John Stranahan; 11-24-2006 at 10:39 AM.
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Old 11-24-2006, 10:27 AM   #596
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Quote:
Originally Posted by John Stranahan
In a pan, I think brushless is the way to go. 500 minutes of runtime with only a bearing replacement. That is really nice. You can limit speed and rubber tire melting torque by selecting one of the stock type of brushless if the track is shorter. I run a Novak 4.5 R with a Sphere Competition speed control. If the track is oudoors I recommend using some front shocks as well as 3 rear shocks at this point in time. I use a 2S 1P Lipo.
Yeah i will be running shocks allround. Since i am doing a scratch build I am going for a double a arm front (both active) a 4 link rear. the engine mounted in the chassis with a belt drive to the axle. this is to get a weight distribution of 50:50 (seeing as under the local rules front and rear tyres will be the same width). I have looked a few RC shocks and i don't like them so I am making my own, a big task I know but I have been spoilt from full size racing and want the option of proper shock tuning and proper rebound damping. when I finish my CAD drawings of the chassis and suspension I will post them up so you can see.
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Old 11-24-2006, 10:42 AM   #597
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Quote:
Originally Posted by John Stranahan
Josh-I don't think there is a rear roll center adjustment on this car as the rear pod is a solid piece. There is a low roll center kit offered for sale which lowers the center pivot of the car. It had no effect on cornering in my experiments. I want the low body to handle just like the tall body. This is the reason for the front roll center change. I don't have much else that I can change to accomplish this. If it gets worse I will know straight off. What I want is some more roll and suppleness to the car so the tires are not shocked.

I have thought of using a 4 link rear suspension so that roll center could be adjusted. It might work out nice. If the motor is not driving the rear axle directly, there will be aome loss of efficiency, though. The brushless may start to have thermal shutdowns in hot weather.

Rear weight is 2-3 ounces heavier than the front. Ideal for me.
John
Adjusting ride hieght a little will affect roll centre hieghts a little. This is a little trick I liked to use. (bare with me as all my experience lies in full size racing) eg car under steers - lowering the front ride HT by 2-3mm lowered the front roll centre by about the same amount (double a arm so your set up will be slightly different ratios) and also gets the front splitter closer to the ground (more downforce but probably not that applicable in this case) - usually worked well, so maybe addy just a little more front ride or a little less rear. Other ideas - more rear droop, less front droop.
When sorting balance issues I like to go straight to springs, for turn in issues I like to use roll centre changes (particularly in the rear) and sway bar. for corner exit I like to use springs and sway bars (rear only).
As I said this is for real racing cars but I can't see why these principals can't apply to RC. That is why I am building my own car (with a rear sway bar).
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Old 11-24-2006, 11:28 AM   #598
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Shocks-any shock with a piston that has holes in it can have a separate bump and rebound adjustment. I bevel the top of the holes in my touring car shocks to have a softer bump that rebound. This works extremely well through the bumps. The shock need not be more complex than this.

Our front droop is basically none with the heavy springs we need to run. The rear is basically non adjustable. The rear pod just needs to be run with soft shocks and springs to keep from spinning out. There are some pictures of the rear pod in the second post on this thread.

Touring cars have all the adjustability that you describe. Our pan cars do not. The way that we lower the front ride height is to shim up the entire front suspension, so in effect the suspension stays exactly as is and the chassis is the only thing lowered. No change to the roll center but some change to center of gravity and thus to the roll moment. I need my full ride height for the chassis to clear the bumps well. This limits my front ride height adjustability. Rear ride height of the pod is adjustable, but I don't notice a big change when I make the adjustment. I make this adjustment just to compensate for different diameter tires as they wear.

I have not run this Peugeot body with the new front shocks and the softer front springs that I can run now, It may be just fine now.
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Old 11-24-2006, 07:34 PM   #599
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Hmm I can see what you say about the piston in the shocks. However I have a fairly simple method of achieve a true independence. Once I have done some drawings of this (may be some time away.) I will also post these. The design will keep the same piston that seems the norm but an extra component else where that will allow me to make bump and rebound adjustments independently.
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Old 11-24-2006, 08:23 PM   #600
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Quote:
Yeah a bigger wheel/tyre will provide a greater moment of inertia making it slower accelerating and acting as a kind of traction control when grip is low.
thats exactly the reason top fuel rails run such tall tyres.
the inertial difference between a 2.25, and 2.15 tire is minimal perhaps not non existant, but minimal.. perhaps the greater advantage is in sheer contact patch. all tires have some degree of flatening under load creating a larger contact patch. without this, the contact patch would be virtually nothing deep by the width of the tire.. larger diameter tires have more "give" and create a larger contact patch for acceleration, but also give more laterally leading to the inconsistancies when cornering.. this is why dragsters have huge soft sidewalls and large diameter, while touring cars have much lower more rigid sidewalls.
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