inpuressa

iTrader: (10)

I always wondered what the optimal weight is for a particular RC car. I think it has a lot to do with the ratio of the tire contact patch vs the size(weight) of the car.

If you look at on-road touring, why are 24mm wide tires the accepted size rather than the 26mm that is more scale? If a 1/1 race car weighs 2000lbs, then a 1/10 car should weigh 200lbs. But that is not the case, not even close. So to exert more force on the tires for traction, the appropriate tire width must be adopted.

Same goes for off-road. There will be a point where the car is too light to put enough static weight on the tires for adequate traction. Being too light also has its demerits. Like they say, rubbin is racing, if you bump someone with a heavier car than you, you will most likely lose and get punted They also have higher chance to flip over or get destabilized, especially if the weight savings come from the bottom of the car (higher center of gravity)

So my question to you guys, what do you think is the optimum weight for a 1/8 off-road car using the tires that we have?

MX304

iTrader: (21)

One thing I find interesting is that a lot of the top driver have begun adding unsprung weight to the cars by placing weights on a-arms and steering uprights. Any thoughts on what that would accomplish?

inpuressa

iTrader: (10)

Ty Tessmann nicely explained what that was for:

"Since I was one of the few guys that actually tried the weight on the front caster blocks I can tell you what it did. The track that we ran on in Argentina was like nothing I have ever ran on before, the layout itself was relatively easy but the incredible high speed combined with all the bumps and rocks made this one of the most difficult tracks I have ever driven. At the beginning we really struggled for a setup, the new D812 is a car that is very nimble and has alot of steering on the track, perfect for US tracks but when we got on this track it was far too aggressive, driving my car on this track it felt like the car weighed nothing and bounced around, because of our lack of experience with this type of track we basically tried everything that we could to make the car easier to drive. The weight on the front was one of the things we tried, what it seemed to do was make the car react slower and feel lazier and easier to drive, the best way I can describe it maybe is that it made the car feel numb, luckily with more time on the track after the race we were able to come up with a setup that worked very well and the weight was no longer needed. I would consider putting the weight on the front as just a quick fix when needed."

Krio

iTrader: (159)

Weight is a function of volume which is a cubed distance, so technically the 1/10 car should weigh 2 lbs. That's somewhat close to what touring cars weigh, not that is has anything to do with what the cars should be for best performance.

Very key point on the contact patch there that many overlook. Most R&D on 8th scale tires is done for nitro cars that weigh in the low 7lb range. This would infer that to maximize traction based on the tire/foam design we should try to match that weight.

Going back to what Shackleford mentioned about sprung to unsprung ratio; yes for the most part. There isn't a key ratio, but it's a balance of two separate variables. The ideal ratio for unsprung weight to sprung weight is 0:1 as the tires would never leave the ground and the chassis would remain perfectly smooth. This obvioulsy isn't possible as no one has invented a massless suspension. The sprung to unsprung ratioshould be as small as possible while you want your tire friction to complete mass of the car to be as big as possible. The balance comes from determining when you are losing traction due to a suspension that can't keep the tires on the ground vs a tire that is overloaded by the mass it has to move.

I feel like 7-7.5 lbs is what the average consumer should aim for to maximize performance (ignoring tagging traffic) between keeping the tires on the ground and providing the highest friction to mass ratio.

That being said, tires/rims/foams/suspensions can be completely re-worked to provide better performance at ligher (or heavier) weights, but since most of us don't have the capitol to invest in retooling anything, we must settle for what is commercially available to us.

Now that the thread is completely derailed from the original post...

inpuressa

iTrader: (10)

yeah, you are right about the weight ratio.

The tires are the limiting factor to traction, so the car weight should be kept at a reasonable limit. My D8, I switched from the stock 4mm to the milled 3mm chassis(lost over 100g from the bottom of the car), and I started to traction roll all over the place. I had to retune the suspension, and use different clutch shoes/springs and pipe to calm the engine output down. Then the car was money again. But when the track gets rutted, then the heavy car is more consistent. Really depends on the track too I guess.

Getting back to topic, what kind of track does OP run on?

8ight-e

iTrader: (58)

I guess it depends on the driver.. I mean I can agree that a 8ight or similar is far more durable. That said the 810 is narly the same as the SCTE and you can run either of these on ANY size track with out any issues. If you crash a lot or don't know how to jump, ok forget about weight savings get the heavy duty full on buggy and go after durability.

First race I pulled out my 810 on a full sized 1/8 track I decided to run it at the last minute, put it on the track in last place for the a-main as I missed all the qualifiers and ended up taking 1st out of a full field. Ran it with a xerun 540 4-pole 3400k on 3s lipo, flipping bullet.

What I would also say is watch out while racing for others; I did break a couple parts but they were due to morons going backward on the track that I sadly couldnt get out of the way of quick enough Easy fixes and shares parts with my SCTE so was back up in minutes. These hits would have broken any buggy though..

Sean Cochran

iTrader: (1)

This was the lightest I was able to get a RC8e, this is from Nov. 2010.
I run at OCRC and WCRC, indoor clay.
I had problems with this car not working well mostly due to not be able to find springs that were soft enough for the shocks I was using.
The light car corners well, feels quicker, and uses less power to do jumps.
At the time I was running a 3s with a 5.5t 2 pole LRP 550.
If I were to do it again I would try a 4 pole 540 with 3s.

Sean Cochran

iTrader: (1)

This photo is my RC8.2E that I built for this years ROAR Nats at WCRC.
Again I messed up and spent most of the race with the wrong springs, not soft enough for the weight and conditions.
I had to add weight to get above the race minimum of 3200g.

3151g=6.94lbs

nv529

iTrader: (53)

your car was looking good towards the end of the weekend, did you end up settling for K-car orange and yellow springs?

Sean Cochran

iTrader: (1)

Orange and yellow is what I ran but later figured out that AE green front and Losi red rear was better. I also had to soften the shock oil some as well.

Lately I have been working with a standard kit RC8.2e, built weight with all kit hardware was 3375g. It took some getting used to and and tweaking set up but it is almost as good as the light car at OCRC.
The light car still feels like it corners better though.

I could also see where a light car would be not optimum for rough and blown out style tracks. I think the heavier car would be more forgiving in those conditions.

Dave H

Good points Chris. Tire traction coefficients, like everything involving rubber, is highly non-linear. Especially with off road tires with any kind of pins that rely on some embedment for traction. This doesn’t mean more and more weight is better, but for the tires we have too light can be less than ideal, at least in some conditions. On road tires are also non-linear, but generally not to the degree off road tires are. On road with a smooth track, get the Dremel out, lighter is almost always king, especially if there is enough traction that tire heat becomes an issue like in full scale. Off road is not so clear cut best I can tell.

Off subject a little, but this is why anti-sway bars, roll center adjustments, roll stiffness with spring setup, impacts understeer/oversteer. If the traction were linear, those adjustments wouldn’t make much difference. Then a 4lb rear end with 3lb on the outside tire and 1lb on inside would have the same grip as 2.5 out and 1.5 in, but they don’t in practice because of the non-linear behavior.

rcgod

iTrader: (33)

You should try the eb48 springs. They are a softer rate than anything else I've seen.