Front wheel width...rules?

[MCKNBRD]

OK, so I've done over an hour of searching the interwebz and can't find the answer.

What is the maximum FRONT wheel width for 2wd buggy?

ROAR rules specify maximum wheel width as 38.1mm; but they don't mention a difference between front and rear. There is an overall max width mentioned, too...but nothing about front wheels.

Is there any REGULATORY reason I can't run 4wd front wheels on a 2wd? With the shift towards hexes on all 4 corners, it makes sense to use all the tools available...and wider wheels/tires provide more grip.

Thanks-
Byrdman
(and yes, I did copy this over from the thread on RCTech.net...but the discussion got derailed onto putting 4wd tires on 2wd wheels...NOT the question!)

[Coelacanth]

I won't comment on rules, but I'll comment on the physics. Width and wheelbase are both implicated when you're discussing turning performance. You can't just go wider and wider as you'll pass an "optimal balance point" and the car will tend to over-steer more and more. If your car is under-steering and pushing forward instead of turning, then increasing width is one way to counter-act that. But as you continue to widen the front track, the car will start steering too much and become too twitchy.

As extreme examples, imagine a car with twice the wheel width and half the wheelbase; that thing would be near impossible to keep going in a straight line, let alone corner predictably & controllably; the same goes for a chassis with super-long wheelbase and super-narrow track width; now you have the makings of a dragster, which hardly steers at all and just needs to go reliably in a very straight line.

[GoMachV]

Well that didn't take long lol!

(and yes, I did copy this over from the thread on RCTech.net...but the discussion got derailed onto putting 4wd tires on 2wd wheels...NOT the question!)

[MCKNBRD]

I won't comment on rules, but I'll comment on the physics. Width and wheelbase are both implicated when you're discussing turning performance. You can't just go wider and wider as you'll pass an "optimal balance point" and the car will tend to over-steer more and more. If your car is under-steering and pushing forward instead of turning, then increasing width is one way to counter-act that. But as you continue to widen the front track, the car will start steering too much and become too twitchy.

As extreme examples, imagine a car with twice the wheel width and half the wheelbase; that thing would be near impossible to keep going in a straight line, let alone corner predictably & controllably; the same goes for a chassis with super-long wheelbase and super-narrow track width; now you have the makings of a dragster, which hardly steers at all and just needs to go reliably in a very straight line.

Well, I can say here what I won't say there...I'm wanting to make a gold tub as competitive as possible; I'm planning on building a W-I-D-E track front end that is as close to max legal width, using as wide an arm as possible (SC10.2). If there aren't any rules prohibiting it (and no one has been able to quote chapter and verse where x.xxx is the max front wheel width), I'm going to run 4wd fronts, as they're going to give me the max contact patch, which is better than less patch area. When you're starting from scratch with front end geometry, there isn't any reason to not max it out, right?

Byrdman

[alien3t]

for car width. they used the take the cars. and a pre made box had to fit around the car. the box was on the inside the legal width of a rc car. at an event. don't know how they do that now.


http://www.roarracing.com/downloads/2013_ROAR_Rule_Book.pdf

if looking at ROAR rules PAge 49 has all the dimensions for 1/10th scale electric buggy.

[jwscab]

the latest buggies have a pretty wide front tire, a B4 or B5 front wheel is pretty wide compared to the classic front wheels. those chassis' are pretty balanced to use a wide front tire, even still, most run some type of ribbed tire at least in combo with other tread design. having too much traction up front will unbalance the chassis.

[Phin]

...I'm planning on building a W-I-D-E track front end that is as close to max legal width, using as wide an arm as possible (SC10.2). ...

There may not be a limit to the front tire width but there is a limit to the buggy's overall width. To use SC10.2 arms and stay under the legal width you'd need to make your front bulkhead about 3/4" wide. (SC10 arms are each ~15mm wider than B4 arms and a B4 front bulkhead is about 47mm wide according to the dimensions on Tower Hobbies)

That's going to cause some problems.


Balance is important in a car's suspension and grasping on to only one aspect of it thinking it's the most important, and then going to the extreme with it, isn't going to do you much good.

A really wide front tire will affect your scrub radius, and camber during suspension travel. In a 4wd car a wide front tire will give more traction to wheels that are trying to pull the car along. In a 2wd car a wide front wheel will increase resistance as they're trying to be pushed along by the rear driven wheels.

[MCKNBRD]

I have 9-3/4" to work with...what I'm planning on doing is getting an SC10.2 front end and seeing what it will take to fit it in the box. If I can't make a bulkhead narrow enough, I'll take all the parts I have and start playing with arm lengths (I have 10T arms here, as well as stock RC10s; there are many options out there) and see what I can make work.

I know that ll things have to balance; believe me, I'm not working on the front end in a vacuum. The rear end will be as wide as possible, too; I'm just starting to play with those options, too.

As far as drag goes, I've noticed that many race cars that only drive with the rears have the same size fronts...so I'm thinking that it won't be that much of an issue. If it is, well, there are plenty of narrow fronts out there.

Byrdman

[terry.sc]

If there aren't any rules prohibiting it (and no one has been able to quote chapter and verse where x.xxx is the max front wheel width), I'm going to run 4wd fronts, as they're going to give me the max contact patch, which is better than less patch area.

The only rule regarding width is that all wheels must be less than 38.1mm wide.

A modern 2wd buggy front wheel is 26mm wide, a 4wd front wheel is 27mm wide. Not much difference between them at all. Modern chassis are designed around these wider wheels and tyres, running old style narrow front tyres on a modern 2wd will mess up the geometry as the contact patch is now further out than normal.

[Coelacanth]

Going too wide front & rear without lengthening the wheelbase will probably result in a car that over-steers way too much. As terry.sc said, the modern cars have very wide track front & rear, but also have much longer wheelbases.

But by all means, there's nothing stopping you from trying it out. I'm sure you wouldn't have been the first, though.

[MCKNBRD]

But by all means, there's nothing stopping you from trying it out. I'm sure you wouldn't have been the first, though.

Having said that, what have folks found to be a COMPETITIVE setup for the gold pan cars against the current crop of mid-motor machines?

I'm planning on doing a mid conversion on my car, but want to have the best suspension geometry possible. Mostly going to race indoor carpet offroad; maybe some outdoor this summer.

Byrdman

[Coelacanth]

I'll have to defer to the RC10 gurus here, I'm a Kyosho guy. There are guys here who run modded vintage Optima Mids and Lazers pretty competitively with modern cars, and those old cars have shorter wheelbases--and correspondingly narrower track--than their modern counterparts, but I imagine the proportions would be similar. If I'm an expert on anything, it's the original Optima series, and those 4WD cars were known for understeering under acceleration. I slightly widened the track width on two of my modded cars (about 5mm longer control arms & ~12mm wider rear track, and wheels with more negative offset compared to stock) and it definitely made a difference; they don't understeer anymore.

The thing is, I was trying to address a common problem with that chassis. I don't think the RC10's were known for understeering, they were a pretty effective design already.

To be competitive with modern cars, you're probably gonna want to pay more attention to modding the suspension, as the cars these days do ridiculous double & triple jumps and race on totally different surfaces than back in the day...so wheel & tire selection will play a big role, too. IMO, the silly big-air jumping you see today is all out-of-proportion to reality, but I guess it's exciting...and breaks a lot of parts.

Since you mentioned racing on carpet, that would be a pretty high-traction surface, wouldn't it? Oversteering on a high-grip surface could mean lots of traction rollovers.

RC10 guys, time for you to chime in!