XRAY - Model racing cars

I've had a somewhat heated debate on rctech.net about using a one-way on carpet with foams. I thought I'd post my reasoning here as well.

Here is why I decided to go down the one-way path for carpet racing, versus the traditional adjustable limited slip differential:

1. A one-way ensures maximum use of the drive train power. An LS diff decreases a percentage of the power to the outside wheel where traction is the greatest and gives it to the inside wheel which is unloaded and "slipping" because of the diff action. Unless the front diff is tightened to the point where it functions like a spool, there will always be a loss of acceleration from the drive train. By tightening the diff you can minimize the loss of power, but then you start to introduce the problem of both front wheels fighting each other in the turns where the inside wheel wants to spin faster than the outside wheel because of the steering arc and their different turn radiuses. This scrubs too much speed on corner entry and also causes the front end to hop/skip/bounce. The beauty of the one-way is that it maintains 100% of the power to the outside wheel, while allowing the inside wheel to spin as fast as it needs to for its own tighter radius around any given corner. You can't count on 100% of the power to be applied in the rear either, since it also has an LS diff which is normally even more loose. The high amount of traction provided by foams on carpet is an advantage that should be exploited for acceleration, as well as cornering.
2. The first knock on the one-way that needs to be addressed in another area of the setup is too much initial turn-in steering where the car can become twitchy. I have found that you can address this effectively by using 0 front toe out (which also improves straight line speed and makes the car easier to drive) and a lot of caster (which also improves another weakness of the one-way, on-power push)
3. The second weakness is abrupt loss of traction as the front suspension reaches maximum roll and load prior to the apex. This can be addressed by using a stiff front anti-roll bar (which is also the best way to improve steering response without making a car twitchy; chicanes anyone?)
4. The last weakness is reduced on-power steering or push. The steep caster angle helps here as well. However, an interesting bit of information that I picked up from a well renowned scale racing book called "Engineer To Win" by Carroll Smith states that although a front anti-roll bar may decrease overall front traction mid corner, it actually increases on power steering by transferring the forces applied to the outside wheel, through the bar, to the inside front wheel which is instrumental in steering out of corners as the cars weight shifts to the rear. Carroll illustrated this to emphasize the benefit of using more ackerman, which increases the steering angle of the inside wheel to gain even more steering on corner exit. Of course the one-way and camber effects from the steep caster angle, which causes both front tires to dig in mid corner as the steering wheel is at or close to full lock (the inside wheel now gains a lot of positive camber for a wider contact patch), assist with providing enough traction mid corner that you would normally lose when using a stiff front anti-roll bar.

Now this type of setup became a little more difficult when I switched from the XRC FK to the FK05 with its new front biased weight distribution, which produces less rear mechanical grip. And coupled with the increase of front traction from the steep front caster angle, the rear needs more traction out of the tires, unless you want to mount a lot of weight to the rear. I found the orange-purple/double pink and plaid/magenta combinations were the ticket here to actually bring the car into, and not out of, balance.

The end result from all this is an easy to drive car that scrubs very little speed on corner entry and mid corner, is extremely responsive but not twitchy, has great steering throughout the entire corner and accelerates faster through and out of the corners. I've even found that dialing down the brakes to 60% allowed me to come in extremely fast and tight to corners for passing and then blip the brakes to rotate the rear end around quickly and smoothly, with minimal scrubing of speed. The one-way then allows me to keep the position by accelerating faster out of the corners.

I welcome anyones feedback and constructive criticism.

It is very interesting and sounds logical. I would welcome a harder front tire anyway since the double-pink/orange wear out soooo fast.

I will definietly try and tell what I found, however when I last tested the one-way on the FK-04 it was very hard to drive because of the tons of steering I had. Hope with the changes you described this problem won't be an issue anymore . Can't wait to test it.

Is this the setup you are talking about? http://www.teamxray.com/xforum/xform/in ... tup=t1fk05

If not, could you post the complete setup?

Thats it.

I forgot to add one more setup option that helped with #4 above. With the on power push problem, one-ways have not been useful for tight tracks. Another option I used to address this issue was minimum wheelbase. I found that the car actually became more stable, the more I shortened the wheelbase. I believe this comes from placing more weight over the FK05's light rear axle. With the minimum wheelbase, the cars steering arc tightened very nicely and the car handles sharp turns with very surprisingly little steering EPA and ackerman effect.

When I used a setup somewhat similar to this with my original FK, I had to use hole #2 and even #1 on the steering blocks for increased ackerman effect to turn sharp mid corner, but this made the car scrub a little too much speed mid corner and the difference was noticeable when the FK05's came out. I was pleasantly surprised to find that I'm able to use the carpet spec hole#3 for minimum ackerman effect when using this type of setup with the FK05.

For Saturday's race I had to use the extension plates with the standard composite blocks. But the angle of the steering rods added bump steer when mounted on top of the plates with no shims. In order to get the rods at the correct angle to eliminate bump steer, I had to mount 2mm plastic shims around the button head screw shaft in hole #2 between the plates and the steering mount holes, use longer in hole #1 and then mount the steering rods underneath the extension plates. I then had to shave a little material off of the side of the steering blocks outside of hole#2 as well as off of the side of the steering rod ball ends to keep them from rubbing.

All of the headache with bump steer above can be eliminated by simply using the and 3 hole composite steering blocks which I should be receiving this week. I threw this car together last week with a lot of my old used parts from asphalt season and had to improvise for Saturdays race.

You should get the wide top deck from xray for free .
Do you think that the standard one with the washers has the same effect on handling as the wide one with posts all four corners?

I've been using a one-way on carpet with foams for the past 2 years. I don't think people give them enough of a chance. The car set-up needs to be different... too many think that you can slap a one-way in, and if it doesn't increase your lap times, a std diff must be better... it just doesn't work that way.

Not true. Off power both wheels spin freely and have no drive. Although I do grant that in the off power situation if any of the front wheels were to have some drive it would be the inside wheel. This is caused by the fact that the inside radius is tighter meaning the inside wheel will turn more slowly than the outside wheel and if the rear diff is set too tight (or working badly) the spinning speed of the outside rear wheel will be transferred to the front through the transmission. Since the outside rear wheel spins faster (larger radius corner) than the inside front that will have some drive in this situation. If the rear diff is set loose and is smooth this does not happen. This is exactly the reason why quite often you will find the car very difficult to drive if the rear diff is set too tight or not working smoothly in combination with running a front one-way.

On power both wheels have 100% power but only the outside wheel will have 100% drive. The reason is that during cornering the inside wheel has less weight on it and therefore will have less grip causing it to be the one to loose some traction and rotate too fast for the corner speed the car has on the inside (tighter turn radius).

I've updated the setup sheet again with a few changes from more testing. While playing follow the leader yesterday, to simulate true racing conditions with traffic, I realized that the rear was just a hair too loose for consistency. Here are the changes that worked to keep the rear end firmly planted during last minute quick transitions and *bumps* in traffic, but still provide great rotation around and out of corners:

* Increased rear toe in from 1.0 to 1.5 degrees
* Moved the rear upper shock mounts out one hole from #4 to #5
* Decreased negative camber .5d all the way around from -2.0 to -1.5

FYI, I plan on continuing to give updates on the one-way setup for anyone who is interested. As most know, this type of setup is uncommon territory for foam/carpet racing and good information is hard to find.

10/1/05 - Club race

I arrived late, but in time for the second qualifier. The car felt pretty good, but needed a little improvement. Using the micrometer, I noticed the front left tire was wearing a little too much more than the front right from the high speed right sweeper at the end of the straight. The rear end also seemed to be unloading too much on corner entry and then rolling to the point of bogging down at initial acceleration out of tighter corners. I wanted to keep the front end feel, so I made the following changes before the last qualifier:

* Raised rear ride height from 4.25mm to 4.5mm (front is 4.0mm)
* Decreased rear droop from 3.5mm to 3.0mm
* Moved the front anti-roll bar ball mounts forward and more onto the bar (almost touching the front axles) to tighten the bars effect for slightly less front end roll

I put a few laps in during the break to confirm that the changes worked and the results were good in the final qualifier where I was able to make the last grid spot in the A main. This was great as it allowed me to focus on rebuilding and tuning the motor for the main. A check with the micrometer after the qualifier showed that the left front was wearing considerably less with the changes and on par with how the rears were wearing.

The results in the main were more than I expected as the changes I made earlier in the week (tighten the rear up a little to prevent sliding out from rubbing fenders in traffic) for consistency helped me to drive like Alonso has the past six Formula 1 races (CAREFULLY keep the advantage and don't take risks ). After a few incidents with all but the first and second place drivers put me into third with a lead on the others, I was able to keep tight lines where I was comfortable and swing a little wide where I was not. The fourth place driver was able to reel me in with a few laps to go, but my strategy paid off and I was fortunate that he had to slow up to let the top two race leaders go by just before the end of the race.

During the last qualifier and the main, I noticed I was slower in a medium speed S section taken at full throttle, where the first part is a double apex left hander going into a double apex right hander. There are two of these S sections, but the first one seems to have more traction. The rear still feels a tad too tight in the double apex left and I seem to lose the front end feel and a few tenths of time in the transition to the double apex right. After conferring with another driver, I believe adding a 1mm rear anti-roll bar may be the ticket and I plan on testing this next week.


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hi, im running a one-way on carpet with rubber(carpet dragon). how would your findings help?? if at all.oh i use the t1r.ian

Well thats somewhat of a loaded question. I had hoped to actually run the car at the local track with rubbers to do some testing, but the track has banned them from being run on carpet.

Here is the baseline setup I was going to start with:

http://www.teamxray.com/xforum/xform/in ... tup=t1fk05

Please let us know all the details of what you find works and doesn't work if you don't mind.

@TeamGP and TonyV

I want to give foam tires a try at a club race on carpet with my FK04 . In previous postings Tony stated that a front diff is the hot setup for foams on carpet because a one-way will result in a very twitchy car due to the busload of grip provided by thees tires.
Is this still valid, or have things changed?

It will be a stock race on a track with a tight infield, two long sweepers and to medium long straights.

If you still recommend the diff I got another question. On this track with the stock motors we dont need to brake when we use a one way and rubber tires. Would changing to a diff create a need for brakes? This would be a negative point because I think it is too time consuming.

Shall I invest in the additional diff?

Any tips are much appreciated!

Here's an update...

I re-built the car last night with the following carpet hopups:

* 3mm chassis
* hard fr/rr arms
* hard c-hubs
* Adjustable top deck with all 3 braces mounted
* 3 hole steering blocks (third hole is used for less ackerman and more corner speed in high grip conditions)

Here are the building details:

* Used a Q-tip to apply a thin layer of CA to all counter-sunk screw holes on the bottom side of the chassis and the top side of the top deck.
* Used a file to round all the sharp edges off the chassis and top deck. Then I filed the battery slots down some more until the IB3800's sat almost flush with the bottom of the chassis. The batteries ended up sitting 1mm lower than the tightened down brace and I had to use single sided "spongy" RC aircraft tape on the bottom side of the brace to fill the gap and keep the pack low and secure.
* Applied a thick layer of CA to all outer edges of the chassis and top deck and let it soak into the CF. Finally, I used a Q-tip to apply a thin layer of CA to the battery slots.
NOTE: I was careful to wipe away the excess CA from the top and bottom flat surfaces after applying the glue
* Used a 3.03 hand reamer to open up both hinge pin holes on all four arms for a perfect fit and free movement of the suspension
* Used the new aluminum shims for all shimming. All shims are noted in the appropriate boxes and in the notes section of the setup sheet.
* I did not dremel any material off the rear arms, since I decided to use the inner holes for mounting the rear shocks
* I cut away 3mm of material from inside the outer corners of the impact absorbing lower front bumper brace to allow the corner of the brace to flex back and inward from front quarter panel hits. I've confirmed that this works well to prevent splitting the front of the chassis, by absorbing front/side impacts better and minimizing the stress applied to the screws securing the brace to the chassis.

Notable setup changes:

* Switched from a Fantom stock motor to a Chameleon 2 to increase corner entry & exit speeds in order to get a better feel for how planted the rear end was
* Tightened the rear diff slightly
* Changed to red springs in the rear to match with the red ones already in use up front
* Changed from XRAY 40wt oil to 35wt oil
* Opened one hole up in the front shocks
* Switched from Parma Plaid & Magenta tires to Jaco Purple/Orange & Double Pink tires front and rear respectively
* Applied Paragon on only 1/4" inside of front tires and full on rear tires
* Kept the 1.5d rear anti-squat, but piggy backed on the common theme of most drivers and lowered the rear roll center in order to use stiffer rear springs. I achieved this by replacing the standard suspension holders with the .75 holders in bulkhead slots #4 & #1.
* Used slightly less front anti-dive (.75d) and raised the front roll center slightly by using .75 suspension holders in bulkhead slots #4 and standard suspension holders in bulkhead slots #3
* Changed from middle to low rear diff height
* Changed rear ride height from 4.5mm to 4.25mm
* Inclined rear shocks two holes to #3 on the rear shock tower

After the first pack, I noticed that the car actually had too much rear end traction all the way through the corners, but especially mid corner. I made the following changes:

* Added 1.5mm shims under front outside camber links. On the setup board, I noticed that this change resulted in matched angles (approximately 2 degrees) of the front and rear camber links.
* Stood up the rear shocks one hole to #4 on the rear shock tower

During the second pack, I still needed more steering. So I increased the transmitter dual rate from 63% to 70%, which worked well. After the second pack, I noticed that the insides of all four tires were wearing a little too much. This was probably due to the car not rolling as much with a stiffer chassis, top deck and rear springs. I made the following changes:

* Reduced front and rear camber from -1.5d to -1.0d
* Applied Paragon to all of the purple compound, and none to the orange, in the front tires

As I expected, during the third pack the car had a little too much steering. So I decreased the transmitter dual rate back down to 63%, which seems to work best for maximum corner speed using the M8.

During the fourth pack, I switched back and forth between the Parma Alfa and the Mazda 6. The Parma was nice and free, but I felt more connected with the front end and more consistent when using the Mazda 6. I decided to add one more wrench to the mix and see if I could get the front end to feel slightly more connected with the Parma Alfa body.

* Added 1mm rear anti-roll bar

I started the fifth pack with the Mazda 6. The increase in rear end rotation was very apparent with more steering throughout the corners. I decided against reducing the transmitter dual rate anymore, since it would provide too little low speed steering in case I had to turn around after an incident. Switching back to the Parma Alfa was the ticket and the cars front end was on rails again and carrying corner speed.



Summary of the final results:

At the end of the day, the steering felt more linear and controllable throughout the three corner phases. Steering corrections in any phase were completed without hesitation, feeling smooth and predictable. The rear felt a tad more stable, but with better rotation through the corners. Although I couldn't get lap times, confirmation of less chassis roll, by needing to use less static camber, indicates that the car should be slightly faster through the corners.