Preventing and Repairing Rear Tyre Chunks

Preventing and Repairing Rear Tyre Chunks

One of the things I like about 1/12th scale racing on foam tyres is that the grip remains consistent independent of the age of the tyres or the number of runs they have done.

In the UK we only run odorless additives which are less aggressive to the rubber, the track and the health of the people at the meeting! I have completed a whole meeting on one set of foams for example. Readers of this article may want to argue about the benefits of rotating to a fresh set of tyres every run within the racing day to prevent the tyres from becoming over soft.

My objective in this article however is to demonstrate how “chunked” rear tyres can be prevented and even repaired. Tyres are expensive and I hate the feeling when you come back from a heat or final with the outside edge of your precious foam rubber with huge bites out of them.

All it takes is a little care and time. You can save yourself some serious beer money!

Preventing Chunking in the First Place

Well the first rule is not to hit anything and do not get hit! Failing this, tyre checking and maintenance is going to be required after every run.

In general only the rear tyres will have chunks ripped out of them. The fronts normally come back chunk free but it still pays to check them over.

Try to peel back the tyre from the wheel on the outside edge using pressure with your finger or thumb against the side wall. If there are any gaps developing between the tyre and wheel (even small ones), you must repair these now. Failure to do this will result in further damage in this “peel” area and eventually whole bits of the unsupported foam rubber will be ripped away from the tyre resulting in the dreaded chunking.








Use the right Glue

Squeeze some contact adhesive (I use Evostick) into the gap and then work it around with the end of a knife or miniature screw driver. Do not use superglue! With foam tyres we need glue that will flex with the tyre. If you use a super glue or cyano type adhesive you will create hard spots in the side wall and the tyre will rip around these spots again.

If you have made a large repair, it might be a good idea to apply some light pressure around the outside of the tyre while the glue sets. I use a strip of “Velcro” to do this but a rubber band will do just as well.



Leave the tyre for a good 30mins before you run it again.



Chunk Repair

Ok, when I explained this trick some of my racing friends; they simply did not believe me! So I have laid it out for you step by step. Unlike my most of my "improvement targets" in the A final , I do not get given tyres so this is all very important to my wallet!

To do this level of repair you are going to need a a tyre truer, a scapel knife, Evostick type contact adhesive and some nasty chunked rear tyres. I persnally find I have the latter in plentiful supply.

You will notice my patient tyre has two nasty chunks which I will repair at the same time (because I am that good nurse!). These chunks are quite small but to be honest, the smaller ones are harder to repair.






Step 1

Glue under any gaps between the damaged wheel and the damaged tyre. There may be small tears in the tyre that you are left with so you need to repair these with contact adhesive before we move on to addressing the bits of foam that are missing.

Step 2

Choose a donor tyre. This needs to be a scrap tyre of the same compound as the one under repair. It also needs to be slightly larger in diameter than the current patient also. This one donor will repair many patients!

With a scalpel knife carve a chunk out of the donor tyre that is a similar shape but slightly oversized compared to the missing bit in the repair tyre. You will be surprised just how bad this match can be! Providing it is “sort of right”, we are going to be able to cram it on in there later! Try to match slants and features in the patient’s “hole” buy cutting these features in the donor chunk. Again do not worry too much but do make sure the donor piece is oversized.




Step 3



Apply Evostick contact adhesive to both the surfaces of the chunk hole in the patient tyre and the donor repair chunk. Make sure all surfaces that will finally come together when the chunk is pushed into place are well coated.

Do not over glue here. We are looking of a shiny glue finish on both parts. Wipe off excess glue. Do not bring the pieces together yet! We need to glue to dry off. This will take around 5-10 mins. Go make a cup of tea.

The idea of a contact adhesive is that the two parts are “tacky” dry before we bring them together.


Step 4

Now fix the repair piece into place. You only get one shot at this:

Hold the repair piece over the hole with slightly too much material than will fit, ready to be pushed into place. Start the contact point from the outside rim of the tyre working in. Really compress the repair piece into place. The contact adhesive (if dry enough) will grip it and hold it fast.

Now wait again. A good 30 mins is needed for the glue to go off. Go make a sandwich!









Step 5

A disclaimer, especially for my international friends some of which have a certain reputation for “compensation culture” attitudes….. Placing your hands near fast spinning objects is dangerous. Do not attempt this unless you are a consenting adult and you have assessed your own risks. I take no responsibility whatsoever!



Place your tyre on your truer and wind the tool in over the repair area until it just starts to cut the oversized replacement chunks. Slowly grind these down until you are at the same diameter as the rest of the tyre. Take a final small cut on the whole tyre.




Now using glass paper, true the side walls very slowly cutting back the protruding repair chunks until the side wall is flat.

Finally round off the edge of the tyre with a small radius as normal.




Congratulations! As the before and after images show. You just saved yourself the equivalent of £14.

Cheers

Mark Payne

CRC Carpet Knife 3.2R Total Tweak Guide

Total Tweak Guide for the CRC Carpet Knife 3.2R

Essential Reading and Prerequisites

In my previous posts on statically balancing the CRC Carpet Knife, setting good ride height and correctly building the Associated reactive castor front end, I was trying to lay the foundations for ending up with a totally tweak free car.

The following three articles are hence prerequisite procedures before moving on to the steps detailed below:



Please note that I only have experience of the CRC Carpet Knife 3.2R. Obviously the Associated front end is common to many cars but my total “system” assumes you have rear pivot ball and side spring type rear end.

Read these first:

http://markpayneblog.blogspot.com/2005/12/building-associated-l4-type-front-end.html

http://markpayneblog.blogspot.com/2005/12/crc-32r-chassis-setup-static-balance.html

http://markpayneblog.blogspot.com/2005/12/measuring-ride-height-on-crc-32r.html

The Coin Trick is Not Enough

Most people use the coin trick on the front tyres whereby you lift the front off the ground via the front center point and adjust rear tweak screws for a simultaneous coin drop. In fact I will end this article with the coin trick as a final tweak check BUT… the validity of the “coin trick” is dependant on other things being right first.

With the coin trick, what you are doing is setting the rear tweak screws to equate the downstops (limit of upward front suspension travel) of the front suspension. This is a factor of how accurate the front lower arms are and the ride height spacers included under them. Why would we set the tweak on the rear springs to fix a potential error in the front arms? The rear tweak should adjust the springs when the chassis is LOADED onto the springs all round, not unloaded at the front.

Don’t worry… if you don’t get this yet… we will go through the whole thing here.

Lets Start

I am assuming you have done the following checks (read the previous posts above):

A. You have a well built front end with all the slack just shimmed out of the king pins.

B. You have a fresh ground set of accurate and equal tyres on the car. The left and right tyres are exactly the same size measured with your calipers.

C. You have set the ride height to 4mm all round using the “five points” system.

Step 1

Remove the whole rear tweak bar assembly and damping tubes. We will be setting up the front end first. Load the car with a set of cells. You will notice that I tape cells into the car. I do not use the CRC “rubber band” cell retention as I believe this can tweak the car.

Now that the tweak bar is off, it is a good time to check the chassis pod is rotating freely. When a CRC car takes a crash, the centre pivot plate can be shifted out of line causing the rear pod action to bind. I now tend to check this every run at important meetings.

Video: Checking Pod Movement

If in doubt, loosen the two pivot plate screws and retighten them with the pod aligned with the main chassis. Recheck the pod action again.







Step 2

Spend some time and make sure the camber on the front wheels is set to 1 degree and is the same left and right. I use a protractor that I have had since my school days (its old!) but there are plenty of camber gauges on the market.



Step 3

Take the front wheels off.
I put the chassis on 5mm blocks. Hudy make these http://www.hudy.net/xhudy/products/proddesc.php?prod_id=153&kategoria=64


Make sure the chassis itself is on the blocks and not any protruding screw heads, cells or cell tape. If the blocks are parallel to each other there should be no “rocking” evident. If the chassis rocks on the blocks then it is twisted. You will never tweak this car out with a twisted chassis.






Now use your calipers as shown to make sure the front axles are at exactly the same height off the glass. If they are not I grind the front A arms until I have the a perfect match. If you have followed my previous front end instructions then there should be no issues here and this is just a sanity check. If the axles are not at the same height to within 0.25 mm (0.01 in) then you have a problem that you must address.

Step 4

Put the rear tyres on (fresh ground). The rear tweak bar and side dampers are still removed. The centre shock and spring is on as normal.

I now place 10mm blocks under the front axles (ok, I use whatever I have at hand to make an accurate 10mm block). The rear end cannot impose any tweak on the chassis as there are no side springs. Even if there is a slight error in the rear tyres, this cannot be transferred through to the front end. The lateral balance of the chassis is now purely a function of the front springs only. You statically balanced the chassis already right? (I do hope you have read the prerequisite steps ;-).


Something to be aware of … we balanced the chassis as a whole earlier but right now the front springs are subject to the full running weight of the car but the weight balance is being imposed by the front part of the chassis only.

I asked myself .. “ what if there is a balance error in the rear pod that we correct in the front (main) part of the chassis?” … Let me save you some time. I took the rear pod off and measured it’s balance complete with motor and pinion etc, it is crack on. An engineering thing that Calandra have got 100% right.

Step 5



Now with the chassis loaded onto the springs at the front, any left/right differences in loaded spring length will result in the chassis not being laterally parallel to the glass. This error will be maximized at the back of the main chassis near where the tweak mounting pods are attached; this is where I measure using a “wedge” type ride height gauge.



Add or remove one or two king pin shims until the left and right reads exactly the same. You will be taking a shim out on the high side or placing a shim in on the low side.

Assuming you (just) shimmed out the slack in the king pins earlier, if you now have to add or remove more than two shims on either side to get things right at the back… alarm bells should be ringing. Something else is wrong here. Go back and check the build of the front end (separate post) and the previous steps in this section.

I generally only accept the addition or removal of one shim on one side or the other at this point to level things up.

Step 6

Its time to put the tweak brace back onto the car. But before we do this I have some tedious details to discuss!

Motor Wire Warning

Be careful. The wires going to the motor can tweak the rear pod, resisting it’s free movement. To help with this I use 14 AWG wire, not the 12 AWG I have in my kit from 1/10th scale touring cars!

I take the wires under the tweak brace. If you go over the top they can catch on the body, again introducing tweak.

Which Side Spings?

I generally use the red side spring as a starting point. I go to the white spring (softer) for less direct steering. There are two main things I have learnt about adjusting these springs.



The first point is to make sure the spring sits on axis with the tweak screw and mounting. If it is off centre, you get unpredictable changes as the tweak screws are adjusted.

Now attach the tweak brace and I will come to the second point:

Start with the tweak screws wound right out (springs tight against the brace). Now wind down the same amount on both sides until the springs just touch the balls. I have read CRC setup notes where it has been recommended that this is how you should run the car. Ie. With the springs just resting on the balls when the chassis is flat. But…. My second main point is that I have found the car much more predictable if I load two full turns of spring tension beyond the “touch down” point.


Leave the side dampers off for now. The front end is still on the the 10mm blocks.

Now look from the rear and lift the rear pod off the ground using the centre point. I adjust the rear tweak screws to ensure both REAR wheels lift at the same time. If you are adjusting the tweak springs by more than ½ a turn on either side, alarm bells should be going off again. Go back and recheck everything because something is wrong.

Step 7

Now look from the front and sanity check the action of the front suspension. As you lift and release the suspension up and down using the centre hole at the front, you should see the same spring compression and expansion working equally on both sides.

Video: Comparing Left and Right Front Spring Action

Well done! You car is now corner weighted and applying balanced pressure left to right.

Step 8

Now comes the “coin trick”!

Why are we doing this?

To be honest, if you have carried out all the steps above, you don’t really need to do this. Remember your car is tweaked out evenly on the springs and the “coin trick” is going to lift the whole front suspension off the springs. Is this how the car drives? I think not. You already know the front axles lift off at the same height, because you checked this at Step 3.

However, assuming that you have laid the foundations by doing all of the above, the “coin trick” is the quickest way of re-tweaking the car to allow for the errors that creep into the tyre diameters after a run. No track imposes even left/right tire wear and we have to reset for this each run.

I swap the left and right tyres over each run to balance the wear rate up. After doing this I will re-tweak using the coin trick rather than do through the total “system” from scratch.

The Coin Trick

UK readers will want to find two 20p pieces or two £1 coins if you are feeling rich! As the CRC CK 3.2R is such a well engineered American car, I like to use $1 coins….. also this prevents me from spending my tweak tools at the tea bar on race day :-)

Attach the front wheels (fresh ground, the same size!) and place the car on your level flat surface.

Now place your money on the top of both front wheels, just slightly beyond the apex (by the same amount). Now lever the car up using the front centre hole s l o w l y.
The wheel that lifts first will drop the coin first. This is the high side.

Video: Coin drop on a car with bad tweak

Correct the high side (coin dropped first on the left here) by winding the tweak screw down on the opposite side (clockwise turn on the right in this case). Alternatively back off the tweak screw (anticlock) on the same side as the “early drop” coin. Adjust by no more than 1/8th of a turn at a time. If you are using more than ½ a turn either way then something is wrong or it is time to re-true your tyres.


Video : Coin Drop with good tweak

Conclusion

If you join me at this point then I congratulate you! You are as “sad” as I am :-)
… but I am confident that you car is going to go straight when you power up!

Happy racing!

Building the Associated L4 Type Front End

Introduction

I use the Calandra CRC Carpet Knife 3.2R but the following build note apply to any car using the Associated reactive castor "L4" type front end.

I am heading towards explaining how to fully tweak out the CRC Carpet Knife 3.2R chassis. Before we look at the final tuning of the whole car it is important that the Associated front end is built as consistently and accurately as possible.

Get it right!

Fractional errors and “left to right” differences in spring lengths, shimming, block moldings and general alignment within the front end will cause unpredictable handing on the track.

It took me a whole season of 1/12th racing to realise that the driving the car is hard enough on its own without the thing wanting to pull right or left under acceleration. I am not an “inch perfect” driver but I am learning to build and inch perfect car. Unfortunately, predictability measured in inches on the track needs accuracy down to fractions of a millimeter or 1/100th of an inch .. or better!

You are going to need a set of vernier calipers to make the required measurements. I use a digital one from Mitutoyo http://www.mitutoyo.co.jp/eng/products/nogisu/hyojyun_01.html

Step 1 Checking the Lower Arms

Using your flat surface (I use a 2.5 sq ft safety glass offcut), make sure the Associated left and right lower arms are the same height. You will be surprised how different these things can be. I have had two arms from the same packet as different as 1mm (0.04 in).






If there is a difference, you will need to “grind” the high one down until they match. To do this, lay a new flat piece of 200-300 grade abrasive paper on your flat surface and apply an even pressure, keeping the molding parallel to the surface. It is important that you do not grind an angle into the block as you remove the material.



In measuring here, the critical height is from the glass plane to the upper surface of the arm where the ball gets inserted.







Step 2 Choose your balls

I have found a much better king pin pivot ball than the standard kit part.
The suspension balls I use are Teflon coated. You will end up with a much freer suspension using these parts while keeping the suspension free of play.

http://rc4less.safeshopper.com/22/389.htm?692

I think IRS do these also. Not cheap but worth it… trust me.



Finally, when you push the balls into the moldings, make sure the collar is orientated towards the steering block. The collar will be pointing up in the lower arm and down in the top ball joint.

If you use pliers to force the balls into location, do use some folded paper to prevent the pliers from causing damage.

Step 3 Trim the lower arms

If you use the CRC front axles (as I am sure you do.. the titanium ones are great!) the securing nut on the inside of the front block will hit the lower arm on full lock so some trimming is needed here. The two photos below show this trim.



Also when the suspension is built the steering block must ride top and bottom on the metal pivot balls, not the lower arm or the upper ball joint plastic moldings.

The castor of the steering block (as a result of the king pin angle) tends to make it bind on back edge of the lower arm while steering, lifting it off the lower ball. To stop this, trim some material off of the lower as shown.











Step 4 Polish the king pins

It’s a simple thing to do but it makes a real difference to the feel of the suspension. Make sure the king pins move freely in the pivot balls. I use a house hold metal polish to do this. After polishing it is important that all residue is removed. If you fail to do this the polish grit itself will become a binding agent in your suspension.

The king pin may be burred where the slot for the E clip has been machined. This makes the king pin slightly oversized and it may scratch the ball as you push it through. A light touch with abrasive paper around the ends followed by a polish will solve this problem.

The king pin moves through the lower arm ball like a piston as the suspension compresses on the spring so it is important to make sure this area runs especially free.

It is also important for the king pin to move freely through the steering block. I use the optional Delrin CRC block. This is more accurate than the stock Associated molding.

http://www.teamcrc.com/crc/modules.php?name=Shopping_Cart&file=product&c_op=viewprod&prodID=7718834

… the reaming for the king pin is really nice and free… but…. There is a problem here. The Delrin blocks include stock axles but if you screw in the CRC threaded axles, the king pin hole tightens up and the suspension will bind.

After screwing in the CRC axles it is necessary to ream out the king pin hole to an accurate 1/8th fitting again. You can do this by hand using a 1/8th drill or (even better) a 1/8th reamer. Alternatively, just stick with the stock axles that come with the blocks.

Step 5 Polish the upper pivot pins

I have started to use the CRC optional Delrin upper arms as they are more accurate than the standard associated ones.

http://www.teamcrc.com/crc/modules.php?name=Shopping_Cart&file=product&c_op=viewprod&prodID=7718833

The upper pivot pin can bind on these arms, mainly because the tolerances in the pivot hole on the CRC part is tighter.



Polish the pivot pins, especially at the ends where the bearing surface is made with the upper arms. You do not need to get the pin to run free in the reactive castor angle block (the 0 degree, 5 degree, 10 degree part), you will probably want to lock the pin in place with a grub screw here anyway to hold the pin fast.

When assembled, the top arm should pivot around the pin with no noticeable friction, damping or binding.

Handling note

I normally run the 10 degree reactive castor block which does offer the most aggressive steering out of the three (10, 5 and 0 degrees). I will reduce to the 5 degree block if the car is too snappy on the front.

Step 6 Shim and Spring the King Pins

Handling note

My starting point is to work with the 0.020 in. front spring. The spring number is referring to the diameter of the wire used to form the coil. The higher the number, the thicker the wire and stiffer the spring. Optional springs are as follows:

0.018 in. Soft
0.020 in. Medium
0.022 in. Hard
0.024 in. very Hard!

Spring Errors



First pick two springs with the same thickness and measure them with your calipers to double check yourself. I know this sounds stupid but I have made mistakes in the past and ended up running different springs left and right!

It is also important that both the springs are the same length to within 0.01 in (0.25 mm) which is the thickness around two shims. If the springs differ by more than this I would assume that one has collapsed and is damaged. Get a new pair.

Adding Shims

I like to shim the king pins so the play is just taken out of the assembly when there is no load on the suspension. The shims are applied to the top of the king pin between the top ball and the upper E clip. Do not use motor shims as their outside diameter is too large and they will bind up in top ball joint molding.
The correct shims for this job have an outside diameter of 4.92mm (0.1940 in).



The two shims sizes I have are 0.11 mm (0.0045 in) and double that at 0.22 mm (0.0090 in).

If you use the Teflon coated balls (as mentioned above) with the 0.020 spring and have trimmed the back part of the lower arm also, a good starting point for the shim stack is 2.22mm (0.0870 in). The problem with giving this measurement is that there are so many variables including the balls, steering arm, king pin length and even the E clip thickness.

Step 7 Final Check

Finally mount the front end on the car.

Adjust the servo links for Zero degrees toe. Then use a camber guage to get to set 1 degree of negative camber.

Always remember, when using the turnbuckles to adjust the front camber, ensure the top ball joint part finishes inline with the king pin. That will mean that it is angled back slightly.

Finally check that the shims have just taken out all of the king pin play and that the suspension has the same free feel when you compare left to right.



Cheers

Mark