22 replies to this topic

[Les Boyd]

I admit right from the start, I have been building Retro chassis just a few months.

 

I have had a hard time with the rear axle tube. I tried bracing it with a U-shaped wire from the top of the motor mount out to the end of the tube but a couple of races and the tube is bent. I also tried using a brace running from the ends of the tube down to the chassis across the back and then back up to the other tube. Again a couple of races and it is bent.

 

Then the only I have found to get back straight is to pull the bearings out and run a tube that just fits inside through it.

 

I began to think that these tubes are a weak design and just putting the bearings in the motor mount would be a better way.

[TG Racing]

I built a R-Geo Speedwagon (tripod) with no axle tubes. I didn't like the way it handled so I put full length tubes on it and they bent. I cut new ones shorter and braced them. No problems so far.

[John Streisguth]

Some people like the idea of bearings directly into the bracket for that reason.

 

Personally, I prefer axle tubes. I brace them with the wire from one tube to the other, going up to the top of the motor bracket and back, and then wires from the main rails up to the tubes both front and back. I've only ever had one tube get out of whack. 

 

I have had a wide bracket get twisted because of lack of bracing. I just rebuilt that chassis. 

 

I can't see how people build chassis without a fair amount of bracing in the bracket area, unless they just never get crashed.

[Tom Harrington]

Les: I think you left your car at Central TN Raceway after the Shootout. I looked at your car and it looked like a good build but all of the rear axle tube braces and other braces were broken loose. You took a bunch of big hits that would have probably done as much or more damage with the bearings mounted directly into the motor box.

 

I think you've said that you only get to race Can-Am's on an oval? Try getting more laps on a road course before you change your cars up. The track at CTR is fast and tough for first timers with minimum road course experience.

 

It was nice to meet you. 

[Steve Deiters]

Silver solder.

[John Streisguth]

 

Les: I think you left your car at Central TN Raceway after the Shootout. I looked at your car and it looked like a good build but all of the rear axle tube braces and other braces were broken loose. You took a bunch of big hits that would have probably done as much or more damage with the bearings mounted directly into the motor box.

 

I think you've said that you only get to race Can-Am's on an oval? Try getting more laps on a road course before you change your cars up. The track at CTR is fast and tough for first timers with minimum road course experience.

 

That's another way to look at it... if you take a good hit, it's better to break loose an axle tube than to badly tweak the whole chassis. It's one of the reasons I don't tie solder joints at the front axle.

[Tom Harrington]

Great point, John, but his chassis was tweaked also. Les needs laps just like we all did at one time instead of trying to crash-proof his cars. His bent axle tube problem will probably go away with more road course laps. He'll get it soon I'm sure.

[Pablo]

Without a picture of the bent tubes, one can only guess at your problem.
You didn't even say if it was an F1 car or Can-Am.
Lots of guys like the bushings into the bracket, no tubes, and they work fine.
I always use tubes, and have never bent one nor had my bushings misaligned. I do a lot of bracing.
The motor box is one area of a car where you want absolutely zero flex.

Here is an article by Steve OKeefe that may give you some ideas.
Even though it's about vintage cars, some of these methods are still being used on Retro cars today.

Inline bracket bracing

[Ramcatlarry]

Are the tubes bent or is the motor bracket bending or the solder joints breaking loose... or all at different times? 

I do not recall an actual bent TUBE, but I have punched the center out of a ball bearing a time or two as well.
 
If you don't crash them, they do not bend...

[NSwanberg]

When you build a car with tubes you usually use a single length of tubing through the bracket and then cut the tube after the bracket and bracing are all solered in place. When you cut the tube away to clear the crown gear sometimes the tubing shifts ever so slightly giving the appearance of a bent tube. You have to be careful with the initial set up and make sure there is no bind between the bracket and the tube.

I prefer using brackets that are as wide as the class will allow and not use a bearing tube which usually means a hand bent and drilled bracket. I like to have the axle supported by the bearings as far outboard as possible. I think this minimizes axle flexing that can upset the handling of the car.

[Les Boyd]

Sorry I have made repairs before I thought about taking pics. But I have had this come up with the first build that I did and also the pre-built chassis that I purchased from another builder.  

My second build had a lot more bracing on it and stood up to the contact that comes with racing. I am the first to admit that at the Shootout the Black Snake bit me pretty hard and most of that was me. (I don't have the reflexes that I did years and years... ago) And running on a tri-oval has spoiled me. 

I see from the replies, and from what I have done that there needs to be a good deal of bracing on the tubes. I thought that a wider motor bracket and shorter tubes would help, but even then... bracing must be used to keep everything straight.

Thanks for all the replies.

[A. J. Hoyt]

I used to build with axle tubes. Now, I have to believe that the wider brackets (with bearings fit snugly into their holes) are lighter and stiffer (except in the section where the motor is, which might be the best reason to use narrow brackets with axle tubes).

 

I used to bend the axle tubes but I used to get crashed by the faster guys passing me (and not being patient as I always try to let them by). Now, I am nearly as fast as the fastest guys, which means I don't see people passing me on either side nearly as often.

 

I have only bent one bracket (it was a bump-out from an existing crash putting me into on heck of a rider wall shot - the place went silent...) but I had ball bearings, which tolerated the very slight misalignment and I didn't even know it until I checked the car out after the race.

 

I think the wider brackets (use .040 half hard brass, .050 is too thick...) are the way to go with the steel mainrails a good distance away from the can and the magnet flux pull they can create if they are too close or have too much volume of steel (big wire).

 

I have pondered making a 5 bend bracket, just wide enough to support the motor can faying surface, then going at a 45 degree angle to the rear upright areas, parallel right where they reach the rear axle bearings. From an engineering standpoint, that would be very stiff but would require special die tooling to make. 

 

Also, the literal tech Nazi's might say that it is not a 3 sided bracket...anyone want to chime in on whether this would be legal? It certainly meets the intent of the rule, methinks.

 

As I look ahead to avoid crashes, I find I am bending my chasses less - maybe that's the secret?

 

Brace it, race it, bend it, mend it.

 

 

Or just keep it in the slot and hope for the best,

 

AJ

[Pablo]

Maybe this will inspire you:

 

[Half Fast]

Now that's some bulletproof bracing!

 

I assume the V shaped notch is for easy soldering of the bearing/bushing

 

Cheers

[Duffy]

I have pondered making a 5 bend bracket...the literal tech Nazi's might say that it is not a 3 sided bracket...certainly meets the intent of the rule, methinks.

 

   We've seen one or two of those at tech, and our local Tech Nasty doesn't blink twice. Think of it as a rather large corner radius in Bizarro World. Works for meee.

 

Seems to me that bracing of a bracket just adds mass, complexity, and possible failure points. I started with a bombproof bracket and whittled it down where I might, until it started feeling less bombproof, then backed off one. (Putting the metal back turned out to be the hard bit. Lesson learned.)

Likewise axle tubes / in-cheek bearings: following a discussion about that a while back, I went to BBs right in the bracket with drill blank axles, telling myself the small "give" in the drill rod would transfer an impact more gently throughout the local structure.

In a couple years' hard use down in The Brutal Mains, I haven't had a failure with these conceits. Maybe it means something.

 

In my neighborhood, we got guys who use both and all applications; and it's my observation that if a guy's car hasn't got pretzeled he'll tell you that that proves out his construction method.

 

Duffy

[Phil Irvin]

Keep It Simple Stupid---KISS ... Me personally. I think you can over do it. 1 angle brace to the tubes and 1 to the motor like Pablo's first  pict.. If you hit hard enough to bend the tube. I think you will have other chassis problems too.

 

                      OLPHRT

                       PHIL I.

[slotcarone]

Les I brace my axle tubes and brackets much like Pablo does and rarely have a solder joint break but in a hard hit you are going to bend something inevitably. The easy way to straighten them is to put another bushing in the tube inside the bracket and slide an axle through and you will see the misalignment from one side to the other.  Bend the tubes until they are straight again and resolder if necessary. Hope this helps.

[Pablo]

"I assume the V shaped notch is for easy soldering of the bearing/bushing"
Correcto. I don't always do that, but I did on that one.

Note, I use different size wires for my bracing, depending on the use.
.032, .047, 055, and , rarely, .063. The 90 degree braces than run out the length of the tube are usually .032.

One thing I feel a lot of guys overlook, is bracing the tube on the INSIDE of the bracket as well as outside.
Try this experiment:
Take a (junk) bracket you don't mind ruining, that braces the outside of the tubes well, but not the insides. Install an axle through one bushing, and grab hold and TWIST. Unless it is braced inside, it will come right off. Having said that, on a 3/4" wide bracket for F1, there is NOT a lot of space on the gear side to do this, in which case, .032 wire is mandatory.

[Les Boyd]

Thanks for the pics Pablo they are worth a thousand words. Indeed they inspire me.

[slotcarone]

Les if you search for "Mike Katz Chassis Builds" you will see more pics of bracket bracing.

[Rick Moore]

There’s more than one way to build a cat, or skin a mousetrap, or mix a metaphor…

 

So here’s yet another…

 

Most folks add the rear axle tube onto the motor bracket. I do it bass-ackwards, and build the rear axle tube as the primary chassis component and add the motor bracket as a secondary structure. I started doing it this way over fifteen years ago when I had no inline motor brackets, and would just build the chassis so the motor could just solder into the frame. And when the subsequent rules mandated the motor bracket, I just started adding them after the fact (which explains my proclivity for narrow motor brackets). I haven’t had an axle tube fail in all that time, which of course doesn’t mean it can’t, but so far I’d say the failure rate is pretty low, so…

 

For inlines I always have at least three main frame rails bent as uprights to the axle tube; on the one pictured there are five per side (two aft side and three forward). I attach them at different angles, instead of only perpendicular with the plane of the chassis. I also always include a “spreader” that I solder atop the wire I use as my “gear guard” wire to add side-to-side strength. Later, the motor bracket is modified to fit the frame (since it has to be there; but the chassis would work just as well without it); soldered in, and then I cut the middle of the rear axle tube out.

 

The first pic shows the axle tube before the spreader is attached; the second pic shows the motor bracket installed.

 

Whatever gets the job done is the right way…

 

Rick / CMF3

 

 

[Danny Zona]

No axle tubes for me. Its just one more part to fix and it adds unwanted top weight IMO. I've never had a car not handle because it has axle tubes or not. That's the last thing I'm worrying about when it come to my car handling better. But everybody has there way of doing things. That's waht make retros so much fun.

[racie35]

I think mike at chicagoland sells a CNC brass motor/axle holder now that oughta be a Hercules when braced...little more money but hey..if your having probs, maybe try it