Author Topic: chassis top rail has bowed outwards  (Read 2610 times)

Offline longtimeracer

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chassis top rail has bowed outwards
« on: November 13, 2020, 05:02:35 AM »
Got a bit of a conundrum with my FED, its a 150" mild steel chassis with an injected Chevy small block. The chassis is 5 years old, built to SFI 7.50. Only done 30 runs as I only do 3 meetings a year, its a hobby car, not ultra serious but I don't stint on what it needs. Its a regular low 9 second car but got it into the 8s in 2019. Taking the motor out, I noticed that the offside top chassis rail(that's the right-side from the drivers seat) is bowed out by about 3/4". At the front the motor mounts on saddle brackets held with Micalor hose type clips. I'm thinking that in a wheel stand the right side front motor mount slipped because of the weight of the motor and then pushed the chassis rail out? Also the torque reaction of the motor would tend to push the motor down and push against the right hand side chassis rail which might worsen the effect of a wheel stand. These are thoughts that I've had. Regarding saddle mounts, wouldn't these work better if dowelled? Of course I've got to get it fixed or may be front halved? I would be very interested in what you guys think, having a bit more experience than me, thanks.

Offline longtimeracer

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Re: chassis top rail has bowed outwards
« Reply #1 on: November 14, 2020, 03:43:56 AM »
Hi Roo Man, what do you think? Value your comments

Offline MaggiFinn

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Re: chassis top rail has bowed outwards
« Reply #2 on: November 27, 2020, 01:59:13 PM »
I think the tube that holds the steering box should be fastened to both top rails...


Offline longtimeracer

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Re: chassis top rail has bowed outwards
« Reply #3 on: November 30, 2020, 05:41:57 AM »
Hi Magnus, the steering box is tied to both top chassis rails, the bowing out started in front of the motor and the chassis builder has concluded that a wheel stand did this when the engine came down with the chassis and the weight sprung the top rail out, its also made a a slight curve on the LOWER frame rail on the opposite side, aren't wheelies great :-\. I'm having to front end the chassis now, with extra bracing.

Offline JrFuel Hayden

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Re: chassis top rail has bowed outwards
« Reply #4 on: November 30, 2020, 03:00:43 PM »
Our Heritage JF chassis had the top rail bow out in front of the engine so we just added a couple of cross bars , helped make the chassis stiffer and helped with the tire shake.
We also added another 20 lbs to the front to control the wheelies better and we run our wheelie bar about 1 1/2" off the track. 
Jon C. Hansen

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Offline Curly1

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Re: chassis top rail has bowed outwards
« Reply #5 on: December 08, 2020, 07:06:56 PM »
You need to be able to control the wheelies. On my 225" FED I can pick up front wheels 4 feet high and carry them out well past 60 foot? or with just 25 Lbs on front and no other changes will pull them up 4 inches and maybe carry them 2 feet? I do not even run my wheelie bar any more I use weight up front to control wheelies.

Offline nitroscott

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Re: chassis top rail has bowed outwards
« Reply #6 on: March 24, 2021, 12:07:08 AM »
I am building my chassis 225" also. Thanks for the info about putting weight up front to control wheelies. Now I just have to figure out a good way putting weight up front. I am building the nose now and it would be real easy to incorporate some kind of aparatus for weight carrying. My engine is a SBC 350 and it is out 50 inches from the center of rear housing. The only thing I can think of is to stack plates on a small pole on a plate attached to the chassis.

Offline Curly1

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Re: chassis top rail has bowed outwards
« Reply #7 on: May 11, 2021, 07:33:08 PM »
So at the end of season last year I was experimenting with taking wheelie bars off and using weight to control wheelies. It sort of worked but created another problem, popping front tires out of beam. A little weight on the front makes a big difference but after trying it with out the wheelie bars I am going back to them. WHY? Well because it was popping front tires out of beams and wheelie bars seem to help that. It was popping out of the beams then it set them back down and after that it did a beautiful power wheelie.  But you can not win if it is popping out of the beams as it goes red and is not consistent. So I have moved the battery from the middle to the nose adding 17 Lbs up front and then putting another weight plate to add 35 Lbs up front and will be running the wheelie bar again.

What I think was happening from the video was the rear tire was wrapping up and front end coming up out of beams, then it goes right back down and slowly comes up. With the Wheelie bar it was hitting before the tire gets completely wrapped up and that helped stop it from popping out of the beams. Then as it get up on rear tire it was still allowing it to do a nice little wheelie about 8 inches high and carry it out about 60 foot. If that makes sense.

 One other issue I was having and the weight may help there to is the front tires simply were not doing much. I was cranking the wheel to the right and it was still going left a little. I believe under full throttle if you could scale my car I bet there was not 25 Lbs of weight under at the finish line.

Offline BK

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Re: chassis top rail has bowed outwards
« Reply #8 on: May 13, 2021, 08:04:57 PM »
I am building my chassis 225" also. Thanks for the info about putting weight up front to control wheelies. Now I just have to figure out a good way putting weight up front. I am building the nose now and it would be real easy to incorporate some kind of aparatus for weight carrying. My engine is a SBC 350 and it is out 50 inches from the center of rear housing. The only thing I can think of is to stack plates on a small pole on a plate attached to the chassis.
If you have a tube front axle fill it with shot. Mine had a couple pipe plugs in it to add and remove shot.

Offline retroboy

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Re: chassis top rail has bowed outwards
« Reply #9 on: May 29, 2021, 03:30:42 PM »
Hey Mate. I bent my chassis doing wheel stands so run a string line along the bottom rails and see if you haven't. Bend the bottom rail upward and the top rail will bend in or out.
Cheers

Offline longtimeracer

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Re: chassis top rail has bowed outwards/front motor mounts
« Reply #10 on: June 01, 2021, 04:39:13 AM »
Thank you all for your comments, very helpful. I had the chassis front halved but now I'm worried about the front engine mounts. The chassis is slightly taller and narrower and so the old mounts do not fit. New mounts mean the plates are at a downward angle which worries as if the engine drops slightly in a wheelie they wont be rigid enough to stop the engine pushing the top rails out - again! I think the mounts should take a vertical load only to prevent this, especially as the saddle mount is hose clipped to the top rail(Micalor clip in fact) but I'm not a chassis pro. I'm thinking of having plates which pick up the water pump bolts as well as the lower air conditioning bolts and having the plates sit over the top rail. What do you guys think, am I overthinking this?

Offline THE FRENCHTOWN FLYER

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Re: chassis top rail has bowed outwards
« Reply #11 on: June 01, 2021, 12:20:10 PM »
I think your plate redesign reasons are sound and the issues are valid.