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Frame geometry

C

Captain Dave

CB750 Member
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411 Surf Rd Melbourne Beach FL 32951
I have a 1980 CB750C 32,000 miles on the spedo. I Purchased the bike used several years ago and finally got it running last year. The motor runs good. The bike was then parked for several years prior to me purchasing it. The seller did not know the history of the bike since it was driven by a family member who is no longer with us.

Since the the first few minutes of my first ride on this bike it has some unusual handling characteristics. The handle bars are bent. I should have a new stright used set soon. Second the bike is unstable drifts and wobbles when I relax pressure holding on to the handle bars when in motion.

I have owned many bikes and they have all (but this one) been very stable. In fact on all my other bikes I am comfortable letting go of the handlebars briefly. Not so with this bike. When I do it drifts mostly right.

I have examined the bike for bends and can't find any obvious problems. On a closer look I ran a straight edge from the outsides of the rear tire forward. The front tire is not on the center-line as I expected it should be. The front tire is about 2 to 3 inches to the right (sitting on the bike looking forward) of center-line.

I first checked the rear swing arm for proper positioning and put 2 new tires on front and rear. I can't adjust the rear tire with the adjusters enough to align with the front tire. I brought the bike to 2 local shops, one said it was fine the other said bike manufactures sometimes design them that way. I find both these answers hard to believe.

My next step is to bring it to GDM Computrack in Atlanta and have them take a look. GDM has equipment to mesure and straighten frames. Before I head down an expensive repair any suggestions, recommendations or advice will be much appreciated.

Thanks in advance,
Captain Dave
 
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I would first check the tightness and condition of your steering head bearings, but that may not yield anything. I would also make very sure that both fork legs are straight, both the chrome upper tubes and the lowers in relation to the uppers.

However, I much more suspect that the impact that bent your handlebars tweaked the alignment of your top and bottom triple trees or bent them relative to each other. I don't remember how exactly they're constructed on this bike, specifically whether the steering "axle" is pressed into the lower triple or affixed in some other way. With a press-fit steering axle an impact can actually rotate it relative to the lower triple, and you cannot fix this without taking apart and reassembling the entire front end.

On a previous motorcycle with a similar front end (a 1993 Suzuki GS500E), I was in a lowside crash that tweaked the triple tree alignment slightly, but not enough to noticeably alter handling. I tried to correct the bend a few times without taking the front end apart too much but they never worked, so it stayed that way for a year until I traded it away to a guy who was doing a complete frame-up rebuild on it anyways.

Both of those shops that you took it to are shops that you should never go back to again. Both of them are either ignorant enough of frame geometry to be dangerous, or are completely apathetic about their customers riding completely unsafe motorcycles.

If you've never disassembled a steering head before and aren't confident in your wrenching abilities, I would just take it to GDM Computrack like you mentioned. This is a serious safety issue; how much is your safety worth to you?
 
GDM has measured the frame and as suspected the steering head is not pointed in the correct direction. There is a 3.3 deg. twist between steering head and swing are pivot.
At axle level offset 42.7mm to right.
front wheel 55.6mm to right of back wheel at ground contact.
 
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Yipes! That'll do it. A bend between the rest of the frame and the steering tube usually means a totalled-out motorcycle, unless GDM or a custom builder is willing to work some bending or fabrication magic.
The less-legal way that people sometimes use to get around this sort of discovery is swapping the VIN plate to another frame.
 
They have repaired the bent frame.
 

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  • Frame Machine.jpg
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  • Frame Stright.jpg
    Frame Stright.jpg
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Captain Dave said:
I have a 1980 CB750C 32,000 miles on the spedo. I Purchased the bike used several years ago and finally got it running last year. The motor runs good. The bike was then parked for several years prior to me purchasing it. The seller did not know the history of the bike since it was driven by a family member who is no longer with us.

Since the the first few minutes of my first ride on this bike it has some unusual handling characteristics. The handle bars are bent. I should have a new stright used set soon. Second the bike is unstable drifts and wobbles when I relax pressure holding on to the handle bars when in motion.

I have owned many bikes and they have all (but this one) been very stable. In fact on all my other bikes I am comfortable letting go of the handlebars briefly. Not so with this bike. When I do it drifts mainly right.

I have examined the bike for bends and can't find any obvious problems. On a closer look I ran a straight edge from the outsides of the rear tire forward. The front tire is not on the center-line as I expected it should be. The front tire is about 2 to 3 inches to the right (sitting on the bike looking forward) of center-line.

I first checked the rear swing arm for proper positioning and put 2 new tires on front and rear. I can't adjust the rear tire with the adjusters enough to align with the front tire. I brought the bike to 2 local shops and one said it was fine the other said bike manufactures sometimes design them that way. I find both these answers hard to believe.

My next step is to bring it to GDM Computrack in Atlanta and have them take a look. GDM has equipment to mesure and straighten frames. Before I head down an expensive repair any suggestions, recommendations or advice will be much appreciated.

Thanks in advance,
Captain Dave
Click to expand...
sounds very close to whats happening to me !
 
WOW !!! How much was for the repair of the frame and do you kow if they are in South Florida ? Thank you !!!
 
$250 to measure your frame, $700 to straighten mine. Unfortunately their shop in south Florida has closed.
You can check your steering head by running 2 strings from both sides of the back tire forward past the front wheel. My guess using this process was within a 1/4 inch of GDM's measurements.
There are several internet videos that show how to do this. Here is one.
 
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