Topic: CR 750 FRAME MODS

[Andy Downes]

Hi , is there someone who could post photos or drawings to modify a std K2 frame into a copy of a CR frame, any help would be gratefully accepted.

[nippon]

Hi,

this side offers many close up pics.

http://daytona70.com/English/sales.htm

nippon

[Andy Downes]

Thank you, please keep sending me any info you have, regards Andy

[Sam Green Racing]

Andy, take a look in this thread, you might find the odd picture showing areas where mods have been done.

http://forums.sohc4.net/index.php?topic=27902.0

Where in the UK are you? and welcome to the forum.

Sam.

[ttr400]

I'm busy with my frame at the moment, will post some pics when it's all welded up.

Kevin

[kos]

Hi,

this side offers many close up pics.

http://daytona70.com/English/sales.htm

nippon

Do not order from this site...you will never get The parts you order. I have all factory engineering drawings and samples made up ready for sale. Please contact me if you want a real proper replica.

m3racing.com










Mark @ M3

[lordmoonpie]

+1 to what Mark says - Daytona seem to have delivery problems allegedly!

[nippon]

Hi,

didn't he want to see pics only in order to make his own modifications?

nippon

[Andy Downes]

Hi, thank you for all your replies. I would like to build a CR, with me making the parts, as all of my family and friends tell me I could not build a Lego set.
Mark, please do not for one minute think I do not want to deal with you, but I have been set a challenge,and if I do not meet it, I will be back in one of our spare bedrooms, as my wife is fed up with me moaning on about a CR.
I live in, Stirling Bonnie Scotland, where all I do is wander about talking to anyone that will listen to me re Honda,s.
If have a few bits already for a start, tanks, seats etc, but I would really like a bare frame thus allowing me to start from scratch, but please all be aware I am a total dreamer, but I have to start somewhere, so I will gladly accept all snippets of info, parts and any help given.

I have read Lordmoonpie,s articles and missed his exhaust on ebay, stunning both pieces, that is really rekindled my interest again, thank you everybody for all your help so far, regards, Andy.

[ttr400]

Andy, if you drop me an email I will forward some drawings that Rob from voxonda kindly sent me.

Kevin

[andy750]

Andy you are lucky to have such great roads to ride on. Years back (early 90s) I used to visit a friend who had a farmhouse outside of Stirling next to Bannockburn. The back roads are excellent and going up to Crainlarch was always one of my favourites (if there is no traffic). I bought my F2 from a guy near Dunlane (in `93).

Good luck with the CR. You are near the Tank Shop so are you going to get a CR tank made by them?

cheers
Andy

[kos]

didn't he want to see pics only in order to make his own modifications?


Hers are pics of real Honda made CR750 parts differentiated from other CB parts with "970" number at end of Honda part number.

KOS

Pic #1 correct steering dampener with correct front mounting point at lower steering stem, and bolt on bracket for rear mount.
Pic#2 correct foot peg and correct foot peg brackets (ie: rear set)

[kos]

Also shown in pic #2 two is the bottom two lower oil tank mounts welded to inside of frame. Most people never get this right...so here is the detail. The reason that these two brackets are flat vs right side mounts that are rounded lugs, is for chain clearance.

Mark@M3Racing

KOS

[kos]

Now here is the point many builders miss...are you building a CR750 replica? or a RC "works" Honda 750 replica? Because if you are building a RC replica... the seat used, is shown in this picture and so are the rear frame mods needed to make it fit as it is a much shorter frame and and seat hump. The oil tank breather resides back in the area under the hump section.

NOTE: A CR750 replica seat is much different, as it bolts to the rear of a standard frame where the fender and turn signal stalks bolt on. AND...there is no central oil filter cut away.

KOS
Mark@M3racing.com

[kos]

This photo also shows the proper upper oil tank mounts. Another area many builders miss.

[voxonda]

Mark,

With all your knowledge and experience with these frames, what is in your opinion 'the weakest link' in this concept.
IMHO the 'short' wheelbase (1455mm) together with the trail (95mm) can be held responsible for the infamous 'dead wobble' at speed.
Next to the weak engine mounts. Honda firmed it all up during the years with a wheelbase of 1480mm and trail 119mm for the F2, who had a much better 'roadholding' though needed some more input entering corners.

Rob

[kos]

There are many reasons for what people have come to call "the death wobble" but a short wheelbase has nothing to do with it. We ran our bikes at every track but Daytona at 55 inches. This, by the way, was the standard length for most every World Superbike and Moto GP bike... prior to the Honda V5's introduction. And that was to control the bikes tendency to wheelie under acceleration. Just like a car, a shorter wheelbase goes around a corner of a given length...faster than a long wheelbase machine.

There are a couple of things to remember about the Honda CB750, It has a frame made from what I call it, "water pipe" tubing. Works great for street use but when raced... starts flexing all over. And it is heavy. Not to worry, gussets at certain points, bronze swing arm bushings at the rear, tapered steering head bearings, forks with longer slider tubes for more fork tube/to slider bearing area help a lot. Also, we raised the rear of the bike over 2 1/2 inches to steepen the steering head angle and when a 18" front wheel was fitted made it even more steep. We probably got to 24-25 degrees of fork angle before we were done. Handled great! No wobbles at all.

Another area was to spend the money for a REAL high quality steering dampener like Ohlins and make a very solid mount for its attachment to the frame. The CR kit bracket that bolts on...looks and is the part, but due to its "clamp-on" arrangement can, and will move during high speed racing actions and inputs. This resulting in some very bad wobbles at times.

Also, remember, most builders are stuck in a "time warm" or just building on past experience. The fork tube offset is another area in which to play with front end geometry. We started with standard Honda CB750 offset, but when we went to 37mm fork tubes...we brought along the alloy CBX top and bottom triple clamps for more strength and lighter weight. Of course we had to bore out to 37mm from 35mm, but this front end is where we stayed for over 5 years.

Last part we looked at was the frame breaking on both of the down tubes right after the gusset strengthen tube stopped approx 11" down down steering head. This we determined was from very hard braking load being feed through the frame. We added gussets all the way around the frame tubes in this area... finishing the job that Honda would have done...if only they knew what these bikes would be doing down the road from their initial inception


And BTW....swing arm was bone stock, save for the bronze bushings..

Any more questions...I will be glad to answer any and all questions.


My next frame will not be built from "water pipe" tubing. It's 4130 Chrome moly from this day on! So I can give away all my secrets.


Mark McGrew
KOS

A note here...when Honda went to Daytona for another go at the 200 mile race, during speed weeks they went from standard CB750 fork offset to the shorter offest offered by using CB500 SOHC four triple clamps.The turn-in was reported to be quicker and this shorter offset also reduces wheelbase.

[von_Wanderlust]

Great info KOS!

Re the shorter frames, do you think that the pics of the frame that Benly Ben placed in the Daytona CR750 thread (the one you were interested in - did you end up getting it?) is an example of this? http://forums.sohc4.net/index.php?topic=27902.msg654366#msg654366

It looks like the stamped section/plate (what's that called?) is further along the the top tubes then normal, (i.e. the top tubes are shorter than normal). In turn, this is also the reason why the down tubes (i.e. the one that runs from the headtube to the bottom of the engine - feel free to correct my terminology) is placed directly on the headstem and not in the middle of the stamped plate, and also why the triangular space for the coils is smaller?

[voxonda]

Thanks Mark,

Great info. But... it is likely that there is more than one or a combo of reasons for that 'wobble'. Though my riding skills are very limited, I had problems with 'wobbles' specially when hitting a bump or whatever. I used a fork, with triples (60mm off-set) from the 35 mm DOHC serie with much bigger overlap between tube and slider, head bearings, oem swing with bronze bushings, 1" over Hagon shocks etc. I decided to build a longer swingarm, changed nothing else and now it is rockstable.
Thanks again for your great input.

Cheers, Rob

[lordmoonpie]

Great info KOS!

Re the shorter frames, do you think that the pics of the frame that Benly Ben placed in the Daytona CR750 thread (the one you were interested in - did you end up getting it?) is an example of this? http://forums.sohc4.net/index.php?topic=27902.msg654366#msg654366

It looks like the stamped section/plate (what's that called?) is further along the the top tubes then normal, (i.e. the top tubes are shorter than normal). In turn, this is also the reason why the down tubes (i.e. the one that runs from the headtube to the bottom of the engine - feel free to correct my terminology) is placed directly on the headstem and not in the middle of the stamped plate, and also why the triangular space for the coils is smaller?

This frame in the photo is on Tony Huck's stand t Stafford and Tony gets a lot of his parts from a guy near Chester who has two ex-Honda France endurance CR750s. This I believe is one of his frames and is a genuine factory item. The narrowing of the traingulation space for the coils is a modification to provide better ground clearance for cornering by effectively changing the headstock to engine distances. I've seen a replica of this same frame produced in Reynolds 531 tubing and it's incredibly light and massively strong too.

When I'm a bit fitter I will scan and post the CR750 frame mod drawings I have copies of from Honda in the early 70's. The drawings were made by a Honda race technician working on he CR750s of the time and even come on HRC headed paper. Most refer to the mods that Mark has already posted pictures of but it's interesting to see the period view of things...

[nippon]

Very interesting,...thanks Mark.

nippon

[kos]

Great info KOS!

Re the shorter frames, do you think that the pics of the frame that Benly Ben placed in the Daytona CR750 thread (the one you were interested in - did you end up getting it?) is an example of this? http://forums.sohc4.net/index.php?topic=27902.msg654366#msg654366

It looks like the stamped section/plate (what's that called?) is further along the the top tubes then normal, (i.e. the top tubes are shorter than normal). In turn, this is also the reason why the down tubes (i.e. the one that runs from the headtube to the bottom of the engine - feel free to correct my terminology) is placed directly on the headstem and not in the middle of the stamped plate, and also why the triangular space for the coils is smaller?

The exact reason for the difference in the frame pictured vs a stock CB750 frame is....


Honda lowered steering head by 50mm when designing these frames. The fork tubes used were also shorter, measured from top of fork tube to axel centerline. A shorter fork tube has less tendency to bend under load. This lower steering head, along with shorter rear shocks, lowered the entire machine...which lowers bikes overall center of gravity. These bikes were built to be used with rather hard compound tires of the period, so there was not much concern with leaning the bike over, so the reduced ground clearance had a minimal effect on corner angles. In fact they did not even change tires during the Daytona 200 Mile race!

The fact that this lowered steering head brought the frame tubes closer in proximity to each other might have had a minimal effect on the frames overall strength...IMHO


KOS
Mark@M3 Racing

[kos]

Thanks Mark,

Great info. But... it is likely that there is more than one or a combo of reasons for that 'wobble'. Though my riding skills are very limited, I had problems with 'wobbles' specially when hitting a bump or whatever. I used a fork, with triples (60mm off-set) from the 35 mm DOHC serie with much bigger overlap between tube and slider, head bearings, oem swing with bronze bushings, 1" over Hagon shocks etc. I decided to build a longer swingarm, changed nothing else and now it is rockstable.
Thanks again for your great input.

Cheers, Rob

Rob, You did not say what steering dampener that you were using and how it was mounted to frame? Another area to look at is tires, front to rear wheel alignment and how forks are set up. Spring rate and fork fluid amount/weight can effect the forks reactions to hitting even the smallest of bumps at speed. At any rate, sounds like you have hit on a combination that works for you...and as you stated there is no one thing that causes a motorcycle to act in a a very different than designed way.

NOTE: on the subject of suspension fluids...never use "fork oil" in racing applications, always go with the "Suspension fluids" label when changing fork fluids. Fork oil is good enough for street use, but when you want a fluid that takes a lot longer to break down and effect handling...the fluids designed to work in the highly worked/leveraged single shock rear suspension units are much superior.  I have found that they do come in correct weights if you look.

KOS

Mark@M3Racing

[voxonda]

Mark,
You are absolutely right about the steering damper. As you can see on the pic that I use a (new) simple damper. Mounting is "solid" with a welded on treaded mount on the downtube of the frame and a alloy clamp on the fork.

You also see the forks I used with the long(er) sliders.
Very good tip on the Suspension fluid.
Thanks.

Rob

[CPHjimm]

Great pictures Kos... just a shame I have to do some rework on my frame now 
how does the top brackets for the oil tank look? I having a hard time imagine how it should be bolted to the oil tank and frame.

also, where can I get a similar front damper, the one I got is a modern one where the piston goes through.

thanks
Jimmy