Topic: Max lean angles for cb750 based race bikes

[gschuld]

Geometry again....

The common approach for factory frame based cb750 road racers is to raise the rear around 1.5-2” for the purposes of lean angle clearance.  The front end is generally either kept at the standard height fork position or raised slightly(maybe an inch).  The 18” front wheel/tire lowers the front end compared to the 19” front tire/wheel in itself.  The tree offset is then tailored to bring the trail back in the “green”(usually between 30-45mm offset)

My question relates to actual grounding points.  The Works CR type exhausts are a severe lean angle limiter with modern race tires.  Raising the rear for these is a given.  Many bikes have pushed to alter 4 pipe exhausts to tick as close and high as possible to reduce this issue.  For those with 4 into 1 exhausts that are up high and tight, is there less need to raise the rear?

The main and obvious grounding points outside the exhaust, belly pan, and toes🙄 is the engine case sides at the alternator and ignition positions.  Assuming these are kept to minimum width (for example: left side of crank cut off with block off plate and slightly thinned down points side cover), how high does the suspension really need to be raised to keep a hot shot skilled rider who likes to test the leaning limits on the track?

George

[gschuld]

Corey Forde and Dean Oughtred in Australia.  Dean is on a respectably faithful cr750 using many original “KIT” parts.  Corey Forde is pretty much the opposite.

Looking at the photo of them leaning together, it seems Dean is a little bit farther over compared to Corey.  But Dean is nearly scraping paint, where Corey has LOTS of clearance to go.

Oddly enough, Dean has nearly dominated his multi bike CB750 based competition in post classic and has won numerous championships over the last few years.

It seems counterintuitive....

George

[gschuld]

Some random cornering shots.

George

[madmtnmotors]

The CycleX road racer used allen socket head screws for the shifter cover. The rider leaned far enough over to the left to bevel the heads of those screws. The picture was a lot nicer before Photobucket started their extortion campaign (eighth picture down):

http://forums.sohc4.net/index.php/topic,108206.msg1212590.html#msg1212590

[gschuld]

The CycleX road racer used allen socket head screws for the shifter cover. The rider leaned far enough over to the left to bevel the heads of those screws. The picture was a lot nicer before Photobucket started their extortion campaign (eighth picture down):

http://forums.sohc4.net/index.php/topic,108206.msg1212590.html#msg1212590

That’s pretty cool, though there are factors that make must be taken into account.  The CycleX bike ran 17” wheels and tires(lowering the bike) on both ends, and a calculated measurement from a side view picture suggests a eye to eye rear shock length of 14”.

This offers a lower center of gravity, but also reduces lean angle clearance.

George

[bwaller]

Unless the head angle is changed, then raising the rear is the only option to help them turn, but I know you're aware of that.

Rearsets and lever bits can be tucked & raised out of the way, so if engine covers are addressed and exhaust isn't a concern, my best guess is perhaps 1" in the rear ought to keep hard things from throwing sparks. But there are a lot of unknown variables also, suspension loading for one.

My old boss raced with a 19" rear wheel back in 70-71. We could get a 4:10x19" Dunlop K81 which he used on the rear. In those days most production based bikes ran these tires on the track. Just to say, he used four pipes (not sure the manufacturer) and didn't have serious issues.

[gschuld]

FWIW

Avon 150/70/17 is 24.2”
Avon 130/70/18 is 25.6” (1.4” taller or nearly 3/4” less radius and ride height in the rear)
If the rear shocks on the CycleX bike are indeed just over 14” eye to eye, that would be like a 13/70/18 rear tire with a 13.25 or so effective rear shock height. (Basically stock)

I may have missed something in the swing arm or something else that makes my assumptions incorrect.  But it seems the CycleX bike was on the low side for a road racer.

Brent, yes, the steeper steering heads angle offered by raising the rear side seems to help quicken the turn in. 

I’m just trying to figure out the right blend between cornering clearance and good overall geometry.

The less one has to raise the rear, the less issues one will have dealing with the worse and worse chain issues resulting from the misalignment of the swingarm pivot.  And the overall center of gravity would be lower.  For those with such highly lifted suspensions, how does a 5’8” rider even touch the ground when seated?  Can’t be by much.

George

[slikwilli420]

With the way my bike is set up (14" shocks and GL1000 forks on custom trees) I find the edge of the tire quite easily and don't drag anything ever.  I could see 17" wheels with more modern rubber causing a headache but on 18" wheels with the standard Avon/Conti tires, there isn't much worry, unless you have those monster meggas to drag, but a 4-1 is tucked in plenty to use all of the tire and  still have some left over. Fresh pavement and fresh Conti's this weekend at Barber makes me think if there is a chance to drag hard parts, this will be it.

[gschuld]

Good luck this weekend👍 

George

[simon#42]

most things in racing are a compromise , i have never liked the long shock solution to change geometry  but it is very easy and that is its attraction . the proper way to do it is to alter the frame , if this is to much trouble you can make up yokes to alter the steering angle . i first spotted a set on a rg500 suzuki in 1976 but doubt they were a new idea then , i think harley davidson used them on some road bikes .

[gschuld]

most things in racing are a compromise , i have never liked the long shock solution to change geometry  but it is very easy and that is its attraction . the proper way to do it is to alter the frame , if this is to much trouble you can make up yokes to alter the steering angle . i first spotted a set on a rg500 suzuki in 1976 but doubt they were a new idea then , i think harley davidson used them on some road bikes .

Simon,

You mean an intentionally mounting a mismatched offset set of trees(homes) to alter the geometry?

Wouldn’t that effectively be the same as using a matched set of shorter offset trees(yokes)?

George

[simon#42]

no changing the offset is not the same as changing the steering head angle . on a set of standard yokes the holes for the fork tubes are bored at 180 degrees to the yoke . if you alter that angle it has the same effect as altering the head angle . its a good tool for developing a frame , once you have the angle that works best you can either alter the frame or more likely make a new one and go back to standard yokes.

[turboguzzi]

George, know the proverb in order to finish first, first youo have to finish? I'll add to it, first you have to finish the bike!

So dont get stuck on grand plans to out-trick everybody else, just grab setup quotes from someone like slikwilli's as a baseline. Everything will change when you hit the track anyway.....

The only practical tip i can give you is to do a check that you have enough clearance. no fun reaching the track and finding your rider is grinding hardware in turns first session out.

Grab a long metal angle and put next to the two wheels. then take some plywood, put it at 45 degs while leaning on the angle's  internal corner.

It will let you visualize in no time what is sticking out.... allow for 50 mm of suspension compression that happens while leaning, see pic with ruler at the end

[bwaller]

I'm more "need to see actual conditions" kinda guy. Once I knew how much travel I had front & rear, I used rachet straps and compressed both ends to maximum travel. Then tipped the bike over using a strap from a secure point on the ceiling. 

Didn't really care what the angle was, but more importantly what would touch down first. If it was the end of a clip-on I knew I was good to go! 

[bear]

That's my kind of engineering. 

[gschuld]

I was wondering how much suspension compression should be taken into consideration when testing lean angle. Yossef likes 50mm(1.95”) at both ends and Brent bottoms both out.  Full travel is in the neighborhood of 3” front to back?

So safe bet is to measure at 2” compressed and 3” compressed at both ends and compare.  Does anyone have a sense for a realistic max lean angle?  I know we are not taking 60 plus degrees like MotoGP guys🙄

45 to maybe 50 degrees?

George

[RAFster122s]

Well, chicken strips will answer the question on a bike that is already racing.
But, then you have to lean the bike like Brent suggested to be able to measure it.

I have surprisingly leaned a little Honda C70 Passport over enough to start scraping the footpegs in corners.  Yeah, I don’t slow down too much for corners. . . If I had the leg shield on it might have dragged too.

Good luck figuring out safe value for the lean angle. It is an interesting discussion.

[bwaller]

Vintage suspensions require all the attention one can muster. Good components, then initial setup, including sag% etc.

My goal is to use as much travel as possible and every weekend spend time working up to that. (as closely as possible) Our total travel is 116mm (4.5") front, 79mm (3") rear. O-rings on fork tubes and shock shafts allow us to see what travel is being used. Even though by vintage rules, we're limited to very "basic" adjustments, we still should make suspension as much a priority as possible. It counts towards better lap times.

George, static measurements will only offer a partial amount of information, but it's a start in the right direction.

[gschuld]

Brent, do you see near the full suspension travel when racing?

George

[bwaller]

Absolutely George. We keep notes with a starting point for each track, and fine tune from there.

[gschuld]

Would you have a guess for your maximum suspension compression in the high lean corners?

George

[bwaller]

Sorry George, missed your last. It's a tough question, but consider static compression sag at 25-28%.

For certain most of the rear travel is used on the throttle hard out of a corner, the front is another matter. If it's a heavy braking entry, (where all the travel is used) I always trail braked to keep the front from popping up so it wouldn't decompress much until on the throttle again.

To your point finally    and a best guess considering the static sag again, in a fast high lean corner where no brakes were applied, maybe 80-85% travel??? More front until the apex, more rear after.

I'm sure there will be other opinions!

[gschuld]

That’s excellent info, thanks Brent👍

George

[Don R]

 My Blue/Green K0 had all of the pegs narrowed (cut and weld) in the brackets, the pedals were all bent in towards the engine. I assume the PO fancied himself quite the rider. The 4-1 was quite flat on the bottom so I assume he jumped a lot of curbs after class at college.