Topic: Chassis setup for Vintage road racing...again...;)

[Dresda500]

I've read through lots of the various posts and picked up on some trends, but wondering specifically where you guys are sourcing the 30 to 35mm offset trees in 35mm? I'm currently racing a CB250 K and buidling a CB500 with Dresda frame. I race in Scandinavia and our track are typically 2.5km long and more technical than high speed compared to some track in UK or USA.

This is the CB250. It is the same as the CB350 K twin which is very  popular to race everywhere. It's a bit shorter than y'alls CB750. My trail numbers seem very short with the CBX 45mm offset trees with the 24deg rake. This is with full droop on suspension, tires just barely touching the ground.

Some notes from my chassis setup excel sheet: I had a low side unexpectedy last weekend, just lost the rear tire in perfect weather right after that pic was taken below.
 
Fork tube dia:      35mm                        
Fork tube width:      180mm c-c From Honda CBX1000 1979                        
Axle diameters:      20mm hollow, both                        
Front Tubes are T45 chromoly from Maxton                              
Front forks have Maxton SD20 Cartridge conversions with Rebound in right leg  (2.5W)and Compression in left leg(5W).                              
 After a low side at Våler and bending the tubes, I have decreased air gap from 160mm to 155mm as well as 10mm of spring pre-load from the 9mm used previously. This is mainly due to forks coming very close if not bottoming out under braking.                               
Tubes replaced with exact as before but am ordering tubes with +25mm length and will put spacers under cartridges to keep interface the same. This will allow room for front tube adjustment in the trees, but need to compensate by raising rear if doing so.                               
Rear shocks are Maxton twin shock adjustables. Year unknown, were on bike when I bought it. Will send to Maxton this winter for rebuild.                               
Swingarm is lengthened by 30mm form stock and tubed with 1.2mm wt Docol R8. Font section is braced with Docol 1.2mm plate.                               
Custom sprockets are made and converted from 520 chain to 415 ERZ from DID.    

                           
Any recommendations or comments?

[scottly]

My trail numbers seem very short with the CBX 45mm offset trees with the 24deg rake. This is with full droop on suspension, tires just barely touching the ground.

The suspension at full droop isn't a real world situation. What are the measurements with rider and fuel aboard, and how much more does the suspension compress in a hard turn? Did the fairing hit the track, causing the low side? 

[Dresda500]

Hi Scottly,
Yes, I agree, full droop doesn't represent live conditions, but it is what every professional suspension guy I know uses as a base and used to check sag. I first thought that maybe the muffler ground out, but I'm not sure now. I think I might have just had it too far leaned over and too stiff rear spring setting. Being a shorter bike one has more rear weight than normal maybe with rider, which one would think would help rear tire grip.  Here are the numbers for weight distribution and for sag on frobnt and rear with and without rider. The rear spring preload might be a bit high. Front looks pretty good in my novice opinion. I didn't feel any sliding or shudder in the rear tire before it let loose, which also made me think it ground out and lifted the tire suddenly, but a stiff spring and a bump in the turn can do the same thing.
Compression on front fork in a race situation is usually in the 65-70mm travel from static with bike sitting on paddock stand and no rider...but this measuremnt will be from hard braking, not mid turn with minimal braking.  This is putting the dust seal pushing the zip tie pretty much rught up against the lower tree. I don't have any more fork length to push the tubes lower in the trees but am going to order a set of +50mm longer tubes to have some adjustablity in the future.  Not sure how much rear travel I'm getting, I need to pay more attention to that going forward.

So with more sag in front than rear, the trail gets even shorter in a race situation, not just braking, but cornering as well.

i'm 5'11" and 185lbs (178cm , 83kg)

[simon#42]

a picture from the side when on the ground would help . with the bike on a stand it looks like you need 35mm offset yokes and shorter shocks . it would also benefit from a longer swing arm try try and get the weight a bit further forward . thats from what i can see but without riding it its very hard to guess .

[Dresda500]

I figured out one thing I fumbled on before last race...I swapped from the CB500 trees to CBX1000 trees as they are aluminum lower and dual pinch bolt. Same 45mm offset and width. Well, the CBX has a thicker and flat top clamp, whereas the CB500 has a dropped top clamp...so I inadvertently shortened my forks by 20mm or so. Here is a pic with the CB500 top clamp installed before sliding the tubes down. This will help a bit by raising the front end this amount, but still think it will need some 30-35mm offset  to get it better. Are there any factory trees with this dim? I have a set of Marzocchi on the shelf that are 25mm offset, but that might be too much. I need about 180mm center to center.
I will say though, it rides quite OK, not twitchy or falling into the corners. When I first got the bike it had CB350 trees which have I think over 60mm offset....it would fall into the corners VERY fast before, the CB500 trees where a huge improvement.
Here's a few pics form the side with weight. This was just before the last race with the CBX trees installed. I lengthened the SA last year an inch or so, but I think it could use a bit more.
Do any of you guys use a chassis program? I don't have one and not thinking to buy one of the super trick ones that you pay a quite large sum as a yearly subscription. I don't mind buying one if it is useful.  Would be nice to input all my measurements and then see how it affects the geometry by changing tripple tree offset for example and to track changes throughout a certain bikes life.
I'll be making som major changes this winter, hopefully includding a new frame and a new tank that is a bit smaller, narrower at the knees to fit my body and lower the height of whole tank a bit. The frame will be lightweight Docol R8 tubing , but basically a copy of the original for the most part as I have to run a stock frame, but "modifications are allowed"... I can move SA mounts, engine position and whatever slightly, but can't build a true one-off race frame.

[turboguzzi]

Well, ill take a shortcut back to your original question,

yes, i think you definitely need 30-35mm offset triples to bring the trail to around 100mm range.

Took me a while to find a pair back in 2008, i think mine came off a honda or cagiva 125 two stroke for the italian market which i adapted, was quite a bit of work.

but today its so easy to have them CNC that id go that way instead of trial and error/modifying existing parts

Mirko form messnermoto.com was  an active member here, he does one off cnc jobs for people all the time, ask him.... mirko@messnermoto.com