I installed some cagiva alazzura triples on my narrow case Ducati to make room for my giant, manly front brake. I gave up about an inch of offset, gaining an unknown amount of trail. Any one have any suggestions? Raise the back? Drop the front? Just do a lot of curls and muscle it around?
I use the Excel rake and trail calculator from DataMC: https://www.datamc.org/downloads/ I use an inclinometer (cheap digital level) to get the rake angle, as I find it easier than any measurement methods. You might be able to just get an app for your phone to do this. That said, I am pretty sure Italian chassis from the narrow-case era were not very scientific. Are you having any handling issues now? As a default, I usually try to shoot for a trail figure between 95-105mm, and I usually prefer more rather than less trail for small bikes. Outside of changing triples or re-welding the headstock angle, changing the ride heights is pretty much all you can do.
Thanks. That program should be a big help. I put the new triples on a spare frame, so I can't just ride it around the block. When I'm done with the fab work, I'm just going to swap my motor and bodywork over. I had to trim the headstock, so I left the original frame alone.
No problem, hope that helps! The other thing that people who are looking at steering behavior tend to forget is that wheelbase will have a big effect. A long bike with too much trail is a proper truck. A short bike with too much trail might feel less twitchy. I bet that Ducati will be fine at anything under 125mm trail, but I still recommend trying for 105-110.
What kind of Ducati? If you 'gave up' (meaning REDUCED) offset of the triples by 1" compared to the units that came off, then you've increased ground trail by that amount...1" (25.4mm). That's massive. Like, night and day. If the chassis was in the 'normal' range prior to the change, say 90-100mm or so, and now you're at 125mm - the bike will feel like a wheel barrow. In order to reduce the effect of this without changing hard parts, all you can really do is lower the front of the bike. If you raise the rear to get a similar result, your adjustment will also affect swingarm angle quite considerably (increasing it).
We're talking about a 1960's Ducati here, the chances of two of them coming out of the factory with the same head angle were quite slim.
Offset is a printing process and you can use one of these to do it And this is a trail and you walk on it, any other weird questions?
Actually, a little more swingarm angle might not be bad. The stock position is nearly horizontal, so a bit more angle might help load the front on corner exit. And keep my exhaust of the pavement. I'm going to establish some baseline numbers and see how the bike feels. It's entirely possible that I'm too old and slow to tell the difference these days.
Modern fork offsets are difficult to use on vintage bikes / frames, here are some details on the reasons, this should also help you to know IF you can/should run the triple clamps you put on the bike. First, the support structure of typical steel tube frame vintage frames lacks the strength to control the induced forces of rake angles less than 25 degrees, with the reduction in the offset of the triple clamps it is almost 100% likely you will have to get under 25 degrees to get the trail anywhere in the range for non-tractor like handling. If you reduce the rake enough to get the trail in range you may have a meeting with GOD during the tank slapper, or loose the front while cornering. The steering head angle and structure on a modern bike is engineered to handle the force generated by steep angles, the steeper the angle the less offset in the triples, some modern bikes have rake in the range of 20 degrees. There are two types of tube vintage frames (my definition) single top support tube and hybrid lowboy / Seeley style frames. these two styles can be seen in period Yamaha frames, the RD 250/350 through 1979 and the TZ 250/350 through 1977 and the RZ350 84/85 and the TZ250/350 79, 80, and the 81-85 TZ250's. then into the modern Ducati trellis frames which have more advanced steering head geometry. For street riding many people get by with modern fork retrofits because in reality they are just cruising, on the race track a bike with excessive trail will be very stable but it will lack the ability to a tighter line at higher speeds. Numbers, RAKE: A vintage frame with a single top tube steering head will not behave well if the rake is less than 24 degrees and some may object to less than 25 degrees. the more reinforced later RZ/TZ steel frames will likely be fine at 24 degrees. TRAIL: Be careful at going below 3.6" on the later style steel frames you may be able to go to 3.5" but test carefully, it may be to twitchy. On my RD400 I ran about 3.75" of trail and it was incredibly stable and would turn on a dime with zero twitchyness. Trail numbers in excess of 3.8" will get progressively slower but will also be very stable. Make sure you know the actual tire diameter when inflated, this is critical to getting the proper numbers when you calculate the trail. ADJUSTING the bike to change the numbers: Without having custom triple clamps made getting the trail into an acceptable range with the clamps you have will be a function of increasing the rake, this can be done two ways and they can both be used to to achieve the maximum adjustment. 1. Raise the rear, this can be done by changing the shock length or moving the mounting location, the positive is it is easy if your shocks have a length adjustment. The negative is there is a limit to how far you can go before the arm angle becomes an issue (chain rubbing on the pivot. If your wheelbase is well under the limit lengthening the arm with the same shock position will decrease the trail (likely a small amount). Wheel base is often misunderstood, Front end geometry has more influence on a bikes ability to turn than wheelbase. Example if adding 2" to the wheelbase and slightly increasing the angle of the swing arm gets the rake and trail in the sweet spot the will turn fine and be stable. I am not uncomfortable with a wheelbase as long as 52" most super bikes are in the range of 54". There is a secondary advantage to the loner arm/wheelbase, it also changes the weight bias placing more weight on the front end which is a positive on older bikes. 2. Lower the front, this gets tricky, there are a couple ways to do this. 1 slide the forks up, 2 run a smaller diameter front tire (same rim diameter) both of these will decrease the rake it will also lessen the ground clearance. A balance of working on the rear and the front can get you in the ballpark. Careful measurements and calculations will define where you are with the new front end, from there you can calculate what is required to achieve an acceptable geometry, it will also tell you if it can be achieved with the clamps you are using: 3. There may be an alternate set of stock clamps that give you the proper offset and are wider to accommodate the brake you are using. This is likely the easiest and cheapest way to get this right. The SR400/500 and XS650 come to mind. In summary, Rake no less than 25 degrees, trail between 3.6 and 3.8, while maintaining the ability to make some + or - adjustments for feel. Feel free to reach out to me with a PM, I will give you my contact # I can help you get to the actual rake and trail #s for the new setup (1" reduced offset is HUGE)
Thanks for the detailed response. I have not yet spoked a rim to my front hub, in case I need to machine it to make it fit. I will try a stock wheel just to put tires on the ground and get some baseline measurements. As it happens, I have an SR500 flattracker in my shop. A quick check revealed it has very similar triple clamp offset to my alazzurra parts. It's not so much that they have a radically small offset as that the stock 250 clamps have a giant one. Eric Watts is running a Ducati single in AHRMA with very little offset and doing well with it.
I do remember that some vintage bikes had excessive offset in the triple clamps, It is strictly the geometry, if you come out in the range of 25 degrees of rake and 3.75 inches of trail you should be golden. The stock TZ/RD triple clamps of that period all have about 40mm/1.575" of offset in them, If the new clams you have are in that range you will be fine, get the rake to about 25 degrees and you should have an awesome handling bike. As far as fork springs, given the size of the bike depending on your weight .85 if you are a light rider, .9 if not. if the forks have modern damping get it adjusted well with the proper oil weight. If not emulators (modify the damper rods per the specs) with the compression set at 4 turns to start and 15 weight oil.
