Gear

Technical FAQ: Rebuilding a Schwinn Paramount, Shimano/SRAM 12-speed mashup, and a math correction

Lennard answers questions about rebuilding a Serotta - Schwinn Paramount with a new fork, mixing and matching Shimano and SRAM components, and making a mathematical correction.

Have a question for Lennard? Please email us veloqna@comcast.net to be included in Technical FAQ.

Dear Lennard,
With more “shop time” resulting from this epidemic, I’ve built up an old Paramount Ti frame built by Ben Serotta. The rear stays can handle 28mm tires no problem, but the front fork balks at anything larger than 25mm, at 63 – 65psi. If I inflate the front tire to anything above 65psi, the crown of the tire makes contact with the inner arch of the fork. The wheels are HED Belgium C2s.

Questions:
1) Will the tire height increase much as the temperature increases — ambient air temperature and road temperature? So, if I start a ride with tire clearance (maybe 2mm), will there be enough clearance later in the ride?
2) Wound Up makes a carbon fork with 28mm clearance, 1-inch threaded steerer, and in four different offsets. If I replace the original fork, how best to decide what fork rake to use? I rode the frame for ten years and loved it before switching parts to another frameset. Some internet-sleuthing suggests it’s 44mm, but I cannot get a sure fix on this. The current fork is a straight design.

In the end, I’d like to ride this bike again with the comfort and speed of 28mm, but if it’s built with 25mm/28mm, my only concern is tire expansion, and increased chance of flats.

What do you think?
— Kevin

Dear Kevin,
In answer to your first question, yes, heat will increase the pressure in the tire and hence its diameter. You don’t want to win a Darwin Award for pumping your front tire up just enough that it clears and then zooming down a mountain on a hot day, hitting the brakes hard for a switchback, super-heating the rims, and flipping over the front and off the cliff when the tire stops the front wheel from turning.

As for your second question, that is a sweet bike you have, and you should indeed get a new fork so you can use it. Serotta made about 500 of those titanium frames to be painted as Schwinn Paramounts. The rear end on them was the same as on a Serotta Legend Ti, and the main tubes were straight-gauge 3Al/2.5V titanium.

When Schwinn decided not to have Serotta paint the frames, they ended up warehoused “raw”, here in Boulder (this was about 1998, when Schwinn/GT was in Boulder). They languished in storage for a long time (a year?) before somebody at Schwinn finally got them painted. By then, many who had ordered them were so disappointed in the long delivery time that they ended up selling for pennies on the dollar. It was another classic example of devaluing a great brand by not honoring it with good service. These frames were built about a year after Questor Partners bought Schwinn from Scott Sports Group, and about a year after they were painted; the company was in bankruptcy.

Schwinn entrusted our painter, JP Burow at Rainbow Cycle Craft, in the tiny nearby town here of Niwot, CO, with painting the titanium Paramounts. Rainbow did lots of work for Schwinn at the time, and constantly had semi-truck trailers outside full of Schwinn frames, both road and mountain. I was poignantly aware of this, because the volume of Schwinn work affected the turnaround time of the paint on my frames, as it did for other small builders in the area, including the Tomac frames made by Doug Bradbury. Schwinn was, by far, Rainbow’s biggest customer, and when the bicycle manufacturer went bankrupt, Rainbow took a steep dive toward its own demise.

Rainbow painted the titanium Paramounts in either candy red, or candy blue (maybe there was a silver option as well). The chainstays and seatstays were left raw, and they had a long, white downtube decal with a script Schwinn on it. Burow thinks the fork had a rake of around 42mm, and he remembers that they were carbon forks. He painted some steel Paramounts, too (made Tim Isaac), and those had steel forks. By the way, a “straight fork” still has offset (rake).

I forwarded your question to Ben Serotta, and he said, “We didn’t make the forks. It may have been an aluminum fork; it looked kind of like one of those Cinelli internal fork crowns. I don’t have the records to those bike, but I think they may have had a steeper head tube angle, like 74 degrees.”

In the case of such a steep head angle, you want to have either the 40mm or 43mm rake (a.k.a., offset) on that WoundUp Road X Fork. This is because the 45mm or 48mm fork offset would make the bike overly twitchy, IMHO. Trail is the lever that rights the bike, and the greater the fork trail, the more stable the bike and vice versa; increasing fork rake reduces fork trail. I personally think that fork trail around 60mm gives a good balance between stability and maneuverability.

With a 74-degree head angle and 700 X 28C tires these are the WoundUp options:
• 40mm of fork offset, resulting in 56mm of trail
• 43mm of fork rake, yielding 53mm of trail
• 45mm of fork offset, yielding 51mm of trail
• 48mm of fork rake, resulting in 48mm of trail

In high-speed riding, you want the stability of trail near 60mm. However, if you ride very slowly, you will probably like less trail. For elderly riders, I build bikes with low trail, unless they are getting an e-bike. At slow speeds, the 48mm-rake fork would be preferable, because, while the trail is low (48mm), the “wheel flop” — the amount the hub drops when you turn the handlebar, is also low, around 12mm, so the bike zigs and zags less at low speed. With the 40mm-rake WoundUp fork, rake is 56mm and wheel flop is 15mm, which is better for high speeds, and less good for low speeds.
― Lennard

Dear Lennard,
I wrote to you (quite) a while back, asking if a Shimano XT-8100 shifter would work with a SRAM GX 12-speed rear derailleur. Approximately six months and 25+ rides later (and countless hours of fun), I am happy to report that all is working fine. After some tweaking and fine tuning, my bike now shifts better than ever. I installed a new Eagle top-of-the-line chain but am using used GX cassette with limited wear.

I replaced the front shifter on my 1×12 mountain bike. It was set up with SRAM Eagle GX all around, but I prefer the shifting actuation, and frankly, the durability, of Shimano. Since I had watched videos where the author claimed success using an XTR with an Eagle X001 (but nothing involving lesser-priced models), I figured I could run an XT M-8100 shifter with a 12-speed GX rear derailleur.

There were some rough moments riding after the initial installation, all of which were cured by my LBS, and a mechanic with a much more trained hand than mine. Basically, the B-adjust screw needed to be backed out. This could be due to the fact that I swapped the drivetrain from a full suspension trail-oriented bike, to a soft-tail Ti mountain bike. Again, all is well, and I am happy to report a happy ending, without any additional expense (for now!)
— Joe

Dear Joe,
Thanks for letting us know about this option.
― Lennard

Dear Lennard,
In article on tubeless tires in the latest Gear Issue of VeloNews, Lennard Zinn makes the following statement:

“Aerodynamic drag increases exponentially with speed. It doesn’t take twice as much power to go twice as fast relative to the air; it takes four or more times as much power.”

While both parts of this statement are correct, the second is wildly inaccurate. Aero drag is proportional to the square of the speed, but the power required is drag times speed, so it is cubic. It takes “eight or more times as much power”. This has been relatively widely known for decades so it surprises me to see this statement in VeloNews.

The graph presented also appears to be somewhat inaccurate in that the difference between “aero power” and “total power” should be increasing with speed but appears to be fixed over a wide range. The numbers themselves seem off as well. I realize that the graph is presented as a visualization, but I expect that a bunch of people will be looking at the graph and taking numbers from it as though it were a chart in an engineering journal.
— Kerry

Dear Kerry,
I cringed when I saw that in print, too. My mistake; I can’t blame it on anybody else’s editing, and I’m hoping when it makes it to the Web that I can correct it.

I originally wrote about the drag going up as the square of the speed, and when I changed the sentence instead to something easier for people to visualize, namely power, I did not put in the correction. As you said, power varies as the cube of rider speed, not as the square.

The graph was courtesy of ENVE, and it is presumably based on its data. It’s possible somebody didn’t draw the curves correctly, because the blue curve is supposed to be the sum of the red curve (power absorbed overcoming aerodynamic drag) and the gray curve (power absorbed overcoming rolling resistance). Indeed, the distance between the red and blue curves should increase with speed, since power lost to rolling resistance increases with speed.
― Lennard


Lennard Zinn, our longtime technical writer, joined VeloNews in 1987. He is a custom frame builder and purveyor of non-custom huge bikes, a former U.S. national team rider, co-author of The Haywire Heart, and author of many bicycle books including Zinn and the Art of Road Bike Maintenance (DVD), as well as Zinn and the Art of Triathlon Bikes and Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists. He holds a bachelor’s in physics from Colorado College.

Follow @lennardzinn