A deeper look into the 9point8™ Slack-R™

A deeper look into the 9point8™ Slack-R™

Okay, it says 1.5 degrees, but what does that mean installed on the bike?

Good question! We know some of you like to geek out on the geometry numbers, and we’d like everyone who buys a Slack-R to understand how it changes the geometry of their bike. It is a simple product, but the effect on the geometry is really complicated to work out! It varies according to your frame’s geometry, including frame size. We’ll use an example to illustrate. The bike we’ve chosen is a Santa Cruz Tallboy 3 in size Large. This is a great candidate for a Slack-R – it uses an IS headset, and while it is a well-regarded, capable bike, with a 68° head tube angle we can see a lot of riders wanting to make it just a touch slacker, just like Santa Cruz did with the latest Tallboy! The size Large Tallboy 3 has a 100mm head tube length – punching that into our calculator, we actually have two Slack-R kits available, the “Bravo” kit which lists a 1.4 head tube angle delta, and the “Echo” kit which lists 1.8 degrees. Those delta numbers tell you what the angle will be between the head tube of your frame and your fork steerer tube with the Slack-R installed, but because the Slack-R also adds some stack height due to the bearing cups, and because slackening the head tube angle actually lowers the front end of your bike, this doesn’t tell you exactly what the end result is. We’ve picked the Bravo kit for our example bike, and the resulting geometry on the Tallboy is shown in Figure 1: Santa Cruz Tallboy Example. Despite only a 1.4° delta between the head tube and the fork steerer tube, the overall change is actually 1.7°! Generally speaking that’s what we’d expect to see on most bikes – a slightly slacker head tube angle than the listed delta, a very small change in bottom bracket height, a slight increase in stack, a slight decrease in reach, and a noticeable increase in wheelbase.

2016-2019 Santa Cruz Tallboy (L)
29x2.3 Tires, Fox 34 120mm 29er Fork (527mm A2C)


Hold up, this changes my reach and stack too?

Yes it does! Since Slack-R effectively adds external upper and lower bearing cups, it raises the front end of your bike by the external height of the lower cup (about 10mm), and the upper cup adds about another 10mm to the stack measurement. Luckily, as you slacken the head tube angle, the front end of your bike drops as the fork kicks forward, and this effect balances out to a large degree the extra height due to the lower cup, which you can also see by the small change in bottom bracket height. On the top side, you still have the upper cup height to deal with, but if you’re running more than about a 10mm spacer below your stem, you can simply remove spacer(s) to get your bar height back right where it was. Keep in mind, because we’re adding stack height, you will need to remove spacers anyway – make sure your fork steerer is long enough! The change in reach is more subtle. Part of the reach change is based on the delta in head tube angle (picture the upper cup offset rearward), and part is related to the change in height of the front end. If you correct your stack height by removing spacers, you’ll also minimize the change in reach. Our example Tallboy shows an increase in stack of 9mm and a decrease in reach of 6mm, but removing a 10mm spacer would result in negligible in stack, and bring the change in (effective) reach down to about 3mm.

What if I want a steeper head tube angle? Do you make a Steep-R?

Yes we do! Actually, you can just run a Slack-R “backwards” with the alignment marks at the front of the bike. This will steepen the head tube angle, however, the effects aren’t quite what you might expect. We got a bit of a “bonus” in the change of the head tube angle on our example Tallboy with the Slack-R installed in the conventional orientation, but the same factors compound in the opposite way when you install it backwards. This basically all comes down to the added height of the cups compounded by the steeper angle raising the front end. Looking again at our Tallboy with the Bravo kit, we’d end up only making the head tube angle 0.5° steeper, and we add significantly to the stack and bottom bracket height, along with a more significant reduction in reach. Again assuming you have enough spacers under your stem, you may be able to compensate for the stack and reach changes, but short of changing your fork or reducing the travel, you’re stuck with the increase in bottom bracket height. While many riders will buy the Slack-R to slacken their head tube angle and be happy with it without ever reading any of these details, we view using it as a “Steep-R” as a niche application where riders may even be primarily interested in changing aspects of their geometry other than head tube angle.

Okay, now I’m just more confused?

If you’re not sure if Slack-R is right for you, or you want to understand exactly what the effect will be on your bike, we can help! Contact Customer Service by emailing service@9point8.ca.

ONLY COMPATIBLE WITH IS HEADSETS. (IS41 or IS42 upper bearing and IS52 lower bearing only.)